Discussion:
Power Supply / Regulator Question
(too old to reply)
Jeff Shapiro
2007-01-18 00:41:29 UTC
Permalink
Hi,

I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was getting very
hot. Depending on the enclosure the customer used and the ambient
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months especially
from places in the country that are warmer (i.e. Texas vs Michigan).
In a later revision, a "quick fix" we did was to add about 6 diodes
in series between the 12V input and the 5V regulator to cut some of
the voltage prior to entering the regulator.

The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some external
parts.

I'm starting a new design using a RCM3700 and will need a regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply rated
at about 1A and be able to supply 12V at atleast 750mA and 5V at
maybe 250mA. The "easy" (and least expensive) way would be to just
use a 7812 to get my 12V and then run that into a 7805 to get 5V but
I'm worried about the heat issues I had with my other product.

Any recommendations for something that won't add too much cost to
the total project?

Thanks!

Jeff
Nathan Johnston
2007-01-18 01:02:19 UTC
Permalink
Use switch mode regulators.

You can get a 5V switchmode that is a pin for pin replacement for the 7805. We have used them in the past to get ourselves out of trouble. I'm not in the office at the moment so can't tell you the details of them.

Regards,
Nathan

________________________________

From: rabbit-semi-***@public.gmane.org on behalf of Jeff Shapiro
Sent: Thu 1/18/2007 11:41 AM
To: rabbit-semi-***@public.gmane.org
Subject: [rabbit-semi] Power Supply / Regulator Question



Hi,

I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was getting very
hot. Depending on the enclosure the customer used and the ambient
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months especially
from places in the country that are warmer (i.e. Texas vs Michigan).
In a later revision, a "quick fix" we did was to add about 6 diodes
in series between the 12V input and the 5V regulator to cut some of
the voltage prior to entering the regulator.

The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some external
parts.

I'm starting a new design using a RCM3700 and will need a regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply rated
at about 1A and be able to supply 12V at atleast 750mA and 5V at
maybe 250mA. The "easy" (and least expensive) way would be to just
use a 7812 to get my 12V and then run that into a 7805 to get 5V but
I'm worried about the heat issues I had with my other product.

Any recommendations for something that won't add too much cost to
the total project?

Thanks!

Jeff
Scott Henion
2007-01-18 01:05:34 UTC
Permalink
Post by Jeff Shapiro
Hi,
I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was getting very
hot. Depending on the enclosure the customer used and the ambient
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months especially
from places in the country that are warmer (i.e. Texas vs Michigan).
In a later revision, a "quick fix" we did was to add about 6 diodes
in series between the 12V input and the 5V regulator to cut some of
the voltage prior to entering the regulator.
The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some external
parts.
I'm starting a new design using a RCM3700 and will need a regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply rated
at about 1A and be able to supply 12V at atleast 750mA and 5V at
maybe 250mA. The "easy" (and least expensive) way would be to just
use a 7812 to get my 12V and then run that into a 7805 to get 5V but
I'm worried about the heat issues I had with my other product.
Any recommendations for something that won't add too much cost to
the total project?
(12-5)*0.15=1.15 watts
At .25a it is almost 1.9 watts

If the input is unregulated, the heat will be even worse. Most 12
adapters start at almost 17v unloaded. Plan a larger heatsink than
necessary. I like to keep them hardly noticably warm in free air.

Use one of the National Semi Simple Switcher supplies, I use the LM2975
on several designs. They work well. I usually get about 75% efficiency.
I have done switching supplies with 90% efficient. The ZW schematic on
the proto boards includes a sample 5v circuit. The data sheet has tables
to pick the correct inductor size. Usually it is not that critical, the
best value will maximize efficiency. The added cost is minimal; usually
less than the labor an parts cost of mounting the reg and installing a
heat sink (all parts can be automatically assembled.)

Be sure to add extra caps before and after a switching supply. I put as
large a cap as I can use before the switcher. Also have Polyfuse PTC
fuse and a cap on the input. Beside being a fuse, it also adds an
addition R/C filter on the input.

For 12V, you would probably want a LDO (Low Drop out) regulator. Most
78xx regulators need 2.5v more in than out to work well. The LDO type
usually only need about 0.6v.

------------------------------------------
| Scott G. Henion| shenion-***@public.gmane.org |
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
today's fortune
<Culus> Hhhmmmmmmmm
<Culus> waterbeds for cows
<Culus> eleet
<cas> Culus: why would a cow need a waterbed?
<Culus> cas: To be comfy warm
Dan Allen
2007-01-18 03:05:43 UTC
Permalink
Yup, I'm with the majority here too.. National Simple Switcher is the way to
go.

I've used the 2575's but am more fond of the 5007 lately for smaller loads.
They're not what I would consider "cheap" though, pushing $8 in parts is a
bit steep IMHO.
I like it/them because they are truly 'simple', the 5007 especially is teeny
tiny,
and quite flexible.. Not to mention readily available from your favorite
distributor.
National even has a feature on their web site that will visually show you a
calculate expectancy of your heat dissipation (very cool).

here's a pic of 1 of 2 of my on board power supplies using the 5007:
Loading Image...
Smaller than a dime, and no heat sink needed...


Dan...
Post by Scott Henion
Post by Jeff Shapiro
Hi,
I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was getting very
hot. Depending on the enclosure the customer used and the ambient
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months especially
from places in the country that are warmer (i.e. Texas vs Michigan).
In a later revision, a "quick fix" we did was to add about 6 diodes
in series between the 12V input and the 5V regulator to cut some of
the voltage prior to entering the regulator.
The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some external
parts.
I'm starting a new design using a RCM3700 and will need a regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply rated
at about 1A and be able to supply 12V at atleast 750mA and 5V at
maybe 250mA. The "easy" (and least expensive) way would be to just
use a 7812 to get my 12V and then run that into a 7805 to get 5V but
I'm worried about the heat issues I had with my other product.
Any recommendations for something that won't add too much cost to
the total project?
(12-5)*0.15=1.15 watts
At .25a it is almost 1.9 watts
If the input is unregulated, the heat will be even worse. Most 12
adapters start at almost 17v unloaded. Plan a larger heatsink than
necessary. I like to keep them hardly noticably warm in free air.
Use one of the National Semi Simple Switcher supplies, I use the LM2975
on several designs. They work well. I usually get about 75% efficiency.
I have done switching supplies with 90% efficient. The ZW schematic on
the proto boards includes a sample 5v circuit. The data sheet has tables
to pick the correct inductor size. Usually it is not that critical, the
best value will maximize efficiency. The added cost is minimal; usually
less than the labor an parts cost of mounting the reg and installing a
heat sink (all parts can be automatically assembled.)
Be sure to add extra caps before and after a switching supply. I put as
large a cap as I can use before the switcher. Also have Polyfuse PTC
fuse and a cap on the input. Beside being a fuse, it also adds an
addition R/C filter on the input.
For 12V, you would probably want a LDO (Low Drop out) regulator. Most
78xx regulators need 2.5v more in than out to work well. The LDO type
usually only need about 0.6v.
------------------------------------------
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
today's fortune
<Culus> Hhhmmmmmmmm
<Culus> waterbeds for cows
<Culus> eleet
<cas> Culus: why would a cow need a waterbed?
<Culus> cas: To be comfy warm
neil2452
2007-01-18 09:40:28 UTC
Permalink
Dear Dan

I looked at the photo you posted, it certainly looks promosing for
the size front except for one thing. Is that big electrolytic
necessary ?

I was thinking of trying Switching regulators after years of using
conventional regulators (ie chuck away any surplus volts as heat).
The capacitors are often a critical factors I have found in the long
life reliability of circuits. In particular I refer to electrolytic
Caps which have a short life expectancy than most other devices. To
this end I have always tried to avoid them where possible. Tantalums
were my original perfered option, although more expensive but now we
have much larger capacity ceramics available which are usually
cheaper and are'nt polarised.

Can your LM5007 design be made to work with just ceramic or tant
caps ? Can you post schematic of the regulator section ?

I have found I can buy the LM5007 from Farnell (NewarkInOne for those
in the US) as one-off for £1.57 (about $3).
Post by Dan Allen
Yup, I'm with the majority here too.. National Simple Switcher is the way to
go.
I've used the 2575's but am more fond of the 5007 lately for
smaller loads.
Post by Dan Allen
They're not what I would consider "cheap" though, pushing $8 in parts is a
bit steep IMHO.
I like it/them because they are truly 'simple', the 5007 especially is teeny
tiny,
and quite flexible.. Not to mention readily available from your favorite
distributor.
National even has a feature on their web site that will visually show you a
calculate expectancy of your heat dissipation (very cool).
http://ssminnow.maesoft.net/reflow/IMG_1021.jpg
Smaller than a dime, and no heat sink needed...
Dan...
Post by Scott Henion
Post by Jeff Shapiro
Hi,
I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was
getting very
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
hot. Depending on the enclosure the customer used and the
ambient
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months
especially
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
from places in the country that are warmer (i.e. Texas vs
Michigan).
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
In a later revision, a "quick fix" we did was to add about 6 diodes
in series between the 12V input and the 5V regulator to cut some of
the voltage prior to entering the regulator.
The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some external
parts.
I'm starting a new design using a RCM3700 and will need a
regulated
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
12V and 5V. I'd like to use an unregulated "wall wort" supply rated
at about 1A and be able to supply 12V at atleast 750mA and 5V at
maybe 250mA. The "easy" (and least expensive) way would be to just
use a 7812 to get my 12V and then run that into a 7805 to get 5V but
I'm worried about the heat issues I had with my other product.
Any recommendations for something that won't add too much cost to
the total project?
(12-5)*0.15=1.15 watts
At .25a it is almost 1.9 watts
If the input is unregulated, the heat will be even worse. Most 12
adapters start at almost 17v unloaded. Plan a larger heatsink than
necessary. I like to keep them hardly noticably warm in free air.
Use one of the National Semi Simple Switcher supplies, I use the LM2975
on several designs. They work well. I usually get about 75%
efficiency.
Post by Dan Allen
Post by Scott Henion
I have done switching supplies with 90% efficient. The ZW
schematic on
Post by Dan Allen
Post by Scott Henion
the proto boards includes a sample 5v circuit. The data sheet has tables
to pick the correct inductor size. Usually it is not that
critical, the
Post by Dan Allen
Post by Scott Henion
best value will maximize efficiency. The added cost is minimal; usually
less than the labor an parts cost of mounting the reg and
installing a
Post by Dan Allen
Post by Scott Henion
heat sink (all parts can be automatically assembled.)
Be sure to add extra caps before and after a switching supply. I put as
large a cap as I can use before the switcher. Also have Polyfuse PTC
fuse and a cap on the input. Beside being a fuse, it also adds an
addition R/C filter on the input.
For 12V, you would probably want a LDO (Low Drop out) regulator. Most
78xx regulators need 2.5v more in than out to work well. The LDO type
usually only need about 0.6v.
------------------------------------------
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
today's fortune
<Culus> Hhhmmmmmmmm
<Culus> waterbeds for cows
<Culus> eleet
<cas> Culus: why would a cow need a waterbed?
<Culus> cas: To be comfy warm
Scott Henion
2007-01-18 10:10:40 UTC
Permalink
Post by neil2452
Dear Dan
I looked at the photo you posted, it certainly looks promosing for
the size front except for one thing. Is that big electrolytic
necessary ?
Lage eletrolytics help reduce ripple.
Post by neil2452
I was thinking of trying Switching regulators after years of using
conventional regulators (ie chuck away any surplus volts as heat).
I have been doing switching regs for many years. Really the only way to
go. I do prefer linear if the board has analog.
Post by neil2452
The capacitors are often a critical factors I have found in the long
life reliability of circuits. In particular I refer to electrolytic
Caps which have a short life expectancy than most other devices. To
this end I have always tried to avoid them where possible.
I have never had any fail. As long as you do not run them hard they are
fine. Spec'ing industrial temp caps help if they are in a warm area.

PC's have had them fail as they are running 100amp supplies with a huge
ripple current. The caps degrade due to the heat caused by continuous
A/C current. The better MB's are now using industrial temp caps.
Post by neil2452
Tantalums
were my original perfered option, although more expensive but now we
have much larger capacity ceramics available which are usually
cheaper and are'nt polarised.
Tantalums have a habit of shorting out and catching fire. They do not
like voltage spikes. They are only good after a regulator. Back when I
was repairing car electronics, it was probably one of the most common
failures I saw. Was hard to repair boards with holes burned in them ;)
Then again cars often have 150V spikes on the 12V. I could not believe
the designers that put them on the input side of the power supplies.

Using wall-wart supplies will not have so huge a spike, but they still
have spikes/noise on them. The PTC/cap filter I use works well.
Post by neil2452
Can your LM5007 design be made to work with just ceramic or tant
caps ? Can you post schematic of the regulator section ?
Probably. I use electrolytics on the input side. For low ripple, I use
electrolytic's paralleled with a ceramic.
Post by neil2452
I have found I can buy the LM5007 from Farnell (NewarkInOne for those
in the US) as one-off for £1.57 (about $3).
The LM5007 look good. Nice to see National came out with MOSFET
versions. That eliminates the inefficiency of the Darlington versions
they had.

------------------------------------------
| Scott G. Henion| shenion-***@public.gmane.org |
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
today's fortune
I only know what I read in the papers.
-- Will Rogers
neil2452
2007-01-18 11:00:03 UTC
Permalink
Yea I have seen Tants go bang and emit acrid plooms of smoke, usually
because I got the polarity wrong, very dramatic. You get similar
results with elec caps too.

A lot of the designs I do are for permantantly running installations.
I recently repair some cicuits I installed 12 years ago by replacing
the Elec Caps. In hind sight I am surprised they lasted so well.

If you look at the data sheets for Aluminium electrolytic caps you
should see an operational life figure quoted in hours. Figures vary
from device to device but figures of 2000-8000 hours is quite
typical. If you look around you can find some in the 30,000h range
but these tend to be much bigger value caps (greater than 1000uf). I
don't think the elec cap fails on the 8001 hour but I guess the
performace starts to fall off. If its had an easy life it should
last a lot longer, ie not too hot and a low ripple current.

Its not to say you should never use elec caps but consider what the
consequneces may be if it fails after 8,000 hours. When your circuit
does'nt work anymore after 5yrs which part are going check first ?
Will it be the CPU with an MTBF of 100,000hours or that Elec Cap with
a stated life of 8000 hours ?
Post by Scott Henion
Post by neil2452
Dear Dan
I looked at the photo you posted, it certainly looks promosing for
the size front except for one thing. Is that big electrolytic
necessary ?
Lage eletrolytics help reduce ripple.
Post by neil2452
I was thinking of trying Switching regulators after years of
using
Post by Scott Henion
Post by neil2452
conventional regulators (ie chuck away any surplus volts as
heat).
Post by Scott Henion
I have been doing switching regs for many years. Really the only way to
go. I do prefer linear if the board has analog.
Post by neil2452
The capacitors are often a critical factors I have found in the long
life reliability of circuits. In particular I refer to
electrolytic
Post by Scott Henion
Post by neil2452
Caps which have a short life expectancy than most other devices.
To
Post by Scott Henion
Post by neil2452
this end I have always tried to avoid them where possible.
I have never had any fail. As long as you do not run them hard they are
fine. Spec'ing industrial temp caps help if they are in a warm area.
PC's have had them fail as they are running 100amp supplies with a huge
ripple current. The caps degrade due to the heat caused by
continuous
Post by Scott Henion
A/C current. The better MB's are now using industrial temp caps.
Post by neil2452
Tantalums
were my original perfered option, although more expensive but now we
have much larger capacity ceramics available which are usually
cheaper and are'nt polarised.
Tantalums have a habit of shorting out and catching fire. They do not
like voltage spikes. They are only good after a regulator. Back when I
was repairing car electronics, it was probably one of the most
common
Post by Scott Henion
failures I saw. Was hard to repair boards with holes burned in
them ;)
Post by Scott Henion
Then again cars often have 150V spikes on the 12V. I could not
believe
Post by Scott Henion
the designers that put them on the input side of the power supplies.
Using wall-wart supplies will not have so huge a spike, but they still
have spikes/noise on them. The PTC/cap filter I use works well.
Post by neil2452
Can your LM5007 design be made to work with just ceramic or tant
caps ? Can you post schematic of the regulator section ?
Probably. I use electrolytics on the input side. For low ripple, I use
electrolytic's paralleled with a ceramic.
Post by neil2452
I have found I can buy the LM5007 from Farnell (NewarkInOne for those
in the US) as one-off for £1.57 (about $3).
The LM5007 look good. Nice to see National came out with MOSFET
versions. That eliminates the inefficiency of the Darlington
versions
Post by Scott Henion
they had.
------------------------------------------
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
today's fortune
I only know what I read in the papers.
-- Will Rogers
Scott Henion
2007-01-18 13:49:59 UTC
Permalink
Post by neil2452
Yea I have seen Tants go bang and emit acrid plooms of smoke, usually
because I got the polarity wrong, very dramatic. You get similar
results with elec caps too.
Electr caps usually just blow the caps off. I have seen tantalums shoot
out a 2" flame for several seconds. I had a batch that were marked
backwards.

Although I have never seen electrolytics blow their can in anything that
did not abuse them.
Post by neil2452
If you look at the data sheets for Aluminium electrolytic caps you
should see an operational life figure quoted in hours. Figures vary
from device to device but figures of 2000-8000 hours is quite
typical. If you look around you can find some in the 30,000h range
but these tend to be much bigger value caps (greater than 1000uf). I
don't think the elec cap fails on the 8001 hour but I guess the
performace starts to fall off. If its had an easy life it should
last a lot longer, ie not too hot and a low ripple current.
The "Failure" is when it fails to meet the specs. I would assume the
capacitance degrades and the ESR increases.

Consider a A/C rectifier filter cap. If it is used hard, as it degrades,
the capacitance will decrease, the ripple will increase and the heat
generated also. If the cap is minimal, it is only a matter of time
before it will fail.

That is why my general rule for electrolytics is find the size you need,
double it. Then see if you can go bigger or higher voltage and still
meet size/cost constraints. They are cheap these days.

Any time I have a cap that is handling any serious current, I use two in
parallel. Makes ESR lower and provides redundancy if one fails. The most
common failure I have seen with electro's is someone knocked them hard
enough to pull the leads out.

I had several switching designs done over 16 years ago. These were 24v,
9 amp, +5 at 5A. No reported failures. Again, these had probably 4 caps
on each output. 2 would have been enough, but this is a system that runs
24/7/365.
Post by neil2452
Its not to say you should never use elec caps but consider what the
consequneces may be if it fails after 8,000 hours. When your circuit
does'nt work anymore after 5yrs which part are going check first ?
Will it be the CPU with an MTBF of 100,000hours or that Elec Cap with
a stated life of 8000 hours ?
I'ld check the user with a Mean Time Before Doing Something Stupid of
about 300 hours and find out what he did ;)

------------------------------------------
| Scott G. Henion| shenion-***@public.gmane.org |
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
today's fortune
It [being a Vulcan] means to adopt a philosophy, a way of life which is
logical and beneficial. We cannot disregard that philosophy merely for
personal gain, no matter how important that gain might be.
-- Spock, "Journey to Babel", stardate 3842.4
Dan Allen
2007-01-18 19:37:25 UTC
Permalink
Big?? :)

That's the 2nd smallest package DigiKey had in a through-hole cap.
I had some low profile ones, but they were much wider and the tall
narrow one worked better in my situation.

Yes, it was just one I had in my box-o-parts...
I usually put a 22uF 63v jobber in there which is /the/ smallest one I can
find.
Is it reallllllly necessary? Well, it was built with a pretty wide range of
input power,
including straight AC power, in which case I'd say definitely yes.
If you're feeding it clean DC power and a consistent load --eh, maybe not.
I'd definitely consider either an input side, or an output side cap though,
current board layout includes both.



Dan...
Post by neil2452
Dear Dan
I looked at the photo you posted, it certainly looks promosing for
the size front except for one thing. Is that big electrolytic
necessary ?
I was thinking of trying Switching regulators after years of using
conventional regulators (ie chuck away any surplus volts as heat).
The capacitors are often a critical factors I have found in the long
life reliability of circuits. In particular I refer to electrolytic
Caps which have a short life expectancy than most other devices. To
this end I have always tried to avoid them where possible. Tantalums
were my original perfered option, although more expensive but now we
have much larger capacity ceramics available which are usually
cheaper and are'nt polarised.
Can your LM5007 design be made to work with just ceramic or tant
caps ? Can you post schematic of the regulator section ?
I have found I can buy the LM5007 from Farnell (NewarkInOne for those
in the US) as one-off for £1.57 (about $3).
Post by Dan Allen
Yup, I'm with the majority here too.. National Simple Switcher is
the way to
Post by Dan Allen
go.
I've used the 2575's but am more fond of the 5007 lately for
smaller loads.
Post by Dan Allen
They're not what I would consider "cheap" though, pushing $8 in
parts is a
Post by Dan Allen
bit steep IMHO.
I like it/them because they are truly 'simple', the 5007 especially
is teeny
Post by Dan Allen
tiny,
and quite flexible.. Not to mention readily available from your
favorite
Post by Dan Allen
distributor.
National even has a feature on their web site that will visually
show you a
Post by Dan Allen
calculate expectancy of your heat dissipation (very cool).
http://ssminnow.maesoft.net/reflow/IMG_1021.jpg
Smaller than a dime, and no heat sink needed...
Dan...
Post by Scott Henion
Post by Jeff Shapiro
Hi,
I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was
getting very
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
hot. Depending on the enclosure the customer used and the
ambient
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months
especially
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
from places in the country that are warmer (i.e. Texas vs
Michigan).
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
In a later revision, a "quick fix" we did was to add about 6
diodes
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
in series between the 12V input and the 5V regulator to cut
some of
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
the voltage prior to entering the regulator.
The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some
external
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
parts.
I'm starting a new design using a RCM3700 and will need a
regulated
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
12V and 5V. I'd like to use an unregulated "wall wort" supply
rated
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
at about 1A and be able to supply 12V at atleast 750mA and 5V at
maybe 250mA. The "easy" (and least expensive) way would be to
just
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
use a 7812 to get my 12V and then run that into a 7805 to get
5V but
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
I'm worried about the heat issues I had with my other product.
Any recommendations for something that won't add too much cost
to
Post by Dan Allen
Post by Scott Henion
Post by Jeff Shapiro
the total project?
(12-5)*0.15=1.15 watts
At .25a it is almost 1.9 watts
If the input is unregulated, the heat will be even worse. Most 12
adapters start at almost 17v unloaded. Plan a larger heatsink than
necessary. I like to keep them hardly noticably warm in free air.
Use one of the National Semi Simple Switcher supplies, I use the
LM2975
Post by Dan Allen
Post by Scott Henion
on several designs. They work well. I usually get about 75%
efficiency.
Post by Dan Allen
Post by Scott Henion
I have done switching supplies with 90% efficient. The ZW
schematic on
Post by Dan Allen
Post by Scott Henion
the proto boards includes a sample 5v circuit. The data sheet has
tables
Post by Dan Allen
Post by Scott Henion
to pick the correct inductor size. Usually it is not that
critical, the
Post by Dan Allen
Post by Scott Henion
best value will maximize efficiency. The added cost is minimal;
usually
Post by Dan Allen
Post by Scott Henion
less than the labor an parts cost of mounting the reg and
installing a
Post by Dan Allen
Post by Scott Henion
heat sink (all parts can be automatically assembled.)
Be sure to add extra caps before and after a switching supply. I
put as
Post by Dan Allen
Post by Scott Henion
large a cap as I can use before the switcher. Also have Polyfuse
PTC
Post by Dan Allen
Post by Scott Henion
fuse and a cap on the input. Beside being a fuse, it also adds an
addition R/C filter on the input.
For 12V, you would probably want a LDO (Low Drop out) regulator.
Most
Post by Dan Allen
Post by Scott Henion
78xx regulators need 2.5v more in than out to work well. The LDO
type
Post by Dan Allen
Post by Scott Henion
usually only need about 0.6v.
------------------------------------------
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
today's fortune
<Culus> Hhhmmmmmmmm
<Culus> waterbeds for cows
<Culus> eleet
<cas> Culus: why would a cow need a waterbed?
<Culus> cas: To be comfy warm
Alexis
2007-01-18 06:39:11 UTC
Permalink
I agree with Scott.

Another option you can consider is the LNK500 family. Most agents also
stock the magnetics required. This solution will allow you to drop the
Walmart adaptor. I do however agree that the simpler solution using a
simple switcher with input protection is the way to go. Maybe the
addition of a transzorb and a reverse polarity protection diode would be
worth considering. An interesting thought - your design is only as
robust as your PSU is. ST also have a part the L5973D, they cost less
than a $ and can supply up to 2.5 amps.

Alexis
Post by Scott Henion
Post by Jeff Shapiro
Hi,
I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was getting very
hot. Depending on the enclosure the customer used and the ambient
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months especially
from places in the country that are warmer (i.e. Texas vs Michigan).
In a later revision, a "quick fix" we did was to add about 6 diodes
in series between the 12V input and the 5V regulator to cut some of
the voltage prior to entering the regulator.
The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some external
parts.
I'm starting a new design using a RCM3700 and will need a regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply rated
at about 1A and be able to supply 12V at atleast 750mA and 5V at
maybe 250mA. The "easy" (and least expensive) way would be to just
use a 7812 to get my 12V and then run that into a 7805 to get 5V but
I'm worried about the heat issues I had with my other product.
Any recommendations for something that won't add too much cost to
the total project?
(12-5)*0.15=1.15 watts
At .25a it is almost 1.9 watts
If the input is unregulated, the heat will be even worse. Most 12
adapters start at almost 17v unloaded. Plan a larger heatsink than
necessary. I like to keep them hardly noticably warm in free air.
Use one of the National Semi Simple Switcher supplies, I use the LM2975
on several designs. They work well. I usually get about 75% efficiency.
I have done switching supplies with 90% efficient. The ZW schematic on
the proto boards includes a sample 5v circuit. The data sheet has tables
to pick the correct inductor size. Usually it is not that critical, the
best value will maximize efficiency. The added cost is minimal; usually
less than the labor an parts cost of mounting the reg and installing a
heat sink (all parts can be automatically assembled.)
Be sure to add extra caps before and after a switching supply. I put as
large a cap as I can use before the switcher. Also have Polyfuse PTC
fuse and a cap on the input. Beside being a fuse, it also adds an
addition R/C filter on the input.
For 12V, you would probably want a LDO (Low Drop out) regulator. Most
78xx regulators need 2.5v more in than out to work well. The LDO type
usually only need about 0.6v.
------------------------------------------
<mailto:shenion%40shdesigns.org> |
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org <http://www.shdesigns.org> |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
<http://www.shdesigns.org/rabbit/>
today's fortune
<Culus> Hhhmmmmmmmm
<Culus> waterbeds for cows
<Culus> eleet
<cas> Culus: why would a cow need a waterbed?
<Culus> cas: To be comfy warm
Dave Moore
2007-01-18 01:15:21 UTC
Permalink
Pop a little switcher on there...take a look at the National LM267X parts.

Don't those diodes get awfully hot? ;-)


_____

From: rabbit-semi-***@public.gmane.org [mailto:rabbit-semi-***@public.gmane.org] On
Behalf Of Jeff Shapiro
Sent: Wednesday, January 17, 2007 4:41 PM
To: rabbit-semi-***@public.gmane.org
Subject: [rabbit-semi] Power Supply / Regulator Question



Hi,

I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was getting very
hot. Depending on the enclosure the customer used and the ambient
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months especially
from places in the country that are warmer (i.e. Texas vs Michigan).
In a later revision, a "quick fix" we did was to add about 6 diodes
in series between the 12V input and the 5V regulator to cut some of
the voltage prior to entering the regulator.

The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some external
parts.

I'm starting a new design using a RCM3700 and will need a regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply rated
at about 1A and be able to supply 12V at atleast 750mA and 5V at
maybe 250mA. The "easy" (and least expensive) way would be to just
use a 7812 to get my 12V and then run that into a 7805 to get 5V but
I'm worried about the heat issues I had with my other product.

Any recommendations for something that won't add too much cost to
the total project?

Thanks!

Jeff
np np
2007-01-18 01:18:31 UTC
Permalink
A quick and cheap fix would be a power resistor.

http://www.ckp-railways.talktalk.net/pcbcad28.htm


Dave Moore <dmoore-f4pv2F5LI2c/CDIEhCN/twC/***@public.gmane.org> wrote:
Pop a little switcher on there...take a look at the National LM267X parts.

Don't those diodes get awfully hot? ;-)


---------------------------------
From: rabbit-semi-***@public.gmane.org [mailto:rabbit-semi-***@public.gmane.org] On Behalf Of Jeff Shapiro
Sent: Wednesday, January 17, 2007 4:41 PM
To: rabbit-semi-***@public.gmane.org
Subject: [rabbit-semi] Power Supply / Regulator Question



Hi,

I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was getting very
hot. Depending on the enclosure the customer used and the ambient
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months especially
from places in the country that are warmer (i.e. Texas vs Michigan).
In a later revision, a "quick fix" we did was to add about 6 diodes
in series between the 12V input and the 5V regulator to cut some of
the voltage prior to entering the regulator.

The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some external
parts.

I'm starting a new design using a RCM3700 and will need a regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply rated
at about 1A and be able to supply 12V at atleast 750mA and 5V at
maybe 250mA. The "easy" (and least expensive) way would be to just
use a 7812 to get my 12V and then run that into a 7805 to get 5V but
I'm worried about the heat issues I had with my other product.

Any recommendations for something that won't add too much cost to
the total project?

Thanks!

Jeff








---------------------------------
New Yahoo! Mail is the ultimate force in competitive emailing. Find out more at the Yahoo! Mail Championships. Plus: play games and win prizes.
Fournier, Pete
2007-01-18 14:08:13 UTC
Permalink
I agree with Nathan: You can buy switchers that just drop into the
LM7805 pin holes. The one I have is a Power Trends 78SR105VC. I think
Power Trends is TI now.
If you have to fix the ones in the field that is the way to go. If you
are going to cut a new PCB then you can add your own with a switcher
chip and maybe save some $. Don't forget that your time is money too.
Make sure you listen to the discussion on capacitors that's been going
on here. Lots of good info from the trenches!!
-Pete


-----Original Message-----
From: rabbit-semi-***@public.gmane.org
[mailto:rabbit-semi-***@public.gmane.org] On Behalf Of Nathan Johnston
Sent: Wednesday, January 17, 2007 8:02 PM
To: rabbit-semi-***@public.gmane.org
Subject: RE: [rabbit-semi] Power Supply / Regulator Question


Use switch mode regulators.

You can get a 5V switchmode that is a pin for pin replacement
for the 7805. We have used them in the past to get ourselves out of
trouble. I'm not in the office at the moment so can't tell you the
details of them.

Regards,
Nathan

________________________________

From: rabbit-semi-***@public.gmane.org on behalf of Jeff Shapiro
Sent: Thu 1/18/2007 11:41 AM
To: rabbit-semi-***@public.gmane.org
Subject: [rabbit-semi] Power Supply / Regulator Question



Hi,

I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was getting
very
hot. Depending on the enclosure the customer used and the
ambient
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months
especially
from places in the country that are warmer (i.e. Texas vs
Michigan).
In a later revision, a "quick fix" we did was to add about 6
diodes
in series between the 12V input and the 5V regulator to cut some
of
the voltage prior to entering the regulator.

The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some
external
parts.

I'm starting a new design using a RCM3700 and will need a
regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply
rated
at about 1A and be able to supply 12V at atleast 750mA and 5V at

maybe 250mA. The "easy" (and least expensive) way would be to
just
use a 7812 to get my 12V and then run that into a 7805 to get 5V
but
I'm worried about the heat issues I had with my other product.

Any recommendations for something that won't add too much cost
to
the total project?

Thanks!

Jeff
neil2452
2007-01-18 14:51:56 UTC
Permalink
I just had a look at the 78SR105VC on Farnell at :
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=485147

I would agree with pete. Its a good solution for designs that have
already been commited too but I would'nt use it in new ones because
its not SMD and its a bit expensive, £20 ($40) each. Its bigger than
the original TO220 package it replaces so check the space you need.

I note from the picture it only uses SMD ceramic caps, so in my
opinion life and reliablity should be very good.

It would be nice to have a schematic of a similar tried and tested
minature circuit design to just drop into my next project.

Neil
Post by Fournier, Pete
I agree with Nathan: You can buy switchers that just drop into the
LM7805 pin holes. The one I have is a Power Trends 78SR105VC. I think
Power Trends is TI now.
If you have to fix the ones in the field that is the way to go. If you
are going to cut a new PCB then you can add your own with a switcher
chip and maybe save some $. Don't forget that your time is money too.
Make sure you listen to the discussion on capacitors that's been going
on here. Lots of good info from the trenches!!
-Pete
-----Original Message-----
Sent: Wednesday, January 17, 2007 8:02 PM
Subject: RE: [rabbit-semi] Power Supply / Regulator Question
Use switch mode regulators.
You can get a 5V switchmode that is a pin for pin replacement
for the 7805. We have used them in the past to get ourselves out of
trouble. I'm not in the office at the moment so can't tell you the
details of them.
Regards,
Nathan
________________________________
Sent: Thu 1/18/2007 11:41 AM
Subject: [rabbit-semi] Power Supply / Regulator Question
Hi,
I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was
getting
Post by Fournier, Pete
very
hot. Depending on the enclosure the customer used and the
ambient
temperature I believe it led to some Rabbit module failures.
It
Post by Fournier, Pete
seemed like we got more failures during the warmer months
especially
from places in the country that are warmer (i.e. Texas vs
Michigan).
In a later revision, a "quick fix" we did was to add about 6
diodes
in series between the 12V input and the 5V regulator to cut
some
Post by Fournier, Pete
of
the voltage prior to entering the regulator.
The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some
external
parts.
I'm starting a new design using a RCM3700 and will need a
regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply
rated
at about 1A and be able to supply 12V at atleast 750mA and 5V
at
Post by Fournier, Pete
maybe 250mA. The "easy" (and least expensive) way would be to
just
use a 7812 to get my 12V and then run that into a 7805 to get
5V
Post by Fournier, Pete
but
I'm worried about the heat issues I had with my other product.
Any recommendations for something that won't add too much cost
to
the total project?
Thanks!
Jeff
Scott Henion
2007-01-18 15:15:56 UTC
Permalink
Post by neil2452
It would be nice to have a schematic of a similar tried and tested
minature circuit design to just drop into my next project.
Neil
Look at the schematics for the RCM3700 prototype board. Also the 3200
and 3300 prototype boards.

I believe the RCM3800 has an onboard regulator.

------------------------------------------
| Scott G. Henion| shenion-***@public.gmane.org |
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
today's fortune

In the X-Men movies, none of the X-Men super-powers are done with special effects. Chuck Norris is the stuntman for every character.
neil2452
2007-01-18 16:17:09 UTC
Permalink
I have RCM3200 proto board and I hate to harp on but its got one of
them big Elec Caps next to the inductor.

See this device : TRACO, TOS 06-12SM
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=1007586

Its another solution I have used else where to power a load of PIC
based LED lighting circuits with 5v. Its a SMD Point of Load DC-DC
converter (basicly a switching regulator)(no siolation between i/p &
o/p). You can get 6A out of the small device, it fits on 2p coin,
all ceramic cap construction, theres no minimum load and you can
program the output voltage with a single resistor so you can have
3.3v, 5v or 12v what ever you want as the O/P. Its not as cheap as
your own reg circuit but the current capabillity it pretty good (UK
cost is £13 ($26)). I encased 25 of these in PU resin for out door
use and they ended up smaller than an AA batery cell.

Neil
Post by Scott Henion
Post by neil2452
It would be nice to have a schematic of a similar tried and
tested
Post by Scott Henion
Post by neil2452
minature circuit design to just drop into my next project.
Neil
Look at the schematics for the RCM3700 prototype board. Also the 3200
and 3300 prototype boards.
I believe the RCM3800 has an onboard regulator.
------------------------------------------
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
today's fortune
In the X-Men movies, none of the X-Men super-powers are done with
special effects. Chuck Norris is the stuntman for every character.
Dan Allen
2007-01-19 01:12:55 UTC
Permalink
eeeeew, Vin Max: 14v

Mine:
Vin Max: 63v (AC or DC)
Vout 1: 5v
Vout 2: 12v (adj w/ one resistor)
Iout 1: Current limited to 200mA (Max 420mA)
Iout 2: Current limited to 400mA (Max 420mA)

Small size: smaller than a dime (double sided board)
Single quantity cost (@DigiKey) ~$10



Dan...
Post by neil2452
I have RCM3200 proto board and I hate to harp on but its got one of
them big Elec Caps next to the inductor.
See this device : TRACO, TOS 06-12SM
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=1007586
Its another solution I have used else where to power a load of PIC
based LED lighting circuits with 5v. Its a SMD Point of Load DC-DC
converter (basicly a switching regulator)(no siolation between i/p &
o/p). You can get 6A out of the small device, it fits on 2p coin,
all ceramic cap construction, theres no minimum load and you can
program the output voltage with a single resistor so you can have
3.3v, 5v or 12v what ever you want as the O/P. Its not as cheap as
your own reg circuit but the current capabillity it pretty good (UK
cost is £13 ($26)). I encased 25 of these in PU resin for out door
use and they ended up smaller than an AA batery cell.
Neil
Post by Scott Henion
Post by neil2452
It would be nice to have a schematic of a similar tried and
tested
Post by Scott Henion
Post by neil2452
minature circuit design to just drop into my next project.
Neil
Look at the schematics for the RCM3700 prototype board. Also the
3200
Post by Scott Henion
and 3300 prototype boards.
I believe the RCM3800 has an onboard regulator.
------------------------------------------
| Consultant | Stone Mountain, GA |
| SHDesigns http://www.shdesigns.org |
------------------------------------------
Rabbit libs: http://www.shdesigns.org/rabbit/
today's fortune
In the X-Men movies, none of the X-Men super-powers are done with
special effects. Chuck Norris is the stuntman for every character.
mehiegl
2007-01-19 11:55:21 UTC
Permalink
Comparable part can be had from Digikey for $14-$18.

78SR105VC 7-30V input, 5V, 1.5A output, $18.05 in single piece.
78ST105VC 7-30V input, 5V, 1.5A output, $18.05 in single piece.
PT78ST105V 9-38V input, 5V, 1.5A output, $14.41 in single piece.

All are TI parts Power Trends line.

They work pretty well though you get a fair amount of relatively low
frequency (khz range as opposed to the 1MHZ switching frequency)
ripple if they are too lightly loaded.

They are bigger than a TO220 without a heatsink but they don't get
anywhere near as hot. There are horizontal mount versions too if
your board is designed to mount the 7805 flat on the board.

Mark
Post by neil2452
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=485147
I would agree with pete. Its a good solution for designs that
have
Post by neil2452
already been commited too but I would'nt use it in new ones
because
Post by neil2452
its not SMD and its a bit expensive, £20 ($40) each. Its bigger than
the original TO220 package it replaces so check the space you need.
I note from the picture it only uses SMD ceramic caps, so in my
opinion life and reliablity should be very good.
It would be nice to have a schematic of a similar tried and tested
minature circuit design to just drop into my next project.
Neil
Post by Fournier, Pete
I agree with Nathan: You can buy switchers that just drop into the
LM7805 pin holes. The one I have is a Power Trends 78SR105VC. I
think
Post by Fournier, Pete
Power Trends is TI now.
If you have to fix the ones in the field that is the way to go.
If
Post by neil2452
you
Post by Fournier, Pete
are going to cut a new PCB then you can add your own with a
switcher
Post by neil2452
Post by Fournier, Pete
chip and maybe save some $. Don't forget that your time is
money
Post by neil2452
too.
Post by Fournier, Pete
Make sure you listen to the discussion on capacitors that's been
going
Post by Fournier, Pete
on here. Lots of good info from the trenches!!
-Pete
-----Original Message-----
Sent: Wednesday, January 17, 2007 8:02 PM
Subject: RE: [rabbit-semi] Power Supply / Regulator Question
Use switch mode regulators.
You can get a 5V switchmode that is a pin for pin replacement
for the 7805. We have used them in the past to get ourselves out of
trouble. I'm not in the office at the moment so can't tell you the
details of them.
Regards,
Nathan
________________________________
Sent: Thu 1/18/2007 11:41 AM
Subject: [rabbit-semi] Power Supply / Regulator Question
Hi,
I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was
getting
Post by Fournier, Pete
very
hot. Depending on the enclosure the customer used and the
ambient
temperature I believe it led to some Rabbit module failures.
It
Post by Fournier, Pete
seemed like we got more failures during the warmer months
especially
from places in the country that are warmer (i.e. Texas vs
Michigan).
In a later revision, a "quick fix" we did was to add about 6
diodes
in series between the 12V input and the 5V regulator to cut
some
Post by Fournier, Pete
of
the voltage prior to entering the regulator.
The input to the board is about 12.5V and the output from
the
Post by neil2452
Post by Fournier, Pete
regulator is 5V at about 150mA driving the RCM2200 and some
external
parts.
I'm starting a new design using a RCM3700 and will need a
regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply
rated
at about 1A and be able to supply 12V at atleast 750mA and
5V
Post by neil2452
at
Post by Fournier, Pete
maybe 250mA. The "easy" (and least expensive) way would be to
just
use a 7812 to get my 12V and then run that into a 7805 to
get
Post by neil2452
5V
Post by Fournier, Pete
but
I'm worried about the heat issues I had with my other
product.
Post by neil2452
Post by Fournier, Pete
Any recommendations for something that won't add too much
cost
Post by neil2452
Post by Fournier, Pete
to
the total project?
Thanks!
Jeff
Fournier, Pete
2007-01-18 15:27:26 UTC
Permalink
Yes it is bigger, but then you don't need the heat sink! Also it is somewhat older, so maybe there is a newer/smaller/cheaper parts out there.

Neil is right, don't use it for a new design. Retrofitting is exactly where we use it. When you have 50 boards in stock and a hundred in the field and you are down to selling only selling 10 a year the finances of the replacement device make sense. Our new boards have more customized designs.

-Pete
Post by Fournier, Pete
-----Original Message-----
Sent: Thursday, January 18, 2007 9:52 AM
Subject: [rabbit-semi] Re: Power Supply / Regulator Question
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=485147
I would agree with pete. Its a good solution for designs that have
already been commited too but I would'nt use it in new ones because
its not SMD and its a bit expensive, £20 ($40) each. Its bigger than
the original TO220 package it replaces so check the space you need.
I note from the picture it only uses SMD ceramic caps, so in my
opinion life and reliablity should be very good.
It would be nice to have a schematic of a similar tried and tested
minature circuit design to just drop into my next project.
Neil
Post by Fournier, Pete
I agree with Nathan: You can buy switchers that just drop into the
LM7805 pin holes. The one I have is a Power Trends 78SR105VC. I
think
Post by Fournier, Pete
Power Trends is TI now.
If you have to fix the ones in the field that is the way to go. If
you
Post by Fournier, Pete
are going to cut a new PCB then you can add your own with a
switcher
Post by Fournier, Pete
chip and maybe save some $. Don't forget that your time is money
too.
Post by Fournier, Pete
Make sure you listen to the discussion on capacitors that's been
going
Post by Fournier, Pete
on here. Lots of good info from the trenches!!
-Pete
-----Original Message-----
Sent: Wednesday, January 17, 2007 8:02 PM
Subject: RE: [rabbit-semi] Power Supply / Regulator Question
Use switch mode regulators.
You can get a 5V switchmode that is a pin for pin
replacement for the
Post by Fournier, Pete
7805. We have used them in the past to get ourselves out of
trouble.
Post by Fournier, Pete
I'm not in the office at the moment so can't tell you the
details of
Post by Fournier, Pete
them.
Regards,
Nathan
________________________________
Sent: Thu 1/18/2007 11:41 AM
Subject: [rabbit-semi] Power Supply / Regulator Question
Hi,
I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was
getting
Post by Fournier, Pete
very
hot. Depending on the enclosure the customer used and
the ambient
Post by Fournier, Pete
temperature I believe it led to some Rabbit module failures.
It
Post by Fournier, Pete
seemed like we got more failures during the warmer
months especially
Post by Fournier, Pete
from places in the country that are warmer (i.e. Texas vs
Michigan).
In a later revision, a "quick fix" we did was to add about 6
diodes
in series between the 12V input and the 5V regulator to cut
some
Post by Fournier, Pete
of
the voltage prior to entering the regulator.
The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and
some external
Post by Fournier, Pete
parts.
I'm starting a new design using a RCM3700 and will need
a regulated
Post by Fournier, Pete
12V and 5V. I'd like to use an unregulated "wall wort" supply
rated
at about 1A and be able to supply 12V at atleast 750mA and 5V
at
Post by Fournier, Pete
maybe 250mA. The "easy" (and least expensive) way would be to
just
use a 7812 to get my 12V and then run that into a 7805 to get
5V
Post by Fournier, Pete
but
I'm worried about the heat issues I had with my other product.
Any recommendations for something that won't add too much cost
to
the total project?
Thanks!
Jeff
Yahoo! Groups Links
Richard Wayman
2007-01-18 23:19:30 UTC
Permalink
Hi Jeff,

Using a linear regulator in either of your configurations is going to generate a lot of heat. The 5V regulator has to drop 7V, and dissipates this as heat (1.05W). The 12V regulator dissipates approximately 2.25W assuming the 12V regulator is a 1-2V LDO type. That's over 3W of heat (lost in overhead for the regulators). That's a 3W penalty to generate less than 10 Watts.

Its primarily the 5V regulator that would gain if a Switching regulator were used. Even at 80% efficiency the 5V regulator would require 150mW, significantly less than 1W. The 12V supply would not see the same efficiency gain since it would still require 1.8W dissipated in its pass elements.

Linear Technology has hundreds of switched regulator designs that would work here. If you don't want to change the circuit board, I believe there are even 3 Terminal replacements for the most common linear regulators.

Rich


----- Original Message -----
From: Jeff Shapiro
To: rabbit-semi-***@public.gmane.org
Sent: Wednesday, January 17, 2007 4:41 PM
Subject: [rabbit-semi] Power Supply / Regulator Question


Hi,

I have an RCM2200 design that I suspect had problems in some
occasions because the LM7805 (TO-220 with heat sink) was getting very
hot. Depending on the enclosure the customer used and the ambient
temperature I believe it led to some Rabbit module failures. It
seemed like we got more failures during the warmer months especially
from places in the country that are warmer (i.e. Texas vs Michigan).
In a later revision, a "quick fix" we did was to add about 6 diodes
in series between the 12V input and the 5V regulator to cut some of
the voltage prior to entering the regulator.

The input to the board is about 12.5V and the output from the
regulator is 5V at about 150mA driving the RCM2200 and some external
parts.

I'm starting a new design using a RCM3700 and will need a regulated
12V and 5V. I'd like to use an unregulated "wall wort" supply rated
at about 1A and be able to supply 12V at atleast 750mA and 5V at
maybe 250mA. The "easy" (and least expensive) way would be to just
use a 7812 to get my 12V and then run that into a 7805 to get 5V but
I'm worried about the heat issues I had with my other product.

Any recommendations for something that won't add too much cost to
the total project?

Thanks!

Jeff
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