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
--- In r..., "neil2452"
wrote:
>
> I just had a look at the 78SR105VC on Farnell at :
> http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKUH5147
>
> 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
> --- In r..., "Fournier, Pete"
> wrote:
> >
> > 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: r...
> > [mailto:r...] On Behalf Of Nathan Johnston
> > Sent: Wednesday, January 17, 2007 8:02 PM
> > To: r...
> > 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: r... on behalf of Jeff Shapiro
> > Sent: Thu 1/18/2007 11:41 AM
> > To: r...
> > 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
>

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...

On 1/18/07, neil2452 wrote:
>
> 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?SKU07586
>
> 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
>
> --- In r... , Scott
> Henion wrote:
> >
> > neil2452 wrote:
> > > 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@... |
> > | 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.
> >
>

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: r...
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

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...

On 1/18/07, neil2452 wrote:
>
> 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).
>
> --- In r... , "Dan
> Allen"
> wrote:
> >
> > 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:
> > http://ssminnow.maesoft.net/reflow/IMG_1021.jpg
> > Smaller than a dime, and no heat sink needed...
> >
> >
> > Dan...
> >
> >
> >
> >
> >
> > On 1/17/07, Scott Henion wrote:
> > >
> > > Jeff Shapiro wrote:
> > > > 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@... |
> > > | Consultant | Stone Mountain, GA |
> > > | SHDesigns http://www.shdesigns.org |
> > > ------
> > > Rabbit libs: http://www.shdesigns.org/rabbit/
> > > today's fortune
> > > Hhhmmmmmmmm
> > > waterbeds for cows
> > > eleet
> > > Culus: why would a cow need a waterbed?
> > > cas: To be comfy warm
> > >
> > >
> > >
> >
>

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?SKU07586

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

--- In r..., Scott Henion wrote:
>
> neil2452 wrote:
> > 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@... |
> | 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.
>

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

> -----Original Message-----
> From: r...
> [mailto:r...] On Behalf Of neil2452
> Sent: Thursday, January 18, 2007 9:52 AM
> To: r...
> Subject: [rabbit-semi] Re: Power Supply / Regulator Question
> I just had a look at the 78SR105VC on Farnell at :
> http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKUH5147
>
> 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
> --- In r..., "Fournier, Pete"
> wrote:
> >
> > 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: r...
> > [mailto:r...] On Behalf Of Nathan Johnston
> > Sent: Wednesday, January 17, 2007 8:02 PM
> > To: r...
> > 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: r... on behalf of Jeff Shapiro
> > Sent: Thu 1/18/2007 11:41 AM
> > To: r...
> > 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
> >
>
> Yahoo! Groups Links

neil2452 wrote:
> 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| s...@shdesigns.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.

I just had a look at the 78SR105VC on Farnell at :
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKUH5147

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
--- In r..., "Fournier, Pete"
wrote:
>
> 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: r...
> [mailto:r...] On Behalf Of Nathan Johnston
> Sent: Wednesday, January 17, 2007 8:02 PM
> To: r...
> 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: r... on behalf of Jeff Shapiro
> Sent: Thu 1/18/2007 11:41 AM
> To: r...
> 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
>

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: r...
[mailto:r...] On Behalf Of Nathan Johnston
Sent: Wednesday, January 17, 2007 8:02 PM
To: r...
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: r... on behalf of Jeff Shapiro
Sent: Thu 1/18/2007 11:41 AM
To: r...
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 wrote:
> 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.
> 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.

> 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| s...@shdesigns.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