Converting a ATX PS to 13.8v
Dave Plowman (News)
rail to 13.8v in reasonably simple terms? I Googled for a couple of hours
last night and found answers from the bizarre to ridiculous. The +12v
output is rated at 20 amps and I only need 8 max.
--
*Ever stop to think and forget to start again?
Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.
Arfa Daily
"Dave Plowman (News)" <da...@davenoise.co.uk> wrote in message
news:50856da...@davenoise.co.uk...
I'm pretty sure that RadCom did an article on doing this not so long back.
With licenses now available on the backs of cornflake packets, I guess it
would have to be pretty simple. I used to keep all my RadComs, but took to
chucking them out over the last couple of years. I'll have a look anyway, as
I still tend to keep them until there's a pile, then lob 'em. If it wasn't
in RadCom, it was in Elektor.
Jacking these PSUs up a couple of volts is not usually that hard. There's
often an 'adjustable zener' IC on the primary side, which is fed from the
secondary side via an opto coupler. This zener serves as the reference to
set the PWM for the desired output voltage. Altering that voltage is just a
case of changing one resistor to alter the reference voltage.
Arfa
Adrian C
> Jacking these PSUs up a couple of volts is not usually that hard. There's
> often an 'adjustable zener' IC on the primary side, which is fed from the
> secondary side via an opto coupler. This zener serves as the reference to
> set the PWM for the desired output voltage. Altering that voltage is just a
> case of changing one resistor to alter the reference voltage.
>
Another resource.
http://www.pavouk.org/hw/en_atxps.html
The above schematic could be frigged about with by selecting a new value
for R25.
Typical 13.8v project.
http://www.webx.dk/oz2cpu/radios/psu-pc1.htm
--
Adrian C
Dave Plowman (News)
> Another resource.
> http://www.pavouk.org/hw/en_atxps.html
> Typical 13.8v project.
> http://www.webx.dk/oz2cpu/radios/psu-pc1.htm
The one I have to hand is a bit more modern and uses an all in one SG6105
supervisor IC.
--
*Acupuncture is a jab well done*
Arfa Daily
There are reference and feedback inputs at pins 11,12,13 &14 on that IC, and
if you examine the internal schematics, you will see that behind those pins
are the 'adjustable zeners' which are often external devices on PSUs which
use a slightly less sophisticated management IC. I expect that if you played
with the resistors attached to those pins, you could alter the output
voltage of the supply. One thing to note is that this IC also has under and
over-voltage lockouts, and if the feature has been implemented on your PSU,
then cranking the 12v up to 13.8v, may result in an unwanted shutdown, so
you might need to disable this feature.
Just as a matter of interest, what are you hoping to run from this PSU ?
Requiring 13.8v would suggest that it is something designed for mobile use.
However, most such devices will happily run from 12v, as this may be all the
vehicle voltage that's available, if the engine isn't running.
Arfa
Dave Plowman (News)
Arfa Daily <arfa....@ntlworld.com> wrote:
> Just as a matter of interest, what are you hoping to run from this PSU ?
> Requiring 13.8v would suggest that it is something designed for mobile
> use. However, most such devices will happily run from 12v, as this may
> be all the vehicle voltage that's available, if the engine isn't
> running.
It is indeed. However I'd like the correct nominal voltage for prolonged
soak testing.
--
*The most wasted day of all is one in which we have not laughed.*
Clint Sharp
<da...@davenoise.co.uk> writes
>Can anyone give a link to a site which covers modifying an ATX PS 12 volt
>rail to 13.8v in reasonably simple terms? I Googled for a couple of hours
>last night and found answers from the bizarre to ridiculous. The +12v
>output is rated at 20 amps and I only need 8 max.
>
voltage reference for the 5 v or 3.3 v rail, your journey will take you
into the realms of voltage dividers and the TL431.
Basically, the PSU will regulate the 5v or 3.3v rail and all the other
rails are set by the transformer ratios. You need to alter the voltage
divider for the 5v or 3.3v rail and all the other rails will change
proportionally (with the exception of the +5v SB as this is not
generated by the main switcher and maybe the -12 and -5v as these are
often provided by 79xx linear regulators). Don't forget, you may need
to add some load to the 5v and/or 3.3v rails to make the PSU stable and
don't mess around with it unless you're sure the main reservoir
capacitors are discharged.
--
Clint Sharp
Dave Plowman (News)
Clint Sharp <cl...@clintsmc.demon.co.uk> wrote:
> >Can anyone give a link to a site which covers modifying an ATX PS 12
> >volt rail to 13.8v in reasonably simple terms? I Googled for a couple
> >of hours last night and found answers from the bizarre to ridiculous.
> >The +12v output is rated at 20 amps and I only need 8 max.
> >
> Ahh, grasshopper, to modify the 12v rail first you must find the
> voltage reference for the 5 v or 3.3 v rail, your journey will take you
> into the realms of voltage dividers and the TL431.
Ain't got one of those.
--
*The only difference between a rut and a grave is the depth.
Plain...@yahoo.com
<da...@davenoise.co.uk> wrote:
>Can anyone give a link to a site which covers modifying an ATX PS 12 volt
>rail to 13.8v in reasonably simple terms? I Googled for a couple of hours
>last night and found answers from the bizarre to ridiculous. The +12v
>output is rated at 20 amps and I only need 8 max.
Without trying to be negative about the project, ATX power supplies
are 'jelly bean' items. The only common points between different
brands and models are the case dimensions, the pinouts of the output
leads, and the AC in connector.
You are going to have to continue what you have started. Analyse the
circuit, locate the voltage regulator and over-voltage protection and
adjust the output voltage by changing the appropriate resistors.
Depending on the availability of a suitable power supply, your
electronics skills, and time available, you might be better off taking
a simpler approach. This supply would be much easier to work on.
http://www.mpja.com/prodinfo.asp?number=17095+PS
PlainBill
Franc Zabkar
>Can anyone give a link to a site which covers modifying an ATX PS 12 volt
>rail to 13.8v in reasonably simple terms? I Googled for a couple of hours
>last night and found answers from the bizarre to ridiculous. The +12v
>output is rated at 20 amps and I only need 8 max.
I've done this with an AT PSU. All I did was to modify the voltage
sensing resistors at the feedback pin of the error amp. In my case the
PSU regulated by sensing a weighted average of the +5V and +12V
supplies, so I removed the +5V sense resistor and reprogrammed the
+12V resistive divider. You may also have to modify the overvoltage
sense circuitry, in my case a zener diode. You could also remove the
components associated with the other rails and then fudge the POK
circuit. I also added an aluminium clad dummy load resistor and
mounted it in the fan's air stream.
- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
Dave Plowman (News)
<Plain...@yahoo.com> wrote:
> >Can anyone give a link to a site which covers modifying an ATX PS 12
> >volt rail to 13.8v in reasonably simple terms? I Googled for a couple
> >of hours last night and found answers from the bizarre to ridiculous.
> >The +12v output is rated at 20 amps and I only need 8 max.
> Without trying to be negative about the project, ATX power supplies
> are 'jelly bean' items. The only common points between different
> brands and models are the case dimensions, the pinouts of the output
> leads, and the AC in connector.
I've certainly found that out. ;-)
> You are going to have to continue what you have started. Analyse the
> circuit, locate the voltage regulator and over-voltage protection and
> adjust the output voltage by changing the appropriate resistors.
I'm not going to waste much time on it - given the cost of new units that
do what I want.
> Depending on the availability of a suitable power supply, your
> electronics skills, and time available, you might be better off taking
> a simpler approach. This supply would be much easier to work on.
> http://www.mpja.com/prodinfo.asp?number=17095+PS
But with respect that's 12 volts. I want 13.8. Plenty on Ebay for not a
lot, if I decide to spend some cash.
--
*The most wasted day of all is one in which we have not laughed.*
Dave Plowman da...@davenoise.co.uk London SW
Arfa Daily
"Dave Plowman (News)" <da...@davenoise.co.uk> wrote in message
news:5085a90...@davenoise.co.uk...
> In article <Ae5DJFCJ...@clintsmc.demon.co.uk>,
> Clint Sharp <cl...@clintsmc.demon.co.uk> wrote:
>> >Can anyone give a link to a site which covers modifying an ATX PS 12
>> >volt rail to 13.8v in reasonably simple terms? I Googled for a couple
>> >of hours last night and found answers from the bizarre to ridiculous.
>> >The +12v output is rated at 20 amps and I only need 8 max.
>> >
>> Ahh, grasshopper, to modify the 12v rail first you must find the
>> voltage reference for the 5 v or 3.3 v rail, your journey will take you
>> into the realms of voltage dividers and the TL431.
>
> Ain't got one of those.
>
Yes it has, Dave. With that particular supervisory IC, it's behind the
reference and feedback pins, and not an external discrete device, as I
previously explained ... If you're not understanding the principles of how
these things regulate, then I don't think you are going to get too far with
this project.
Arfa
Jeff Liebermann
<da...@davenoise.co.uk> wrote:
>But with respect that's 12 volts. I want 13.8. Plenty on Ebay for not a
>lot, if I decide to spend some cash.
The most popular official voltage is apparently 13.5VDC, not 13.6VDC.
I bought a pair of these, for bipolar stepper motor power:
<http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180378998292>
The output is tweakable from 12V to 15V, which is handy for also using
it as a battery charger. It belches some HF RFI but not too horrible.
Yeah, I know it delivers more amps than you need, but the extra power
might be useful for something else.
Franc Zabkar
<da...@davenoise.co.uk> put finger to keyboard and composed:
>In article <7dqhp6F...@mid.individual.net>,
> Adrian C <em...@here.invalid> wrote:
>> Arfa Daily wrote:
>
>> > Jacking these PSUs up a couple of volts is not usually that hard.
>> > There's often an 'adjustable zener' IC on the primary side, which is
>> > fed from the secondary side via an opto coupler. This zener serves as
>> > the reference to set the PWM for the desired output voltage. Altering
>> > that voltage is just a case of changing one resistor to alter the
>> > reference voltage.
>> >
>
>> Another resource.
>
>> http://www.pavouk.org/hw/en_atxps.html
>
>> The above schematic could be frigged about with by selecting a new value
>> for R25.
>
>> Typical 13.8v project.
>
>> http://www.webx.dk/oz2cpu/radios/psu-pc1.htm
>
>The one I have to hand is a bit more modern and uses an all in one SG6105
>supervisor IC.
Datasheet for SG6105:
http://www.datasheetarchive.com/pdf-datasheets/Datasheets-6/DSA-104265.pdf
There is an application circuit on page 8.
It seems to me that your +13.8V modification would be very easy.
Just remove the 5V sense resistor from the SG6105's IN pin. Then
reprogram the 12V and SS resistors so that the voltage at the IN pin
is equal to +2.5V when the 12V rail is at +13.8V.
Gnd o-- 2K5 --o-- 11K3 --o +13.8V
|
|
IN
Remove the connections to the V33, V5, and V12 over/undervoltage sense
pins and then construct a potential divider (see below) with the
+13.8V output rail at the top end, and taps at +12V, +5V, and +3.3V
connected to each of the sense pins. In this way you would be
monitoring only the +13.8V rail and faking the others.
Gnd o-- 3K3 --o-- 1K7 --o-- 7K --o-- 1K8 --o +13.8V
| | |
| | |
V33 V5 V12
Franc Zabkar
<arfa....@ntlworld.com> put finger to keyboard and composed:
AISI, the main rails are regulated via the error amp at pins 16, 17,
and 18. The OP would need to remove/modify the resistors at the IN
pin.
The TL431's at the FB2 and FB1 pins regulate the +3.3V and +5VSB
supplies. I don't believe they need to be touched. I suspect, though,
that the +3.3V regulator will be working flat out to bring the output
down, if indeed it can.
Dave Plowman (News)
Arfa Daily <arfa....@ntlworld.com> wrote:
> >> Ahh, grasshopper, to modify the 12v rail first you must find the
> >> voltage reference for the 5 v or 3.3 v rail, your journey will take
> >> you into the realms of voltage dividers and the TL431.
> >
> > Ain't got one of those.
> >
> Yes it has, Dave. With that particular supervisory IC, it's behind the
> reference and feedback pins, and not an external discrete device, as I
> previously explained ...
And it's a TL431? If you mean a shunt regulator that would be ok.
> If you're not understanding the principles of how these things
> regulate, then I don't think you are going to get too far with this
> project.
I doubt I'm alone in that. Try Googling on 'modify ATX 13.8v' and see just
how many different approaches there are.
Which is why I asked here.
--
*Of course I'm against sin; I'm against anything that I'm too old to enjoy.
Dave Plowman (News)
Jeff Liebermann <je...@cruzio.com> wrote:
> >But with respect that's 12 volts. I want 13.8. Plenty on Ebay for not a
> >lot, if I decide to spend some cash.
> The most popular official voltage is apparently 13.5VDC, not 13.6VDC.
13.8 is the usual voltage on a car with the engine running and things
stabilised. But of course most equipment will tolerate a wide range and
still work.
--
*Gun Control: Use both hands.
Dave Plowman (News)
Franc Zabkar <fza...@iinternode.on.net> wrote:
> AISI, the main rails are regulated via the error amp at pins 16, 17,
> and 18. The OP would need to remove/modify the resistors at the IN
> pin.
> The TL431's at the FB2 and FB1 pins regulate the +3.3V and +5VSB
> supplies. I don't believe they need to be touched. I suspect, though,
> that the +3.3V regulator will be working flat out to bring the output
> down, if indeed it can.
Google gave this hit:-
http://www.chirio.com/atx_14_volt.htm
Which says to use 1N4448 in the 3.3, 5 and 12v error sensing rails, which
I did.
No difference. Tried changing the resistor feeding 'IN' with a pot and
altering that from the fixed value cause shut down.
--
*If I throw a stick, will you leave?
Dave Plowman (News)
> >> voltage reference for the 5 v or 3.3 v rail, your journey will take
> >> you into the realms of voltage dividers and the TL431.
> >
> > Ain't got one of those.
> >
> Yes it has, Dave. With that particular supervisory IC, it's behind the
> reference and feedback pins, and not an external discrete device, as I
> previously explained ...
Right - having read the spec more carefully it does indeed state it
contains two of these. So please accept my apologies.
> If you're not understanding the principles of
> how these things regulate, then I don't think you are going to get too
> far with this project.
Well, if I were an expert on them I'd hardly need to ask here.
It's not for some life support mechanism, you know.
--
*Bigamy is having one wife too many - monogamy is the same
Dave Plowman (News)
with the SG 6105 proved a red herring in this case. As the spec covers the
range 10.1 - 14.5v (typical) without mods.
All that was needed was to alter the voltage on the 'IN' pin. But with the
fan plugged in. Without it any alteration to that voltage caused the PS to
shut down. With it, I've got my 13.8v (approx) without problems. 270K was
the value (original 120K) needed in this case. But probably won't help
others since this is a generic PS with no maker's name.
--
*Acupuncture is a jab well done*
Dave Plowman da...@davenoise.co.uk London SW
bw
"Meat Plow" <me...@petitmorte.net> wrote in message
news:318l0t....@news.alt.net...
> On Wed, 05 Aug 2009 10:11:03 +0100, "Dave Plowman (News)"
> <da...@davenoise.co.uk>wrote:
>
>>In article <mfmh751qj1nok16bf...@4ax.com>,
>> Jeff Liebermann <je...@cruzio.com> wrote:
>>> >But with respect that's 12 volts. I want 13.8. Plenty on Ebay for not a
>>> >lot, if I decide to spend some cash.
>>
>>> The most popular official voltage is apparently 13.5VDC, not 13.6VDC.
>>
>>13.8 is the usual voltage on a car with the engine running and things
>>stabilised. But of course most equipment will tolerate a wide range and
>>still work.
>
14.0 volts is the best. You will lose a tenth or so in the leads and
connectors
Plain...@yahoo.com
<da...@davenoise.co.uk> wrote:
>In article <ngvg75p1o3peqscfj...@4ax.com>,
> <Plain...@yahoo.com> wrote:
>> >Can anyone give a link to a site which covers modifying an ATX PS 12
>> >volt rail to 13.8v in reasonably simple terms? I Googled for a couple
>> >of hours last night and found answers from the bizarre to ridiculous.
>> >The +12v output is rated at 20 amps and I only need 8 max.
>
>> Without trying to be negative about the project, ATX power supplies
>> are 'jelly bean' items. The only common points between different
>> brands and models are the case dimensions, the pinouts of the output
>> leads, and the AC in connector.
>
>I've certainly found that out. ;-)
>
>> You are going to have to continue what you have started. Analyse the
>> circuit, locate the voltage regulator and over-voltage protection and
>> adjust the output voltage by changing the appropriate resistors.
>
>I'm not going to waste much time on it - given the cost of new units that
>do what I want.
>
>> Depending on the availability of a suitable power supply, your
>> electronics skills, and time available, you might be better off taking
>> a simpler approach. This supply would be much easier to work on.
>> http://www.mpja.com/prodinfo.asp?number=17095+PS
>
>But with respect that's 12 volts. I want 13.8. Plenty on Ebay for not a
>lot, if I decide to spend some cash.
Glad to hear it. It's always better to start off with something
designed for the job IF the price is reasonable.
PlainBill
Arfa Daily
I didn't intend any offence, Dave - probably could have picked my words
better. All I was trying to imply, is that switch mode power supplies are a
bit of a black art, as you have discovered, and unless you are working with
them all the time, and thus understand the subtleties of some of the
circuitry and regulation techniques which they employ, then this project is
quite likely to tie you up in knots. I must admit that I didn't look too
closely at the application schematic to see how exactly they went about
regulating the thing, but when I saw the two undedicated adjustable zeners
in there, I assumed that they were probably making use of at least one of
them in the primary feedback loop. It is a common technique to use an
(usually external) adjustable zener to achieve main rail set voltage and
regulation.
Franc is probably right in this case with his proposal of altering the
resistors on the IN pin. You may well find that when you put the pot in, you
actually had the desired effect of increasing or decreasing the output
voltage, but then fell foul of the under / over voltage lockout feature,
which I think I said that you might have to disable. If you put a digital
meter on the output, and watch very closely, you might see the voltage start
to alter up and down, before it trips out.
Arfa
Franc Zabkar
<da...@davenoise.co.uk> put finger to keyboard and composed:
>Just to say it's sorted. All the guff about the overvoltage protection
>with the SG 6105 proved a red herring in this case. As the spec covers the
>range 10.1 - 14.5v (typical) without mods.
>
>All that was needed was to alter the voltage on the 'IN' pin.
You are not actually altering the IN voltage. It remains at +2.5V due
to the action of the error amp. The other input to the amp is internal
to the IC and is fixed at +2.5V.
>But with the
>fan plugged in. Without it any alteration to that voltage caused the PS to
>shut down.
Were you testing the supply without a load?
>With it, I've got my 13.8v (approx) without problems. 270K was
>the value (original 120K) needed in this case. But probably won't help
>others since this is a generic PS with no maker's name.
If you did not remove the +5V sense resistor, then the output may
fluctuate as the load varies.
Dave Plowman (News)
Arfa Daily <arfa....@ntlworld.com> wrote:
> > Well, if I were an expert on them I'd hardly need to ask here.
> > It's not for some life support mechanism, you know.
> I didn't intend any offence, Dave - probably could have picked my words
> better.
No offence taken - it is Usenet, after all. ;-)
> All I was trying to imply, is that switch mode power supplies are a bit
> of a black art, as you have discovered, and unless you are working with
> them all the time, and thus understand the subtleties of some of the
> circuitry and regulation techniques which they employ, then this project
> is quite likely to tie you up in knots.
Which is why I'm happy to experiment and hopefully learn.
> I must admit that I didn't look
> too closely at the application schematic to see how exactly they went
> about regulating the thing, but when I saw the two undedicated
> adjustable zeners in there, I assumed that they were probably making
> use of at least one of them in the primary feedback loop. It is a
> common technique to use an (usually external) adjustable zener to
> achieve main rail set voltage and regulation.
Snag is my actual circuit differs considerably from either of the examples
given in the spec sheet - including a sprinkling of external zenners. But
it's beyond my patience/available time to trace it all out properly.
> Franc is probably right in this case with his proposal of altering the
> resistors on the IN pin. You may well find that when you put the pot in,
> you actually had the desired effect of increasing or decreasing the
> output voltage, but then fell foul of the under / over voltage lockout
> feature, which I think I said that you might have to disable.
The network between actual outputs and sensor inputs is far from simple -
the spec sheet just shows a direct connection.
> If you put a digital meter on the output, and watch very closely, you
> might see the voltage start to alter up and down, before it trips out.
Indeed.
--
*I can see your point, but I still think you're full of shit.
Dave Plowman (News)
Franc Zabkar <fza...@iinternode.on.net> wrote:
> On Wed, 05 Aug 2009 14:48:07 +0100, "Dave Plowman (News)"
> <da...@davenoise.co.uk> put finger to keyboard and composed:
> >Just to say it's sorted. All the guff about the overvoltage protection
> >with the SG 6105 proved a red herring in this case. As the spec covers the
> >range 10.1 - 14.5v (typical) without mods.
> >
> >All that was needed was to alter the voltage on the 'IN' pin.
> You are not actually altering the IN voltage. It remains at +2.5V due
> to the action of the error amp. The other input to the amp is internal
> to the IC and is fixed at +2.5V.
Right - so what is happening to alter the output voltage?
> >But with the fan plugged in. Without it any alteration to that voltage
> >caused the PS to shut down.
> Were you testing the supply without a load?
No - I was loading it with a 55 watt car headlamp bulb.
> >With it, I've got my 13.8v (approx) without problems. 270K was
> >the value (original 120K) needed in this case. But probably won't help
> >others since this is a generic PS with no maker's name.
> If you did not remove the +5V sense resistor, then the output may
> fluctuate as the load varies.
Seems pretty stable - at least for my purposes.
> - Franc Zabkar
--
*Age is a very high price to pay for maturity.
Franc Zabkar
<da...@davenoise.co.uk> put finger to keyboard and composed:
>In article <ma6l75p6jjhfpoiui...@4ax.com>,
> Franc Zabkar <fza...@iinternode.on.net> wrote:
>> On Wed, 05 Aug 2009 14:48:07 +0100, "Dave Plowman (News)"
>> <da...@davenoise.co.uk> put finger to keyboard and composed:
>
>> >Just to say it's sorted. All the guff about the overvoltage protection
>> >with the SG 6105 proved a red herring in this case. As the spec covers the
>> >range 10.1 - 14.5v (typical) without mods.
>> >
>> >All that was needed was to alter the voltage on the 'IN' pin.
>
>> You are not actually altering the IN voltage. It remains at +2.5V due
>> to the action of the error amp. The other input to the amp is internal
>> to the IC and is fixed at +2.5V.
>
>Right - so what is happening to alter the output voltage?
I don't know much more about the black art of switchmode PSUs than the
average tech, but AFAICS in this particular case all one needs to
understand is that the error amp inputs must be equal when the supply
is regulating correctly. If the output is too high, then the error amp
reduces the drive to the chopper (by narrowing the pulse), thus
bringing the output back down. Conversely, if the output is too low,
then the error amp increases the drive to the chopper.
>> >But with the fan plugged in. Without it any alteration to that voltage
>> >caused the PS to shut down.
>
>> Were you testing the supply without a load?
>
>No - I was loading it with a 55 watt car headlamp bulb.
>
>> >With it, I've got my 13.8v (approx) without problems. 270K was
>> >the value (original 120K) needed in this case. But probably won't help
>> >others since this is a generic PS with no maker's name.
>
>> If you did not remove the +5V sense resistor, then the output may
>> fluctuate as the load varies.
>
>Seems pretty stable - at least for my purposes.
>
>> - Franc Zabkar
I mentioned this possibility because I had a fake 400W PSU (really
only 200W) where the +12V rail moved about in response to changes in
the +5V load. Also ISTR that some HDD enclosures include a dummy load
resistor for load balancing.
spamme0
> In article <3BWdm.125758$GX7....@newsfe23.ams2>,
> Arfa Daily <arfa....@ntlworld.com> wrote:
>> Just as a matter of interest, what are you hoping to run from this PSU ?
>> Requiring 13.8v would suggest that it is something designed for mobile
>> use. However, most such devices will happily run from 12v, as this may
>> be all the vehicle voltage that's available, if the engine isn't
>> running.
>
> It is indeed. However I'd like the correct nominal voltage for prolonged
> soak testing.
>
Certainly depends on the power supply, but there needs to be enough flux
in the transformer to make the outputs work.
If the 5V is the regulated output...or 3.3...or whatever,
and there's no load, there won't be enough flux to make the 12V work at
any significant load.
You're gonna need to redesign the circuit to sense the 12V output
so you can get the current you need. The rest is just resistor ratios.
But wait...if there's no load on the other outputs, you run the risk
of transients charging the caps till they explode.
And depending on the physical location on the circuit board you
pick to connect your new components, you may make significant changes
to the transient response. With high-currents, one point on a trace
ain't the same as another. You can compensate the feedback loop,
but load transient induced spikes can/will cause you grief if your
layout doesn't consider it. Building the transient test fixture
to test your design is non-trivial.
There's an interesting ratio. How much you are saving on the power
supply (including costs of
design/debug/labor/testing/documentation/training)
divided by the cost of blowing up all the stuff connected to it.
And what do you do when it breaks...or you need another one
and the original PS product is no longer available?
Few people understand the subtleties of power supply design.
It's a black hole (art) that sucks in everything around it.
Go buy a real honkin' power supply and use a slew of small circuit
breakers to power your individual test stations.
I don't know your burdened design labor rate, but modifying
PC power supplies is often false economy.
Your time might be better spent
simulating the automotive power
environment for your tests.
littelfuse appnote an9312 is a place to start.
Are we having fun yet?
mike
Dave Plowman (News)
spamme0 <spa...@netscape.net> wrote:
> > It is indeed. However I'd like the correct nominal voltage for
> > prolonged soak testing.
> >
> Why wouldn't you soak test at worst case numbers rather than nominal?
Worst case for soak testing wouldn't be lower than nominal.
--
*Eagles may soar, but weasels don't get sucked into jet engines *