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'[EE]: 125 VDC Input ATX Supply'
2000\09\20@115023 by Mark Peterson

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I need to power a PC from a nominal 125 VDC source.  The voltage may vary
from 110 to 140 VDC.  I want a 300 W ATX supply that will be a direct swap
with the existing 120 VAC unit in the PC.  I'm finding that this type of
supply is very rare.  A couple vendors in China responded to my email
inquiry stating that they would consider modifying their standard units.
They lost interest when I told them I only need three or four of them.

I've considered modifying a standard supply, which are abundant and cheap.
I searched for schematics but found that they are nearly impossible to get.
It appears that the first thing a standard ATX supply does is rectify the
AC input, filter it a bit, and then present it to the switching part of the
supply.  I'm wondering if I can simply connect the 125 VDC source just
ahead of this switching section.  I'm going to do some experimenting but I
wanted to check if anybody knows of a source of such a supply or if they
have ever tried what I'm planning to attempt.

Thanks.

Mark P

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2000\09\20@115813 by staff

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Mark Peterson wrote:
{Quote hidden}

Yes, just connect the DC supply to the ATX unit as you mentioned.
This will work fine, the front end AC section in switchmode supplies of
that type is just a formality that makes the DC the unit works from.

Normally these supplies have good regulation, many will work ok
from 240vac as they do from 110vac with no adjustment.

Roman

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2000\09\20@120033 by xandinho

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       No, first you have to double this voltage, to connect after the rectifier. The rectifier uses a voltage doubler to raise the input to +250 (or more) volts when you connect it to 110vAC. It would be better to modify the PSU, just don't ask me how! I have some schematics, if you want to take a look,


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       All the best!!!
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2000\09\20@121108 by M. Adam Davis

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The only problem you are lilely to have is associated with the actual DC voltage
present at that section just after rectification. 120VAC rectified and filtered
with caps produces a nominal 120*sqrt(2) or 170VDC.  Many good PC power supplies
will operate off of as low an AC voltage as 90VAC, giving the DC section a lower
limit of 127VDC, which MAY mean that you could put 125VDC in and it would work
out OK.

I wish you the best of luck, and please post your results here!

-Adam

Mark Peterson wrote:
{Quote hidden}

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2000\09\20@121940 by staff

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M. Adam Davis wrote:
>
> The only problem you are lilely to have is associated with the actual DC voltage
> present at that section just after rectification. 120VAC rectified and filtered
> with caps produces a nominal 120*sqrt(2) or 170VDC.  Many good PC power supplies
> will operate off of as low an AC voltage as 90VAC, giving the DC section a lower
> limit of 127VDC, which MAY mean that you could put 125VDC in and it would work
> out OK.
>
> I wish you the best of luck, and please post your results here!
>
> -Adam

Please not this argument! What 120vac means is the average
rms voltage available under load, which is the same as 120vdc.
Makes it easy for dumb electricians! :o) Under no load conditions
120vac will recify to 120 x 1.414 = 170v but that really doesn't
apply here! 120vac rectified into a load is 120vdc. :o)

Roman

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2000\09\20@124517 by Simon Nield

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The certainty of your reply made me pause a minute there Roman, but if you full wave rectify mains
you get the peak voltage of the waveform under no load exactly as Adam says. Any drop you see under
load is due to the capacitors being too small to supply the current required before the mains rises
to near peak again...
The output voltage from a rectifier (half or full wave) will be a DC voltage somewhat under the
peak, with an ac component at the rectification frequency (in the UK; 100 Hz for fullwave and 50Hz
for halfwave). As capacitors are a lot more expensive than diodes in most cases I would be stunned
to see half wave rectification here.
So to get '120vdc' out of a rectified 120vac supply you would be seeing 100% ripple... i.e. no
capacitors at all!

120vrms actually refers to the amount of energy disipated in a resistive load being the same as for
120vdc... and a big capacitor is not terribly resistive.

Don't shoot people down in flames unless you are sure of your aim...

Regards,
Simon
(I have donned my flak jacket for the ensuing flame war)






M. Adam Davis wrote:
{Quote hidden}

Please not this argument! What 120vac means is the average
rms voltage available under load, which is the same as 120vdc.
Makes it easy for dumb electricians! :o) Under no load conditions
120vac will recify to 120 x 1.414 = 170v but that really doesn't
apply here! 120vac rectified into a load is 120vdc. :o)

Roman

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2000\09\20@125249 by staff

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Simon Nield wrote:
{Quote hidden}

My humble apologies Simon, you are absolutely correct!

I thought about it after my post and yes, the 240vac supplies
we work on here everyday have 330vdc approx on the main filter
cap. I was *way* too hasty with the ac to dc conversion...

Please accept my apologies everyone concerned. :o}

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2000\09\20@133253 by Chris Carr

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Just a word of warning. Some of the ATX supplies we have are dual voltage.
Full wave
rectified mains on 240 volts AC. Voltage Doubling on 115 volts AC making the
DC Line always around 300 volts.

We use an unmodified ATX supply on 240 volts and feed it with a cheap DC-AC
Inverter with a modified sine wave output. To improve the efficiency and
improve the reliability we modify the inverter by bypassing the output
switching so we get 300volts DC output.

Regards
Chris Carr

{Original Message removed}

2000\09\20@144930 by Oliver Broad

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I see the problem of the voltage doubler has been mentioned. A PSU intended
for the US market might have a straight bridge rectifier front end, which
would accept DC probably up to 190v?, though you are still a bit low. Dual
range supplies would be useless although they would accept around 350v DC.
I'm uncertain but I've seen bus voltages of 385v DC described, though maybe
this is the absolute max.

There is a power factor correction and universal input circuit shown in
databooks which converts almost anything to 380v, which I believe is fitted
to some of the larger industrial PSUs. An ATX psu with this type of true
universal input might be available at a premium. A module might be available
to put in front of the bog standard PSU but expect to pay 5 or 10 times the
cost of the PSU. If I remember there was one from either Astec or Vicor.

I've an idea how you could make a simple voltage booster but it would be a
last resort!

{Original Message removed}

2000\09\20@162803 by Bob Ammerman

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You will note that some say it will, some say it won't. It really depends on
the particular power supply you are working on. To be able to just bypass
the input stage you need a supply which:

1: Uses a bridge input configuration rather than a voltage doubler input
configuration.

2: Can regulate down to a relatively low AC voltage

You may be able to tell if condition 1 is met by checking the working
voltage on the caps in the input section.

You could tell if condition 2 is met by attaching the supply to a variac,
loading it realistically, and cranking down the voltage. (btw: make sure the
supply will properly start at the lower voltage).

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)


{Original Message removed}

2000\09\21@041919 by Alan B. Pearce

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>The only problem you are lilely to have is associated with the actual DC voltage
>present at that section just after rectification. 120VAC rectified and filtered
>with caps produces a nominal 120*sqrt(2) or 170VDC.  Many good PC power supplies
>will operate off of as low an AC voltage as 90VAC, giving the DC section a lower
>limit of 127VDC, which MAY mean that you could put 125VDC in and it would work
>out OK.

If the power supply is of the type that will accept a wide range of input voltage, then it would be OK to just operate it off the DC. If it has a switch to select between 115 and 230 modes, then it will not be OK to run it in the 115 volt mode. It may be worth trying with the switch set to 230V mode to see if the supply has enough regulation range, especially if you run significantly below maximum wattage, but I think you are wanting too much power to be able to do this.

Be careful if there is no switch on the outside of the supply. If it does not specifically state it will accept a wide input voltage range, then there will be a jumper of some sort inside to change voltage.

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2000\09\21@072945 by Mike Pearce

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Be careful of assuming that the bride rectifier can handle the full load
current through only one half of its diode matrix, as this will be twice the
original design current!.
regards

Mike Pearce
Neutronics

I see the problem of the voltage doubler has been mentioned. A PSU intended
for the US market might have a straight bridge rectifier front end, which
would accept DC probably up to 190v?, though you are still a bit low. Dual
range supplies would be useless although they would accept around 350v DC.
I'm uncertain but I've seen bus voltages of 385v DC described, though maybe
this is the absolute max.

There is a power factor correction and universal input circuit shown in
databooks which converts almost anything to 380v, which I believe is fitted
to some of the larger industrial PSUs. An ATX psu with this type of true
universal input might be available at a premium. A module might be available
to put in front of the bog standard PSU but expect to pay 5 or 10 times the
cost of the PSU. If I remember there was one from either Astec or Vicor.

I've an idea how you could make a simple voltage booster but it would be a
last resort!

{Original Message removed}

2000\09\21@122800 by hgraf

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> I need to power a PC from a nominal 125 VDC source.  The voltage may vary
> from 110 to 140 VDC.  I want a 300 W ATX supply that will be a direct swap
> with the existing 120 VAC unit in the PC.  I'm finding that this type of
> supply is very rare.  A couple vendors in China responded to my email
> inquiry stating that they would consider modifying their standard units.
> They lost interest when I told them I only need three or four of them.

    Since most (all?) switchmode power supplies immediately convert the
incoming AC to DC with a bridge rectifier they should work fine if you
supply them straight DC. Because 120RMS is about 170V peak, supplying it
with only 125V DC may be a little low, but it might work. I suggest you try
it out, ATX power supplies are cheap, hook one up and see if it supplies the
correct amount of voltage and current. Of course things could go wrong so
please be careful but I believe it should work. TTYL

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2000\09\21@154656 by Bob Blick

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>      Since most (all?) switchmode power supplies immediately convert the
> incoming AC to DC with a bridge rectifier they should work fine if you
> supply them straight DC.

Yes and no. Most have a 115-230 volt selector switch. In 230 volt mode,
you use the full bridge rectifier and develop 300+volts. In 115 volt mode,
the neutral is connected to the center of 2 big capacitors to achieve
voltage doubling, and you get 300+ volts.

Not to say that some power supplies won't wake up with only 125VDC applied
to them, but don't expect any real power out of them.

Cheers,

Bob

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2000\09\22@152536 by Oliver Broad

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Something spooky seems to have happened, I recognise my message but I never
saw it in the piclist, I have echo on, and my other messages came back. I've
seen some other strange effects on Piclist which I might raise in a seperate
message.

Good point about the rectifier, Though I believe the devices are in reality
limited by their thermal characteristics, not by an absolute max current
(whatever the datasheet says). Provided the PSU is only ever used for DC
input a fused diode or two would probably improve it, I'm sure they almost
always fail short.

I'm thinking that if all else fails/is too expensive then a boost supply
could be made using a largeish inductor and a Power Integrations topswitch
switchmode chip, TO220 package three leads, switch ground and control. I
haven't used one yet but they seem to be the lowest part-count SMPS parts
I've seen yet.

I think modifying the power supply could mean redesigning and rewinding the
transformer among other things, I know the turns ratio does not fix the
output voltage but it does set a range over which the supply can work.

Oliver.

{Original Message removed}

2000\09\22@162355 by Chris Carr

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Sorry but me braincell is in overload. I do not understand the logic behind
Mike Pearce's statement regarding currents. Power dissipation I would
understand as with AC it would be spread across 4 diodes instead of 2 with
DC.

My previous message regarding Inverters appears to have not been posted, I
appologise for this repeat of information if it has. We retail a 300 Watt
modified Sine Wave Inverter for #56. The "modified Sine Wave" bit of the
circuitry is done by chopping up the output of a 12 (or 24) to 350 volt DC
converter. If you want to start modifying products, eliminate the chopping
circuitry by cutting 2 track in the pcb (one if the 350 volts the other is
the 12 volts to the waveform generating logic) take the 350 volts dc to the
output. Then in the ATX PSU identify the outputs of the bridge
rectifier/voltage doubler and connect those points to the modified Inverter
output. This will eliminate unneccesary stages and improve efficiency a
little. Its not the most elegant solution but its quick, relatively cheap,
and it works.

Regards
Chris Carr

Oliver Broad wrote:

> Something spooky seems to have happened, I recognise my message but I
never
> saw it in the piclist, I have echo on, and my other messages came back.
I've
> seen some other strange effects on Piclist which I might raise in a
seperate
> message.
>
> Good point about the rectifier, Though I believe the devices are in
reality
{Quote hidden}

the
> transformer among other things, I know the turns ratio does not fix the
> output voltage but it does set a range over which the supply can work.
>
> Oliver.
>
> {Original Message removed}

2000\09\22@194931 by Harold M Hallikainen

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On Thu, 21 Sep 2000 12:26:18 -0400 Herbert Graf <.....hgrafKILLspamspam@spam@EMAIL.COM> writes:
{Quote hidden}

       However, switching supplies that have a 120/240VAC input select switch
have a voltage doubler circuit that doubles the voltage to 340VDC when in
the 120VAC position or doesn't double but still gets 340VDC when operated
off a 230-240VAC line. These supplies CAN operate on DC if they are in
the 240VAC position and you have an appropriately high voltage. In the
120VAC position they will not work on DC.
       If, however, the input is "universal input", the incoming AC is full
wave rectified and never voltage doubled. Instead the switcher can accept
a wide range of DC. This will work fine off DC in the 150 to 350 VDC
area. You may be able to go lower. My employer used a universal input
switcher in his electric car to run 12V loads off his 132V battery. I
think, however, that most computer power supplies are not universal input
because these are more costly than those with a voltage selector.

Harold




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2000\09\23@114201 by Olin Lathrop

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> Something spooky seems to have happened, I recognise my message but I
never
> saw it in the piclist, I have echo on, and my other messages came back.
I've
> seen some other strange effects on Piclist which I might raise in a
seperate
> message.

This happens to me too on occasion.  I spend time writing a reply that I
intend everyone to see and then it never shows up on the PIC list.  I think
I finally figured out what is going on.  Some people have their REPLY
address set.  The list server will fill in the REPLY address to the list,
but only if it is not already set.  When other people do a reply to this
kind of message, that reply only goes to the poster, not the whole list.

I find this very annoying.  Assholes that set their REPLY address should
either get their messages bounced, or have the list server just substitute
its own REPLY address.  It's pretty arrogant to think that you are so
special that responses to your comments should go back to you only.

Unfortunately James not only likes it this way, but is also the worst
offender, so this isn't likely to get fixed any time soon.  I wonder how
many others responded to James' request for what you would like to see in a
selection guide thinking everyone would read your response, only to have it
never show up on the list.


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Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, .....olinKILLspamspam.....cognivis.com, http://www.cognivis.com

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