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Author Topic: ATX Power supply as bench power supply  (Read 6337 times)
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Hoping someone here can help me with this as I don't really know where else I can ask it.

I'm trying to convert an atx power supply into a bench power supply.  As a test, I was attempting to power a car amplifier.  (150w x 2  @ 4 ohms amp)

Everything tested fine with the multimeter (+12v on the 12 volt line, +5v on the 5 volt line) but the amp isn't turning on.  I had a +5v wire feeding into the remote (even tried a +12v one in case it had to have 12 volts on the remote) but the amp didn't turn on.  The PSU was good, I took it out of a working computer, and it came on and stayed on (used a 10 ohm 10 watt resistor between +5v and ground for the dummy load).  And the amplifier was working when I removed it from a vehicle not to long ago, so I'm pretty sure both devices are functioning properly.

Could this simply be that the power supply wasn't allowing enough amperage for the amp? I was using a 350w PSU with a single 12v rail rated at 18 amps.  Or do you have any other ideas as to what the problem might be?

Thanks
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ATX power supplies regulate off the 5V rail, they dont regulate the 12V rail.
If you want to use one as a general power supply, you have to modify the supply so the
regulation is done from the 12 V rail.
If you dont the 12 V rail will sag when you put any kind of load on it.
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also even small amps can quite easily hit 15-30 amps depending on output and quality

PS: I have never heard of an remote on a car amp running off of 5v, 12v is a better bet
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In case you didn't see this -
http://dangerousprototypes.com/docs/ATX_Breakout_Board
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ATX power supplies regulate off the 5V rail, they dont regulate the 12V rail.
If you want to use one as a general power supply, you have to modify the supply so the
regulation is done from the 12 V rail.
If you dont the 12 V rail will sag when you put any kind of load on it.


I'm not sure what you mean by regulate off the 5v rail and modifying it to regulate off the 12v rail?  I hooked up the supply like I read on 3 or 4 different guides online on how to convert an atx power supply into a lab bench power supply, none of them mentioned anything about regulation or modifying the regulation?  I'm pretty sure the 12v rail is not sagging, because I tested the voltage of it while I had a load on it, and it remained 12v (and the 5v remained 5v).

That ATX breakout board is neat, but not useful to me now since I've already cut off and thrown away the connectors on this supply hehe.  And if I get another supply for this purpose it's going to be one with multiple 12v rails, so that breakout board would only be pulling power down one rail not giving me the maximum current capabilities as using all the rails would.
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Your power supply is rated for a total of 350W.  You won't get all 350W out of the 12V supply.

As a comparison I have a 400W ATX Supply on my desk.  There are two +12V supplies (as specified by the ATX spec), one rated for 14A and the other for 15A (which can vary).  Since you can't just tie two regulated outputs in parallel, we have to go for the higher current supply.  12V * 15A = 180W.  That sounds weak for a 300W audio amplifier.

Also, keep in mind you may need to load your 5V supply (like with your resistor) so the supply will turn on and regulate its other outputs.
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Your power supply is rated for a total of 350W.  You won't get all 350W out of the 12V supply.

As a comparison I have a 400W ATX Supply on my desk.  There are two +12V supplies (as specified by the ATX spec), one rated for 14A and the other for 15A (which can vary).  Since you can't just tie two regulated outputs in parallel, we have to go for the higher current supply.  12V * 15A = 180W.  That sounds weak for a 300W audio amplifier.

Also, keep in mind you may need to load your 5V supply (like with your resistor) so the supply will turn on and regulate its other outputs.

Hmm really, you can't tie the separate rails together?  One of the guides I read specifically answered that question and said you could..?  My supply I'm using right now, currently only has one 12v supply, I was thinking about using another backup supply that I have that is an 850w supply with four 12v rails in it, but I was planning to tie the rails together so that I'd have more current available.  (It's rated for 18A on each of the rails).

I still don't understand what the previous person was talking about with regulating though, I haven't been able to find anything on that.
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One of the guides I read specifically answered that question and said you could..?
Not everything on the Internet is correct.  When it comes to many things people try something and it appears to work "fine".  Regulated supplies in parallel with each other start to play tug of war with each other.   It will work in some cases, especially cases where you aren't fully loading the supply, but will likely cause one of the regulators to fail eventually.

(It's rated for 18A on each of the rails).
Yes, but not all at the same time...
P = I * E = (18A * 4) * 12 = 864 Watts.  (That number look strange?)

I still don't understand what the previous person was talking about with regulating though, I haven't been able to find anything on that.
The original ATX Specification did not require the 12V rail to be regulated.  Initially it was intended to drive motors and fans which weren't really sensitive to the exactly voltage being supplied.  However, as more of the VRMs started running on the 12V rail, I would expect the 12V rails to be regulated now.
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One of the guides I read specifically answered that question and said you could..?
Not everything on the Internet is correct.  When it comes to many things people try something and it appears to work "fine".  Regulated supplies in parallel with each other start to play tug of war with each other.   It will work in some cases, especially cases where you aren't fully loading the supply, but will likely cause one of the regulators to fail eventually.

(It's rated for 18A on each of the rails).
Yes, but not all at the same time...
P = I * E = (18A * 4) * 12 = 864 Watts.  (That number look strange?)

I still don't understand what the previous person was talking about with regulating though, I haven't been able to find anything on that.
The original ATX Specification did not require the 12V rail to be regulated.  Initially it was intended to drive motors and fans which weren't really sensitive to the exactly voltage being supplied.  However, as more of the VRMs started running on the 12V rail, I would expect the 12V rails to be regulated now.

While I can definitely agree with you that not everything on the internet is correct, I'm left with the dilemma that this message board is in fact on the internet.  So I guess I'm simply going to have to experiment for myself and see which page on the internet is the correct one smiley  No offense intended of course, I wish I had a masters in electronics to know for sure what the correct answer is, but I don't and that is why I'm here asking and searching for other answers on the net as well.

As for the number looking strange, not really.  Should it, if so why?  Looks like it's close enough to the peak power output of the PSU, since I've never seen an 864watt PSU but have seen 850w and 900w, it leads me to believe that the rating is rounded off to the nearest 50w interval.

And the last part, umm that really didn't help explain what the regulating is.  Context clues lead me to believe it's something like the 3.3v sense wire that some PSUs have that ensures a constant 12v is maintained, but I'm really not sure what it has to do with regulating 12v instead of 5v, or exactly what is done to achieve this.
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As for the number looking strange, not really.  Should it, if so why? 
It was just to point out that just because all 4 outputs are capable of 18A, they aren't capable of it at the same time.  Since 864W is more than 850W.  The supply is only capable of a total of 850W across its combined outputs.

And the last part, umm that really didn't help explain what the regulating is.
A "regulated" supply means that the supply has been designed to stay close to its rated value.  As load increases on a rail and more current flows, the voltage will begin to drop.  A regulator will then make a change (switching supplies are PWM and start switching faster) to increase the voltage to meet the demand.  Logic circuits demand stable voltages, this is why supplies are regulated.

Because current must be flowing for a voltage to appear on the output, a minimum load is necessary for the regulator's feedback to work. 

Related:  It is very common to see "Wall-Wart" adapters that are rated for 12V that measure 15 or 17V with a multimeter and no load.  This is because the regulator cannot regulate the output voltage until current flows.  ATX Power Supplies also have minimum load requirements.
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for future reference, since you cut off your connector, the Panel Mount mating housing to a 24 Pin ATX connector would be Molex PN:

39-01-2241

http://www.digikey.com/product-detail/en/39-01-2241/WM1029-ND/284603

The older 20 Pin ATX power supply types would be Panel Mount mating housing PN:

39-01-2201

http://www.digikey.com/scripts/dksearch/dksus.dll?vendor=0&keywords=39-01-2201

Some older ATX power supplys, and switching power supplys in general, do not like no load on the output and auto-shut down. I put a low ohm 25 Watt resistor on my 5V rail, I forget what value I used, but I'm covered by the 25 Watts.

Maybe the power supply isn't liking no load before you turn your amp on?

I threw on fuses/fuseholders rated roughly 5A below my 5V and 12V output ratings (something rediculous like 30A that I do not need) and put on some nice banana accepting binding posts on the bad boy, otherwise Sparkfun sells a kit do do all this...

http://www.sparkfun.com/products/9774
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All the supplies are regulated after a fashion, the reg reference in some supplies is in fact the 5V Rail, particularly the XT and most of the early AT supplies, were carefully wound transformers that were tightly coupled. A heavy load on any of the main outputs is reflected back to the 5V rail whose voltage will suffer due to the load... Thus one of the reasons why there must be a minimum load on the PSU, in the ATX supplies it is my understanding that some special regulation is separate but still the primary reg is the 5 V Rail... again since all the loads are part of the control feedback any of the outputs... even the - 12V can be considered regulated although in the XT and early AT's the -12V rail was considered the worst as it was ... back then as I remember (It was a long time ago and I Won't fix those things, simple though they are) used I think as a - rail for some old Pmos things... ram I think Very low current back then, I do know it never went off the mother boards but all the ISA bus's had a -12V supply rail and it is a part of all the old post testers, new ones too. It really isn't very important though...

Doc
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I've done it with el cheapo power supplies with good luck.  I cut the wires inside, and added a piece or romex wire from each rail and made it stick out the grill with a small stripped end.  Makes for a decent solid place to put crocodile clips for testing.

I've run stuff off the 12 and 5 and 3.3 rails no problem including low resistance electromagnetic coils for magnetising screw drivers.  The +12v rail is the best for this but +5 works fine too.   Where things get sketchy is when you try to use the -5 or -12 connections too.  They are highly regulated.  You can get 24 volts out of it but not much amps. 

Electrolysis with salt water using the 24v rail is quite entertaining though.
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Try this one too as a delux alternative to other kits - all posts are spaced 0.75" for standard dual banana jacks, and are color coded. PCB is 0.75um copper with gold plate for higher current capacity.
http://jordandsp.com/ATX-bench-top-power-supply-adapter.php
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