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ATX power supply question

R

RU482

Lifer
I was wondering, does there have to be a load on each power line of an ATX power supply??

I have an application, running some blower fans on some "test" battery packs at work (automotive). To buy a lab supply that can put out a decent amount of current, you have to spend hundreds, if not thousands of dollars (actually, you CAN buy cheaper ones, but they fail alot sooner...and SUCK) Anyway, an ATX supply can be had at a fraction of the cost of a lab supply (due to sales volume, I would guess). All I really need is ~24A on the 12V side and less than 500mA on the -12V side. 5V, 3.3V and 5VSB would not be used.

1) So, my first question is, do I need to put a load on the unused (5V and 3.3V line), since it is a switching PS?

2) Can you put 2 or 3 switching power supplies in parallel (to comfortably acheive the current required)??

Thanks!!
 
1) No on the 5V and 3.3V line unless a load is required for the power supply to turn on - in which case just put a 10k resistor on there to trick the supply into thinking there's a load.

2) NEVER DO THAT unless you want to see a firework show. Reason is that the two 5V lines aren't exactly at 5V. One might be at 5.2 and the other at 5.15V and that extra 0.05V will burn out the power supplies.

Why do you need 24A@12V for? Is this going to be used to test car amplifiers?

I dont think computer PSU can go up to 24A on the 12V line - more like 10A or 15A. But whatever you do, DO NOT PLACE TWO DIFFERENT POWER SUPPLIES IN PARALLEL!
 
I am powering blower fans (250CFM, even under considerable static pressure) that pull ~ 6A each @ 12V . There are three of these fans (18A+) and then you need more current to get them all started (24A was my estimation). I'm thinking I can probably use 1 supply for each fan..thanks
 
in my experience, the 12V rail will be weak unless you load the 5V rail also. and computer power supplies aren't going to work for loads like those!
edit: I guess you could use one per fan, but I think it would be worth your while to just buy a dedicated 12V power supply.
 
a solution to paralleling multiple supplies is to place a small equal resistance in series with each output. i.e. 0.10-0.33 ohm. It isn't very practical in this application though due to the really high currents. You could also just either manually or with a circuit offset the starting times of the different fans so they all dont suck max current at the same time.

also, why not build your own supply? i dont think these things need to be tightly regulated or even regulated at all.

toroid or EI > diode bridge > filter caps >fuses > output. surplus you could have a +/-12 or 24V 20A+ supply for less than 50 bucks.

jt
 
I have many times paralleled up outputs on AT switchmode power supplies.

I have a 12v load that requires 70 amps and I have 11x 230w AT power supplies all connected together. I found without sufficient supplies the startup current from the switchmodes is not sufficient and they wont start. 5 of these supplies will happily run the load but I had to keep adding supplies to get adequate startup current.

The length of wire is an adequate resistor since the 12 volt rail is not very well regulated, up to a volt difference is OK. With my supplies, some deliver over 10 amps (I can only guess because my analog ammeter only meters up to 10amps) while others are supplying around 2-6 amps.

Cheers
 


<< I have many times paralleled up outputs on AT switchmode power supplies.

I have a 12v load that requires 70 amps and I have 11x 230w AT power supplies all connected together. I found without sufficient supplies the startup current from the switchmodes is not sufficient and they wont start. 5 of these supplies will happily run the load but I had to keep adding supplies to get adequate startup current.

The length of wire is an adequate resistor since the 12 volt rail is not very well regulated, up to a volt difference is OK. With my supplies, some deliver over 10 amps (I can only guess because my analog ammeter only meters up to 10amps) while others are supplying around 2-6 amps.

Cheers >>



I have a 15V 100A PSU(1,500W!). I got it from a surplus shop and it is as compact as two or three ATX PSU's together. There is an adjustment knob somewhere and I have it set to 13.8V to simulate line voltage in a car with engine running. I never had problem with lack of starting current, but I never tried to drive a starter motor with it or anything like that.

 
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