Need help with resistors for 5v load, Turning ATX PSU into bench supply

Thread Starter

Dr. Jay Parallel

Joined Oct 30, 2022
5
it is a 1250 watt PSU and its 104amps on its 12v and it has 25amps on its 5v and another 25amps on the 3.3v . im really at a loss on what resistors to use on the 5v to give it a load. its also a modular supply if that matters. i have quite a few 0.5 watt resistors left over from college classes and well its been a long time since i was good at it. most i do anymore is add the resistance of 2 speakers lol. Any help on what i should use would be greatly appreciated.
 

dendad

Joined Feb 20, 2016
4,251
A good power supply test load is an incandescent lamp. I use head light globes and have a few in my workshop that have one beam blown. So when a lamp fails in my car, I do not toss the old lamp but keep is for use as a load, like you need.
A lamp when cold, places a large load on the power supply and as the filament heats, the resistance increases. This surge on first powering up is a good test, like driving a load with large capacitors, or starting a motor.
A number of lamps in parallel can be used to increase the load.
But one 12V lamp in the 5V rail would be quite ok for the load I think. Tail light lamps are lower power and they may be ok too.
 

Thread Starter

Dr. Jay Parallel

Joined Oct 30, 2022
5
ty and so you think that would be good for what they call a dummy load? i just didnt wana send this thing up in smoke, as its pretty powerful n gona make heat
 

dendad

Joined Feb 20, 2016
4,251
Yes, the lamps are a great dummy load. They are cheap, easily available and are designed to run hot.
And the positive temperature coefficient of the filament material is an added bonus. Just remember their resistance is not constant over the voltage range so is you are after a fixed load then a power resistor will need to be looked at. Or an active load based around a power transistor of some sort.
But for a generic load that the current is not critical, a lamp is very hard to beat.
 

Thread Starter

Dr. Jay Parallel

Joined Oct 30, 2022
5
ill be honest, i have no idea lol...not my area, i know just enough to really mess something up. it will be pulling all it can from the 12v cause ima hit it hard least few times with a amp n subs lol and run a few other things. but based on what i said above the specs of the PSU, you all agree a 5w blub would be sufficient? i also got one of the larger blue resistors, its 46ohms, but i have no idea what watt it is. i think it could be 5w but i dont wana bet this on my skills/knowledge. i did charge and discharge a capacitor with said 46ohm resistor tho but it did get a lil warm by the end.
 

Jon Chandler

Joined Jun 12, 2008
432
Do you really need 25amps @5V for some reason? If you're working on a small microcontroller or IC project, a mistake in wiring with 25amps can leave smoking craters where your chips used to be.

I use a lot of 5 volt USB supplies or former cell phone chargers (nearly all of which supply 5 volts) for powering projects. For 12 volt needs, a switching power supply "wall wart" can using be found at a thrift store or PC recycler for a couple bucks.
 

dendad

Joined Feb 20, 2016
4,251
A lot depends on the power supply.
I use old server power supplies for my ham radios and the only load on the 5V is an LED to show power on. Does your power supply need a load on the 5V? Have you tried it without one?
I did add an extra load to the 5V but that was later. The original 12V fans screeeeeeemed so I just run them on 5V now and they are almost silent. The power draw is not at max and only high during transmitting so the slower fans are fine.
 

Thread Starter

Dr. Jay Parallel

Joined Oct 30, 2022
5
i dont need 25 onm the 5v thats just what it said was on it. but yah i dont imagine i would be using anything but th 12v
and i will test to see if it will come on and work without a load on it or just a led or light.
 
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