Unit to stress test PSU's?

[dodger12]

Hi guys,

I'm wondering if you can help me out.

I test quite a few PSU's and at present i place load on them by wiring them to a series of 100w 12v car headlamps. As you can imagine, this is a bit of a pita, as they have to be handled with care, and i often smash em.

I've heard you can stress a PSU by using a series of resistors, but it was over 10 years ago i took electronics at college, so i'm totally out of touch.

At a guess, i'm going to say that i'm going to need several 1ohm 150w power resistors for stressing the 12v rail...but i'm probably wrong so feel free to correct me and mock me :)

Help MUCH appreciated.

[Diverge]

E=I x R = Ohm's Law
P=I^2 x R
P=I x E

E=volts, I=current, R=resistance, P=power (in watts)

if you put a 1ohm resistor on the 12V rail, it would draw 12Amps of current and 144 watts.

if you want to check PSU specs, then your voltage is known, your current is most likely known too. use E = I x R to find your load. lets say your max current on the 12V rail is 20amps: 12V=20A x R; R= 12/20 = 0.6 ohms. then you'd need to find what kind of wattage you'd need for that resistor (your load). P = I x E (or the other forumla)... P=20A x 12V; P= 240watts. So you'd need at least a 0.6 ohm resistor rated for 240 watts to check a 12V rail rated at 20amps. this is just an example :)

[dodger12]

Yeah thats what i was guessing. So would it be Ok to grab say 3 of those resistors, run them in series and stress 30a on the PSU?

I'm just worried that if i used too small resistors, there would be too much heat from them. Also if i went too small could it trip the PSU (might think there was a short or sommit)?

And last question, do you know anywhere in the UK that sells power resistors cheap?

[crodan85]

http://www.rapidelectronics.co.uk/ or ebay are your best bet

[Diverge]

Yeah thats what i was guessing. So would it be Ok to grab say 3 of those resistors, run them in series and stress 30a on the PSU?

I'm just worried that if i used too small resistors, there would be too much heat from them. Also if i went too small could it trip the PSU (might think there was a short or sommit)?

And last question, do you know anywhere in the UK that sells power resistors cheap?

resistors is series add up, and are less of a load, therefore draw less current. resistors in parallel will always be less the the lesser resistor in the parallel circuit. so if you wanted to draw more current with whatever resistors you had, you'd want to put them in parallel. I'm sure you could google this stuff pretty easily, but your gonna have to do the math for what your looking for and what you have in terms of loads, ect. How can you forget basic electronics, even if 10 years ago? I took it in 1993 :p: ;)

[dodger12]

http://www.rapidelectronics.co.uk/ or ebay are your best bet

Rapid only seem to do up to 100w, but if i'm going to be using a 1ohm resistor on 12v, i'm going to need a 150w or above aint it?

[Diverge]

if you want to stay with 1 ohm, you could buy 4 of those. Group them in to parallel pairs (so 2 are in parallel, and other 2 are in parallel), then put the 2 groups in series. it will still be 1 ohm, but could handle ~400 watts.

[dodger12]

if you want to stay with 1 ohm, you could buy 4 of those. Group them in to parallel pairs (so 2 are in parallel, and other 2 are in parallel), then put the 2 groups in series. it will still be 1 ohm, but could handle ~400 watts.

Confusing. I need to read up on this. What kind of load would that place on the psu?

[dodger12]

Right, before i go ahead with my order, does this sound reasonable

2x 150w 1r0 - for placing a 24a load on 12v rail
2x 50w 0.5r - for placing a 20a load on 5v rail
2x 50w 0.33r - for placing a 20a load on 3.3 rail

[Diverge]

Right, before i go ahead with my order, does this sound reasonable

2x 150w 1r0 - for placing a 24a load on 12v rail
2x 50w 0.5r - for placing a 20a load on 5v rail
2x 50w 0.33r - for placing a 20a load on 3.3 rail

2x 150w 1.0 ohm - for placing a 24a load on 12v rail - 2 R's in parallel should handle 300 watts; and would draw 24 amps, and 288 watts.
2x 050w 0.5 ohm - for placing a 20a load on 5v rail - 2 R's in parallel should handle 100 watts; and would draw 20 amps, and 100 watts.
2x 050w 0.3 ohm - for placing a 20a load on 3.3 rail - 2 R's in parallel should handle 100 watts; and would draw 22 amps, and 72.6 watts.

it seems like it should work for you, if those are the currents you wish to draw from the rails. you also have to take into effect that the rails aren't exactly 12V and 5.0V and 3.3V, they have some tolerances to them. The 3.3 rail will be drawing a little more current then you said above. I'm not an expert by any means, so i dunno if anything is being left out. I hope it works out for ya if you order them :)

[dodger12]

Cheers mate, order placed.

Are these things going to need some cooling? I was thinking 'yes'.

If so, do you think a stock AMD cooler strapped to them all with some TIC would do the trick?

[Diverge]

Honestly, I am not sure. They will probably get nice and hot though. I couldn't find the resistors you mentioned above from that site, so I dunno what they look like. I've seen various types, some with heatsinks around them, some just made of of metal and ceramic material. But I guess you'll find out how hot they get when you try them... hopefully your powersupplies handle those loads, if that is what they are spec'd to handle.