Load on 5V rail?

[matttheniceguy]

I am making a TEC water chiller which is powered by 2 really crappy 300 watt ATX psu's.

http://www.ocforums.com/showthread.php?t=297739

I have the psu's connected up so I can turn them on to test them (just grounded the green wire) and the 12V line is only producing about 10.4 V with almost no load on it. I connected a 50 ohm resistor across the 5V line and ground and restarted the supply, the 12V rail went up to about 10.8 V.

I always thought you needed some load on the 5V rail for the psu to work right, but I just read that you only do on an AT supply. Do I need to load the 5V rail on an ATX psu? If so, approximatly how much? I thought about 1 amp sounded like a good number, but I can't seem to find a resistor about that size.

Also, is there any easy way to adjust the output voltage of a psu, so I could run it at slightly more than 12V?

Thanks, here is a picture of the chiller under construction.

[I.M.O.G.]

Looks like a good start.

Some PSU's have adjustable pots which allow you to change the voltage. Haven't done it myself, nor do I know what they look like. If the PSU's are crappy though, I wouldn't count on finding them in there.

As for the voltage, that isn't too bad for running a TEC, it will be slightly more efficient running at just below 12v, though have slightly less total power. If you can't get it up any further, I would expect performance to be quite similar to what it would be at a solid 12v.

[matttheniceguy]

I did some more in depth testing with one of the supplies. Here is the 12V output with different loads on the 5V line:

.00A - 9.7V
.25A - 10.6V
.56A - 10.9V
.66A - 11.1V
.96A - 11.2V

Obviously as I increase the load on the 5V line the 12V voltage goes up. Note that there is no load on the 12V line aside from a case fan.

What I want to do is put a consistant 1 amp load on the 5V line, but I can't seem to find any resistors, or even a combination of resistors that can handle the amount of heat they produce (about 5W). I know I could buy a resistor like this, but the only place around that has them is radioshack and they are like $4 each.

Anyone know of something I could put on my 5V line?

[uthungover]

The older style auto tail lamp bulbs should work. They have two filaments that you can parallel together, and they're cheap.

[matttheniceguy]

The older style auto tail lamp bulbs should work. They have two filaments that you can parallel together, and they're cheap.


NICE CALL!! I was thinking of using some kind of light, but all i could think of was chirstmas lights (120V) and some other random small stuff, but I didn't think of a tail light. I have quite a few of them around, and they are old and crappy enough that they should draw plenty of current.

[HiProfile]

I was thinking about using one of the ATX PSU's as a POWERFUL portable 12v source for a new R/C car battery charger (costs $80+ for just a 7amp 12v supply in this hobby!), and read all about loading the unused lines. I remembered the PSU Test Station that tom's hardware used for their PSU tests, and had just ordered stuff from www.bgmicro.com ... Low and behold, this 15ohm 25watt Power Resistor (http://www.bgmicro.com/prodinfo.asp?sid=0664004629629636531111129&prodid=RES1078&page=1&cri=power+resistor&stype=3&time_out=19:58). It has an aluminum case for disipating the heat, and would be just right for loading the lines. This power resistor (3.3ohms 7watts) (http://www.bgmicro.com/prodinfo.asp?sid=0666932870370376531111129&prodid=RES1079&page=1&cri=power+resistor&stype=3&time_out=19:50) might be a bit better - assuming it has the same alum. case (no picture in link). The only problem I can see with the alum cases is that if you screw the alum. case to the PSU case, I'm not sure if the alum. case in someway connects to the core/wires - which would short the PSU. BTW just search their site - or google - with the string "power resistor".

If you look at virtually any PSU rail ratings on the respective mfg's website, you'll see each line has a "Minimum Load Rating" - usually around 1amp. Voltage*amperage=minimum watts needed for power resistor. Also, try to keep the ohm rating low, or that will mess with the voltage of that rail, which will probably mess with the 12v rail too. Back to cooling the resistor - with 4 fans, it shouldn't get too hot - a maxed out LM317T voltage regulator uses a heatsink smaller than the pictured power resistor in the 1st link, which is more than enough. The power resistors w/o the alum. case might be a problem though...

[Diggrr]

I've used a taillight bulb before too.
If you find one that's working for you, note it's wattage, and jog down to Autozone for a blue one to accent the psu case too...

[JKeefe]

This thread makes me wonder about something: How does rail loading affect PSUs such as the Antec TruePower series, where the 3.3, 5, and 12V rails have independent voltage regulation?

[matttheniceguy]

I grabbed a couple cheap tail light bulbs and have one of them wired into one of the supplies. It is getting me 11.6V out of the 12V line. The bulbs have 2 filiments, with a resistance of 1.1 ohms when they are put in parallel and the bulb is off. I have no idea what the resistance is when they are on though. I guess I could just check the current through them, but I don't really care as long as it works. The bulbs are just clear, as my only options were clear or orange, and clear was half the price. I think I may just paint them black with BBQ paint. I don't really want to have 2 big white lights sitting in the chiller.