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Old 03-28-2015, 05:13 PM   #1
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Default PC Power Supply Resistor Question

Hello all!

I have a question about the resistors used in a PC power supply. I just wired up an old 250w supply just for the heck of it since the power supply I really want is on backorder. I have a "somewhat" good understanding of electronics, but I'm no electrical tech by any means haha.

After wiring everything up, I initially used a 10ohm 10w resistor on the 5v rail, and I couldn't get a reading above 11.75v on the 12v output. I was able to charge at roughly 15a on an iCharger 406duo, but the sub-12v level had me perplexed. It dropped to 11.3v under a load... So just for giggles, I removed the 10ohm resistor, and replaced it with 2 1ohm-10w resistors (the two wired together parallel with the negative on one end, 5v on the other)

So my question is this, when I have the 2-1ohm resistors wired in, I have a stable 12.3v on the power supply, but the resistors get up to around 350 degrees in a matter of a couple minutes. Is that safe?

I realize that I was probably fine with the 10ohm resistor, and it barely got warm to the touch after 30 minutes but my inquisitive nature is getting the better of me. Thanks for any and all input, I appreciate it!
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Old 03-28-2015, 05:27 PM   #2
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No not safe. 2 1ohms in parallel is only .5ohms. That's a heating coil more than a resistor.
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Old 03-28-2015, 05:58 PM   #3
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Hey, thanks! Heating is definitely what it was doing! For my own knowledge, how would I wire the 2-1ohm resistors? 1 each on 2 separate 5v lines? Or bridge the two ends of the two resistors with a negative and positive on the other opposing ends? Or should I shut up and just put the 10ohm back in, lol

Even a single 1ohm resistor seemed to get pretty flippin hot. I was hoping two should help share the heat/load which was shown on a web site when i was tring to research this. But that just shows my ignorance of the fine details with these things.
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Old 03-28-2015, 06:23 PM   #4
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Yes that's ~25W on each 10W 1Ω resistor, not good. Could put them in series end to end, but they would still be pushed pretty hard and get fairly hot.

I'd try multiples of the 10W 10Ω resistors in parallel until some voltage rise is achieved.
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Old 03-28-2015, 06:35 PM   #5
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Yeah, will-do. I did try 2 10ohm resistors but it actually didn't do anything as far as increasing voltage. It's cool, even at 11.3v with a single 10ohm resistor I was charging at 15a, so I'm probably wasting my time chasing 1v. I was trying to read up in more detail on how these things work but it was a bit like reading Chinese for me. Anyhow, thanks a lot guys, I appreciate the help. Have a good weekend
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Old 03-31-2015, 04:50 AM   #6
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I've read up a bunch about using the correct resistor. I just made a power supply using a fairly new and nicer pc power supply left over from a custom pc project. I did not use any resistors in the 5v side. I get 12.25v under no load, and under a 5 & 10 amp load there is no voltage drop. I'm going to continue to run it without any resistors and see if it holds up. Just wanted to offer my findings.
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Old 04-03-2015, 05:13 PM   #7
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I dont bother with the resistor. Never had any issue, never seen the need for one.
Everyone just assumes its needed
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Old 04-03-2015, 06:06 PM   #8
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No resistors on mine either.been using it for10 years with no issues.
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Old 04-04-2015, 04:17 AM   #9
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PC power supplies vary greatly. Some need, some don't. Some don't like the on/off surge that RC charging can involve. Partially why server supplies have become a thing.
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Old 04-07-2015, 09:01 PM   #10
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Default HP Proliant DL580 G5

Has there been anyone successfully turned this PS on without a resistor?
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