# Heat dissipation

I have a computer power supply and I'm wanting to make it a variable voltage power supply but I have to have a load on it which requires some resistance.

I have connected a 10 ohm 10 watt at 10% between ground (black wire) and 5 volts (red wire) but it overheats!

Can I add a second of equal value to fix this overheating?

• You should make sure you did everything right. P = V^2 / R gives: P = (5V*5V)/10Ohm = 25V^2/10Ohm = 2.5W. Which is much less than 10 watt. So if it gets hot to above 80degree C, it's either: 1. Not 10Ohm, but less. 2. Not 10 watt, but less. If it were 3 watt or 4 watt, it might become quite hot, but without damage, but 10watt should dissipate so much it doesn't become insanely hot. Commented Oct 4, 2014 at 18:24
• Can you place the resistor in the airflow coming out of the PSU? Commented Oct 4, 2014 at 18:53
• Yes but I tried that, fan isn't a coolant(although it does cool) it's more a air circulator. If that makes sense? Commented Oct 4, 2014 at 19:19
• Forced air will almost always draw heat away from the component via keeping the ambient temperature down. Commented Oct 4, 2014 at 19:20
• Fan does no change to resistor temp. Commented Oct 4, 2014 at 19:23

No, since current will also double. Putting just 2 resistors with double the value (or 3 with triple, etc.) will help, since the total current will remain the same. Note that the same amount of power will be consumed, but it will be distributed across both resistors instead of just one.

• If I continue to add resister will it eventually cool? Commented Oct 4, 2014 at 18:24
• Well, no, because you're continuing to put power into it. The best you can do is dump heat from the resistor to ambient as quickly as it's generated. Commented Oct 4, 2014 at 18:26
• So heat sink? Or higher resistor? Commented Oct 4, 2014 at 18:40
• Either or. Heat sink will draw heat away, higher resistor will get hotter slower. Commented Oct 4, 2014 at 18:59

Two resistors in parallel will each get as hot as your original resistor, so that won't work.

You can make a 10 Ohm 40 W resistor out of four 10 Ohm 10 W resistors by connecting two resistors in series, and two of these strings in parallel. The final resistance is then (10+10) parallel (10+10), 10 Ohms. As the power is split equally between them, the power rating is increased by 4 times.

Be sure to keep them well separated, not touching, otherwise the heat won't be able to escape each resistor.

Finally,if all you're trying to do is load the 5 V output a bit, perhaps 20 ohms will also work. Dissipation in each resistor will be 1/4 of what you have now.

I used two resisters at 10 Ohm with 10 watts each in parallel and it cooled of tremendously and is now usable. Thanks for all who commented and offered advice