Author Options:

what can be used as the dummy load on a converted power supply Answered

I dont have a 10 watt 10 ohm resistor for the dummy load but I do have several others like 7 watt 6 kohm ,or 5 watt 300 ohm and some 10 watt 7 kohm can I use any of these if so how to wire them 

5 Replies

rickharris (author)2012-11-11

A light bulb of suitable voltage and wattage or close often makes a good dummy load + you can see the PSU is on.

Select as Best AnswerUndo Best Answer

oob42 (author)rickharris2012-11-11

Yea I tried that I didnt like the way it looked If I decide to do something In It has to be correct . It is a 300 +watt power supply from an old dell computer. I custom built a metal box it has 6 120 v ac plugs 4 usb charging ports molex plugs sata power and others

Select as Best AnswerUndo Best Answer

frollard (author)2012-11-10

Dummy load just needs to draw a few watts from the 5v rail. It doesn't have to be specifically 5 watts (10 ohm at 5v). So long as the ohm rating doesn't draw enough current to over-watt the resistor you're fine. Try to draw 2-10 watts and you'll be fine.

Select as Best AnswerUndo Best Answer

oob42 (author)frollard2012-11-10

So one 5 watt 300 ohm or one 7 watt 6.2 k ohm should work fine i knew the wattage needed to be between 5 and 10 watts but wasnt sure about the ohms

Select as Best AnswerUndo Best Answer

frollard (author)oob422012-11-11


It's not the watt rating, it's the 'make sure you draw 2-10 watts' part that is important. The watt rating is just how much heat it can dissipate without exploding!

Using ohms law you can calculate how much power it will dissipate:
Volts = Amperes (Current) * Ohms (Resistance)
at 5 volts:
300 ohms
5 = I*300
I = 5/300 = 0.0166 Amperes
V*I = Watts
5 * 0.0166 = .083 watts. <<< NOT ENOUGH!

Say you want to draw 3 watts, work it backwards.
at 5 volts, 3 watts is 3W/5V = 0.6 Amps

V = IR
5 = 0.6 R
R = 5/0.6
R = 8.3 Ohms.

You need an approximately 8 ohm resistor RATED FOR better than 3 watts, 5-10 watts is a safe rating to make sure it doesn't blow up.

Select as Best AnswerUndo Best Answer