ATX to Lab Bench Power Supply Conversion

Step 5 Power Resistor

Most modern ATX power supplies require a small load to stay in the ON mode. I added a 10 ohm, 10 watt resistor between +5V and ground to provide this small load. It is strapped to the back wall of the power supply, where it should get plenty of air flow. It doesn't actually even get warm during normal operation so it's not a big deal.

Comments

[aessam1]

there is a problem with me
the 10 ohm 10 watt resistor is not available in my country
what can i use instead of it
i have grounded the green wire
but the fan didn't start
and i measured the outputs with a multimeter
and they were 5v and 11.7 and this is good
but a small led with a resistor across the +5 and GND
didn't glow
so what can i use as a replacement for 10 watt resistor ?
PS
my local store is
http://ram-e-shop.com

[matthewbeckler]

That is strange. If your multimeter measures the 5v output as 5v, then the LED should work just fine, I would expect. Have you tried that LED on a known-good power supply? You could use a bunch of larger resistance resistors in parallel to emulate a smaller resistance resistor with more power capacity.

[aessam1]

really strange isn't it ?
i have tried the led on another known power supply
and it worked fine
i don't know where is the problem
and a question plz
how many resistors should i use and there values ? (1/4 watt)
and thanks for your reply

[matthewbeckler]

Good to know that your LED is working. With the LED in place, try measuring the voltage across the 5v output, the voltage across the LED, and the voltage across the resistor.

There are two things to be concerned with when trying to build a more power-capable resistor from smaller 1/4W resistors. The first is the final resistance. Combining resistors in parallel reduces the resistance. Combining N resistors of the same resistance R in parallel produces a total resistance of R / N. If we want a 100 ohm resistor, we could make it from 10 resistors each with 1000 (1k) ohms of resistance: 1000 ohms / 10 resistors = 100 ohms.

The second concern is the power capacity of the combined resistors. If we have 10 resistors with 1/4 watt power capacity combined in parallel, then the total power capacity is 10*1/4W = 2.5 W.

The current through a 100 ohm resistor at 5 volts is given by the following:
V = I * R (ohms law)
I = V / R = 5 / 100 = 0.05 amps

The total power dissipated in a resistor is the product of the voltage and current, P = V * I = 5 * 0.05 = 1/4 watt

This means that you can take 10 1kohm resistors and combine them in parallel to make a 100 ohm resistor with 2.5 W power rating, and safely put it across 5v to draw 50 milliamps and 1/4 watt of power, which is spread across all the resistors.

Good luck!

[aessam1]

thx very much for the idea ,it has worked
but :D i made two biiiiiiiiiiiiiiiiiig mistakes

first : i didn't notice that the fan works by the time i ground the green wire :D

second : about the led i have tried it again and it has worked , maybe there was connection error on the bread board

and i don't know what to say except that it was all my fault
but at all thx for all

[matthewbeckler]

Mistakes are part of life, don't worry about it. It's very exciting that you got your project working! Congratulations, and good luck with the rest of it!

[aessam1]

thanks for your reply
i will try the led test now
and I'm going to buy resistors tomorrow ( in sha2 Allah )
and i will try them and tell you
thanks again

[motoruser]

what will happen if I did not add 10 ohm resistors 10 watts? whether the reduced performance of the power? thanks
please CC to my e-mail at: [e-mail]

[Sitnalta]

You will damage the semiconductors in the PSU, which will lead to a very erratic output voltage, which may damage whatever you're trying to power.

[matthewbeckler]

I have heard that some power supplies require a minimal load on the +5v rail in order so that the voltage-regulation circuitry works correctly. I have also heard of people having no problems if they didn't add a power resistor, so you might be able to get by without one. Also, if you are going to use your power supply in a setup where there will always be a load connected to the +5v rail, then you don't really need an additional load anyway. You could also use a fan or light on the +5v rail to provide that load. Good luck, let us know what you find out!

[lokivoid]

I have been using a illuminated rocker switch rated at 12VDC/30A for both toggleing power and putting draw on the 5v rail.

[Valence_4]

Do not tie thw 10 watt bleeder resistor with insulated wire. The resistor may melt the wire's insulation and will come loose. It will also smell bad !

[matthewbeckler]

Over the past couple years since I built this project, I've never had any heat issues with the power resistor. It looks like the wire is holding up just fine, with no deformation in the insulation. Good point about the melting though, I should probably replace it with something better.

[pjharro]

Your telling me you have access to a drill, wire strippers, fuseholders, terminals, soldering iron, and LEDs, but you don't have any wireties!!

I find that very amusing :)

[matthewbeckler]

Amusing indeed. Since moving out of the dorms (and across the country) I've since acquired a pile of wire ties for these sorts of things. Since the wire is working just fine, I haven't had a need to replace it. Thanks for your comment!