Which Resistor do I need to keep my PSU output stable??

Thread Starter

Khaleef

Joined Mar 22, 2017
67
Hello,

I am making a Bench Power Supply from a Computer PSU and according to several video tutorials I watched, there is a Choke Resistor (Dummy load) to connected to the 5V rail. Someone else said it would be connected to the 12V rail.

So I picked up 2 PSUs from a local computer store to make mine. I have packed all wire colours and built in my LEDS and Switches. According to the videos I watched, a 10W 47ohm Resistor is used.

But my problem is; Without the Dummy Load Resistor, the PSU powers on and all output works but noticed the 5v rail outputs only 4.8/4.9V and 12v rail outputs 11.9v. I was testing and while having my Voltmeter connected to the 5V wire, connected a 12v LED florescent to the 12v rail and my voltmeter changes to 5.03v.

If I connect the 10w Resistor to the 5v rail, it heats up so much and begins to smoke and I have to quickly turn off the unit and remove the resistor. The same if I connect to the 12v rail.

I checked in my box and found a 5W 47ohm Resistor and tried it in place of the 10w, it heated too but not like the 10w and hooking it with the Heat sink in the PSU, it should be fine.

Connecting the 5w Load on the 12v rail, 5v rail reads 5V but the 12v rail reads 11.7v... with the resistor on the 5V rail, the 12V rail reads 12.1v but 5V reads 4.88v

What Resistor would I use or what to do to keep both rails Output stable?

Thanks.
 

mvas

Joined Jun 19, 2017
538
It is very likely that the 12 Volt rail and 5 Volt rail are averaged together, for the feedback.

Typically ...
a 5 watt to 20 watt load on the +5 Volts supply
a 10 watt load on the +12 Volt supply
a 1 watt load on the +3.3 volt supply
it varies by size of p/s ...

All positive voltage can vary +/-5%
All negative voltages can vary +/-10%

Do not expect +/- 1% regulation like a Bench P/S.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
19,186
Also check to see if the P.S. common is referenced to earth GND, as PC types usually are, not always desirable in a bench P.S.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
19,186
Tower and desk-top PC's have the common rail of the DC supply connected to "Mains earth", GND.
In some instances this is done in the PS itself and/or the mother board ground plane.
If you do not want this feature for a bench supply, then the GND reference has to be removed.
Max.
 

Dodgydave

Joined Jun 22, 2012
8,471
Hello,

I am making a Bench Power Supply from a Computer PSU and according to several video tutorials I watched, there is a Choke Resistor (Dummy load) to connected to the 5V rail. Someone else said it would be connected to the 12V rail.

So I picked up 2 PSUs from a local computer store to make mine. I have packed all wire colours and built in my LEDS and Switches. According to the videos I watched, a 10W 47ohm Resistor is used.

But my problem is; Without the Dummy Load Resistor, the PSU powers on and all output works but noticed the 5v rail outputs only 4.8/4.9V and 12v rail outputs 11.9v. I was testing and while having my Voltmeter connected to the 5V wire, connected a 12v LED florescent to the 12v rail and my voltmeter changes to 5.03v.

If I connect the 10w Resistor to the 5v rail, it heats up so much and begins to smoke and I have to quickly turn off the unit and remove the resistor. The same if I connect to the 12v rail.

I checked in my box and found a 5W 47ohm Resistor and tried it in place of the 10w, it heated too but not like the 10w and hooking it with the Heat sink in the PSU, it should be fine.

Connecting the 5w Load on the 12v rail, 5v rail reads 5V but the 12v rail reads 11.7v... with the resistor on the 5V rail, the 12V rail reads 12.1v but 5V reads 4.88v

What Resistor would I use or what to do to keep both rails Output stable?

Thanks.
I always use a 4.7 Ohm 10W resistor on the 5V rail, this will give a minimum standby current of 1 A.
 
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