Controlling PSU fan

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Controlling PSU fan

After my PSU died and released its magic smoke, I got the XP Power JPS250PS12 which can produce 250W (21A @ 12V), and cranked the voltage up to 13.8V using the gray pot.

Even on 12V the fan was loud and on 13.8V it much more.

I wanted to have temperature control for the fan so when I have high load it’ll spin faster to cool the PSU.
I found that the pot controls both the DC terminals and the fan header so it always on full speed (couldn’t find any reference for thermal management).

So I asked few people and a fellow ham came to the help.

Parts

I modified the list a little bit.
The original scheme suggests using 2.2KΩ resistor and I used potentiometer.
Also, in the original scheme there’s 100nF capacitor between the 12V and GND.

1 x LM317T
1 x NTC Thermistor 470Ω
1 x 5KΩ Potentiometer (Linear)
2 x Screw Terminals

Scheme

PSU fan control circuit PSU fan control circuitSorry about my poor circuit drawing 🙂

The schematic is pretty easy on the left is the input (12V or more check the LM317T datasheet or more information) and on the right is the output to the fan.
I soldered everything on a copper PCB.

I’ve used Molex 2 pin headers because I didn’t want to cut the fan wires.

Testing

I tuned the pot to get the fan to spin more silently.
Attached the thermistor to the power transistors (with some CPU thermal paste cause that what I had).

I noticed the fan have voltage threshold and if I give 3V it can’t spin unless I give it a little push, that means I can’t make the fan go from 0 rpm to full speed because of the low voltage change.

After all of the tests I 3D printed small case to fit in and attached the wires permanently.

[notification type=”alert-warning” close=”false” ]Please note: the LM317T body is positive so don’t make it touch any metal part. I used some tape on the metal to prevent contact.[/notification]

I aligned the circuit so the fan will blow the air in the LM317 direction, which can get to 66°c on full speed.

What’s next?

I ordered IR Thermometer from eBay so I can measure the temperature on the power resistors and check what’s the status on high load.
Then I’ll check if the resistance of the Thermistor is getting low enough under load to get the full fan speed.

73!

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