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PWM issues when powering 4pin 12v fan without a motherboard

happirt

Aug 22, 2018
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I am trying to setup a 12v 4pin fan with a potentiometer to regulate the speed. This is not for a PC so I don't have a motherboard, instead just a 12v battery and a NE555 timer based PWM controller.

fan pwm2.png
My problem is when the green colored ground connection from PWM controller OUT to common ground is connected the PWM is sending a constant 12v not pulsing as it should. What is wrong here?
 

Harald Kapp

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Nov 17, 2011
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Welcome to EP.
Obviously you're using an off the shelf PWM module, do you? Give us the type of module or even better a schematic. Otherwise we cannot judge what the correct connections should be.
 

happirt

Aug 22, 2018
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Thanks for the welcome. I'm just a real amateur DC electronics guy with a pwm fan problem....

I'm using a cheap ebay PWM controller, so there is no diagram :)

https://www.ebay.com.au/itm/12V-DC-PWM-Manual-Knob-Dimmer-Switch-With-Button-for-LED-Strip-Light-ZJ/282887045160?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649

I had to open it up to find it was NE555 timer chip.

Someone suggested it was switching on the ground not positive, which sounds right to me. So I don't know what I can do now.

Ultimately I'd like a LM7555 timer based PWM (for power consumption efficiency), so I was thinking of getting this
https://www.jaycar.com.au/5a-12vdc-pwm-motor-speed-controller-kit/p/KD6102

and replacing the NE555 with a LM7555.
https://www.jaycar.com.au/lm7555-cmos-timer-ic/p/ZL3455

Would it work? Would that switch on the ground?
 

WHONOES

May 20, 2017
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Have you tried connecting the fan ground through the controller unit leaving out the extra connection from the fan to your PSU?
 

happirt

Aug 22, 2018
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Have you tried connecting the fan ground through the controller unit leaving out the extra connection from the fan to your PSU?

Yes I've tried that, and the pwm works, the problem is the pwm controller will also switch the +12v constant to pin1 of the fan (so its pulsing not constant), so I think it's just turning the fan on/off really quickly rather than using the PWM pin as intended. It also gives a humming noise.
 

WHONOES

May 20, 2017
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I think the 12V on the fan pin 1 should be constant i.e. wired direct to you PSU with the controller connected as suggested. I expect the controller is intended to control a motor directly whereas the fan just want a PWM signal to do the deed.

Edit:
As an after thought, the controller may need a pull up or pull down resistor to make it work correctly if it is intended to control the motor directly.
 

Harald Kapp

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This controller is intended to control LED brightness and therefore is very likey, as stated, switching the '-' output whereas the '+' output is connected to the '+' input (can be easily verified by yourself using an Ohmmeter).
To use this controller on the PWM input of a fan you need a pull-up resistor and the following setup:
upload_2018-8-22_14-28-20.png
When the output of the PWM controller is active, it will pull down the PWM input of the fan to ~ 0 V.
When the output of the PWM controller is inactive, resistor R1 will pull up the input of the fan to 12 V.
The value of the reistor may have to be adapted to best match the fan's requirements.
 

Nim

Nov 3, 2022
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Hello, I'm having this same problem. Was able to fix it using this solution but the problem now is that I get 100% duty cycle the fan is stopped and at 0% is at full speed? Is there way to fix this?
 
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