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Capasitive start and run.? Answered

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I have build this circuit and works great for a 12v pc fan on the bench for PWM. When i hooked this into my ATX power supply to regulate the fan speed the fan doesn't like to run unless i touch some portion of the pcb i build the circuit on. I spliced the wires from the ATX power supply into the PWM circuit, then out of the PWM circuit back to the fan. I guess its a  capactive issue.
What could be the fix for this?

The other An EE suggested i add several diodes inline to slow the fan down....hhmm maybe

Tags:ATX

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user
iceng (author)2012-08-11

I think the oscillator does not want to start..
Try a 4.7 or 10 meg resistor in parallel with C1.

A

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user
iceng (author)iceng2012-08-12

I would still put a diode across the fan motor..

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user
WWC (author)iceng2012-08-13

Problem solved.
I tried the resistors, no change because the problem was some where else.
I had ran the ground the same as original as far as an electrical connection goes. I had to run another separate ground back to the case, you can see it it the pic. The black wire on the bottom right. For the most part it is fixed. The ground still is not as good as touching the board. But i can live with the fan not going complete to stop. Slow down was the goal anyway.
I had put a diode in the circuit b4 but with the pics i proved you could not see it, so i included one pic of where i have put the diode.
There was not much room inside so stuck it on the outside. That side goes to the back and partly under a shelf so it can get some protection.

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