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PWM fan DIY control circuit

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Eamoex

New Member
Joined
Dec 1, 2018
Hi All,

I'm in the process of restoring an old computer (Amiga 2000) and I want to replace the PSU fan. Because it's old. Because it's noisy. Because it's fun!

I'm planning on the big thing, with a thermal probe in the PSU, and a knob control on the front panel to adjust fan speed. I found this (rather old) page by Dave on the main website. It shows a circuit using a 556 timer used as a pulse generator to control a PWM fan. The circuit diagram is courstesy of bing. I may use the same generator to control bigger fans so I chose the 556 option over the 555 one.

Now my breadboard is full with the components, I feed the circuit with 5V, but my oscilloscope hooked up to the PWM output and GND pins shows a constant +5V. No oscillation whatsoever. Turning the pot does not do anything. I checked the circuit several times and it looks like everything is right, or at least like on paper.

I haven't checked the datasheet for the particular 556 chip I have. I sort of assumed that all 556's are the laid out in the same way because it's such an ubiquitous piece of hardware.

Anybody has an advice as to what to check to debug this?
 
OP
E

Eamoex

New Member
Joined
Dec 1, 2018
Okay, my bad, one of my breadboarding leads was bad.

The circuit works and I get a nice, 5V square pulse. But when I hook the PWM output to a 12V PWM fan, the RPM range I get is relatively short. I should check frequency with the sense wire from the fan, but I can tell something's weird because when I set the pot to max speed, observe the speed and then disconnect the PWM control wire from the fan, it immediately spins up to a clearly faster speed. How come the fan doesn't run at max speed when the pot is set to the "max" position?