I couldn't think of a way to explain all this in one line, so apologies for the slightly vague title.

I've built a PWM Fan Controller for a DIY Laptop Cooling Pad Project. It theoretically works, and I am able to vary the duty cycle and achieve a change in fan speed from about 10% to a 100%.

However, the fan speed only changes when I rotate my Linear 100K Potentiometer to the half-way mark, and it increases really rapidly afterward. Changing which components, if any, would help make the change in speed slower and smoother?

I built this taking reference from an Overclockers.com forum thread about the same thing: http://www.overclockers.com/forums/showthread.php/641111-Building-PWM-Controller-for-4-wires-PWM-fan?p=6473318&viewfull=1#post6473318 Schematic

  • \$\begingroup\$ A log pot connected the right way round? \$\endgroup\$ – Brian Drummond Sep 17 '17 at 14:11
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    \$\begingroup\$ A schematic would help, but I'd guarantee that your ancient 741 is causing your trouble. There are 10's, if not 100's, of questions on this stack about the misuse of 741's. Your particular issues are caused by running it from a 5V supply, and expecting it to work with input and output voltages close to the supplies (0V & 5V). \$\endgroup\$ – brhans Sep 17 '17 at 14:12
  • \$\begingroup\$ @BrianDrummond I tried attaching a log pot, but with minimal effect. The speed still changes waay too fast. \$\endgroup\$ – Abj Sep 17 '17 at 14:19
  • \$\begingroup\$ @brhans I've edited my post with a link to where I drew my inspiration from. While their forum thread suggests a 311, I couldn't find one of those, but I did find a 339. Would this solve the issue? Here's the link to the forum again: overclockers.com/forums/showthread.php/… \$\endgroup\$ – Abj Sep 17 '17 at 14:25
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    \$\begingroup\$ This is one the most illogical ways to present a problem with layout errors and lack of reference designations. make sure all used inputs are connected. Like LM431-p2 is a jumpered to a location that goes no where in your diagram. Use a logic diagram and understand what it does. Compares average DC from PWM input with average PWM to Fan output. so both must be linear Voltages reduced to a smaller range centred around V+/2 to avoid invalid CM range of Op Amp. DO YOU have a 4 wire fan? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 17 '17 at 14:28

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