I came up with this circuit to control a 12V 6A fan's speed, with a 5V PWM from another circuit. I'm not sure if it's going to work (I'm not very experienced nor knowledgeable in electronics) and I have a few questions:

  1. To compensate for Q1's Vce, and turn off Q4 completely and reliably (when Q1 in on and A is low) I added one extra diode in series with Q4's base (total 2). Is this a good idea? Is it necessary, as I thought?

  2. Are D4, D5 and D12 necessary in this circuit? (I put them there as part of the Baker clamp for each transistor, to charge/discharge the base faster when switching.)

  3. Is there anything else that you think is unnecessary/lacking/wrong?

schematic diagram

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    \$\begingroup\$ Seems like overkill What kind of fan? brush or brushless and what is commutation rate at max RPM and PWM rate? Show datasheet of fan \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 3 '19 at 2:30
  • \$\begingroup\$ @TonyEErocketscientist they are two Delta AFB1212EHE paralleled with each other. \$\endgroup\$ – Microsoft Linux TM Jan 3 '19 at 2:47
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    \$\begingroup\$ where did you read the 6A current rating? \$\endgroup\$ – jsotola Jan 3 '19 at 2:55
  • \$\begingroup\$ 0.460A definitely overkill. Any transistor can do that but you will have aliasing issues with fan and its internal cap may wear out faster than normal \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 3 '19 at 3:04


simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Doesn't the inductor damage the MOSFET when it switches off? \$\endgroup\$ – Microsoft Linux TM Jan 4 '19 at 20:42
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    \$\begingroup\$ ah yes it would without diode TY \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 4 '19 at 21:11
  • \$\begingroup\$ Thanks, is there a formula for capacitor's value? \$\endgroup\$ – Microsoft Linux TM Jan 4 '19 at 21:53
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    \$\begingroup\$ aye , that would be C1 * f >= 1 with L1 * f >=5 and DCR< 0.1 but if you go higher in f , a faster diode is used and if you go too slow the LC's get large. and if you don't use a filter you have to wonder why the fan makes noise. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 4 '19 at 21:59
  • \$\begingroup\$ As for coil, would 35 turns of 0.75mm wire around a T106-26 toroid core be enough? (sorry, my first attempt to build an inductor :) as i can't find inductor of this current rating) \$\endgroup\$ – Microsoft Linux TM Jan 5 '19 at 11:42

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