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In my car the blower motor speed is controlled by 4 valie resistor pack. I want to have fine tune speed control so I am planning to add a pwm fan speed controller ( Generic). My question is if I connect the resistor pack input to the pwm controller input and pwm output to the blower fan( without removing the resistor pack). My concern is if I connect both in parellal (resistor and pwm controller) will it work? Will that damage the pwm controller even if I use a diode at pwm output to prevent the voltage coming back to the controller?

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  • \$\begingroup\$ what type of DC motor? \$\endgroup\$ – Jasen Feb 28 at 21:01
  • \$\begingroup\$ @Jasen Just a typical brushed dc motor. Maybe 50w max. Skoda fabia interior blower motor to be specific. \$\endgroup\$ – Vigneshwaran.m Mar 1 at 5:18
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You need to disconnect the resistor pack. If the stock control circuit calls for high speed on the fan, You will short 12 V to ground while the low side MOSFET is turned on in the PWM controller. You do not want two controls connected in parallel. If you put the two in series, you will only be able to make the motor slower than the stock control is calling for. It may be cumbersome to try to use two control methods in series.

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  • \$\begingroup\$ Pretty much this. Do not connect in parallel under any circumstances. \$\endgroup\$ – DKNguyen Mar 1 at 1:17
  • \$\begingroup\$ Thanks for the answer @ScienceGeyser. But I was little confused when u say high speed on the fan? I will only connect positive lines of the resistor and pwm controller. Just confused how there will be a negative/ground in that line.I am just a hobbyist so I may sound stupid. \$\endgroup\$ – Vigneshwaran.m Mar 1 at 5:16
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    \$\begingroup\$ @Vigneshwaran.m By fan, I meant the blower, but this is sometimes referred to as fan speed on some environmental controls. I made the assumption that we were talking about a blower motor in the car interior environmental control and not some sort of electrically powered intake blower. On most PWM controllers, there is either a half bridge or full bridge output. This is an arrangement of MOSFETs that allow you to switch the output from VCC to ground to control the average current in a load. If you only have a high-side switch, you'll need to be concerned with the back EMF of inductive loads. \$\endgroup\$ – ScienceGeyser Mar 2 at 2:41

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