Kickstart DC motor

I have a computer with a small fan controller built into the case for the case fans. The DC brushless motors require a higher voltage to start than to keep running, so what I'd like is to mod the existing circuit to do that ie supply a full 12V (or close to it, it never actually needs 12V to start or run) and then drop back to the value as set by the controller pot.

The circuit is what I've traced off the controller and made assumptions about re the transistor type from what it's connected to. My question is would dropping a cap in where dotted act as I want ie turn the transistor on fully to get the motor going and then drop back to normal running voltage?

I've seen heaps of designs using linear regulators, or PWM, I just want some simple low cost thing, hopefully using what's already there. The fan load is small, a couple of hundred mA.

emueyes said:

I have a computer with a small fan controller built into the case for the case fans. The DC brushless motors require a higher voltage to start than to keep running, so what I'd like is to mod the existing circuit to do that ie supply a full 12V (or close to it, it never actually needs 12V to start or run) and then drop back to the value as set by the controller pot.

The circuit is what I've traced off the controller and made assumptions about re the transistor type from what it's connected to. My question is would dropping a cap in where dotted act as I want ie turn the transistor on fully to get the motor going and then drop back to normal running voltage?

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No. The capacitor would need to be between the base (+) and ground to fully turn on the transistor initially. You also need a resistor in series with the capacitor to limit the peak current through the capacitor and stretch the time the transistor is fully turned on. Try about 1kΩ in series with 470μF to start.

Thanks, I'll do that. I'd assumed that because the transistor base is connected to a resistor divider it'd never see the full 12V unless the pot happened to be turned that way. My knowledge is, obviously, very basic. It does make me wonder; the controller, and 3 fans, and all the wiring involved with that were supplied with the (fairly expensive) case. All the cabling was well made, routed through the case tidily, with a nice little PCB for the controller. But, these particular fans have quite a big hysteresis range, surely the people who designed the case would be aware of that and could've easily added the few extra components.

The transistor you showed is a PNP and thus requires a current flowing into the emitter and out of the base to turn it on. Since the base is connected to +12V, you need to connect the base to ground (through a resistor) to cause the desired base current flow and turn the transistor on.

I'm not sure whether it'll work, but by using PWM, a cap parallel to the motor starts to turn at a lower duty cycle. Whether a cap in parallel to the motor works for this case, you can tell me.