Lowering voltage to a brushless motor

[randino]

I have a brushless motor powering a water pump for a PC. It starts up
@12 volts, but not 7. How can I lower the voltage to around 8 (BTW, I
have ground, +5, and +12 available)? I tried 5 Ohms of resistance, but
it still wouldn't start. Would a capacitor after the resistor help
anything? I'm guessing the motors resistance is not constant due to the
switching in a brushless motor. So would a capacitor make the input
voltage more stable?

I know all about PWM circuits, but I'm looking for something simple.
PWM is either expensive and bulky (around $30), or cheap (IC), but you
need to purchase 100 of them.

 

[Frank]

The ATtiny series by Atmel...

 

[Franc Zabkar]

I have a brushless motor powering a water pump for a PC. It starts up
@12 volts, but not 7. How can I lower the voltage to around 8 (BTW, I
have ground, +5, and +12 available)?

If the motor current is less than 1A, use an LM7808 regulator or
equivalent.

I tried 5 Ohms of resistance, but
it still wouldn't start. Would a capacitor after the resistor help
anything? I'm guessing the motors resistance is not constant due to the
switching in a brushless motor.

The motor's current would vary with the load. A resistor is not the
way to go.

So would a capacitor make the input
voltage more stable?

I know all about PWM circuits, but I'm looking for something simple.
PWM is either expensive and bulky (around $30), or cheap (IC), but you
need to purchase 100 of them.

Brushless motors use electronic commutation. I haven't tried it, but I
suspect that PWM would play havoc with the motor's Hall effect
sensor(s). At the very least you would need to smooth the output with
an L and a C.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.