dinofx
New Member
I am trying to control the voltage going into an 18W brushless water pump via a PWM control signal provided on my PC's motherboard, and a 12V power source. Here is what I've research so far:
SOLUTION: Use a MOSFET to quickly turn the motor on and off
PROBLEM: Tachometer stops reporting RPMs correctly. I'm not even sure if a brushless motor is capable of running off of a pulsed power, since my understanding is that it has some controller circuitry.
SOLUTION: Use a MOSFET to charge a capacitor in brief spurts, thus providing a continuous voltage to the motor
PROBLEM: I don't see how this would work. First, let's say the motor consumes 1 amp, and the signal is on 25% of the time. What is to prevent the transistor from sending 4 amps of current to the capacitor, allowing it to provide a constant 1amp to the motor? Let's pretend the capacitor were infinite, it would simple charge to 12 volts and then run the motor at the full 12 volts.
Does anyone know how to do this correctly? I have been thinking about using a voltage regulator, a VFET, or adding a low resistor into to the charging of the capacitor to limit how fast it can charge.
EDIT: I guess what I don't really understand yet is how to model the resistance of a capacitor. Obviously a capacitor charged to 12 volts has infinite resistance in the presence of 12 volts being sent to it. I don't know its "instantaneous" resistance at any other value. For example, let's say I'm trying to run the motor at approximately 8 volts. When the MOSFET switches on, what is the rate at which it would begin to charge a .1 uFarad capacitor?
SOLUTION: Use a MOSFET to quickly turn the motor on and off
PROBLEM: Tachometer stops reporting RPMs correctly. I'm not even sure if a brushless motor is capable of running off of a pulsed power, since my understanding is that it has some controller circuitry.
SOLUTION: Use a MOSFET to charge a capacitor in brief spurts, thus providing a continuous voltage to the motor
PROBLEM: I don't see how this would work. First, let's say the motor consumes 1 amp, and the signal is on 25% of the time. What is to prevent the transistor from sending 4 amps of current to the capacitor, allowing it to provide a constant 1amp to the motor? Let's pretend the capacitor were infinite, it would simple charge to 12 volts and then run the motor at the full 12 volts.
Does anyone know how to do this correctly? I have been thinking about using a voltage regulator, a VFET, or adding a low resistor into to the charging of the capacitor to limit how fast it can charge.
EDIT: I guess what I don't really understand yet is how to model the resistance of a capacitor. Obviously a capacitor charged to 12 volts has infinite resistance in the presence of 12 volts being sent to it. I don't know its "instantaneous" resistance at any other value. For example, let's say I'm trying to run the motor at approximately 8 volts. When the MOSFET switches on, what is the rate at which it would begin to charge a .1 uFarad capacitor?