So, for a small home-made engineering project I have cannibalized a 14.4 V DC 20 A motor from a Bosch battery powered screwdriver/powerdrill to spin a flywheel. I'm still using the trigger (drillswitch) from the powerdrill to control the speed of the motor but I want to switch this control for a variable resistor w/ potentiometer for preciser control. In other words, I am trying to make a speed control. How do I do this? What do I need? I will still be using the 15 V 1.5 Ah battery.
Electronic – Speedcontrol for a 14.4V DC 20A motor
battery-operateddc motormotor controller
Related Solutions
Can you instead make a tacho using a slotted opto switch and a disc with perforations. Alternatively you could get some kind of "once per rev" signal from a cheap inductive probe and a small magnet on the shaft.
Once you have this pulse waveform coming back you need to compare its frequency with a frequency that is equivalent to your demand. You mention you have a potentiometer as the "demand" - this could be used in a circuit such as a Voltage-to-frequency converter.
In effect, your demand has been translated to a frequency and that frequency is what you expect to get from your tacho (described above) when it has settled at the correct speed. Using a simple Exclusive or gate or the type II phase comparator as used in a 4046 phased lock loop would work - are you allowed to use logic chips?
As Ignacio said, use a DPDT switch in an H-Bridge configuration. Also, you are just wasting power using resistors. Change to a 1.5V battery and you don't need a resistor.
simulate this circuit – Schematic created using CircuitLab
For a variation on your circuit, you could use two 1.5V batteries and stick with your SPDT switch:
The resistors you are using are not only wasting power, but they are also limiting the current too much so that the motor will not turn.
If you must use a 9V battery, you could use a smaller value resistor per Saidoro's suggestion and put it in series with the motor. Then either use the DPDT circuit above, or the SPDT circuit with two 9V batteries.
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Best Answer
If it is truly a DC motor you simply apply power to and it runs, then you can PWM the DC. I'd use a suitable N channel MOSFET as a low side switch controlled by whatever generates the PWM. Put a Schottky diode in reverse across the motor to give the inductive kickback current a safe place to go. Start with a PWM frequency in the 24-30 kHz range.
If it's a brushless DC motor, then you need something more complicated because the circuit has to do the commutation too.
The drill already has a suitable motor controller in it. The first approach might be to see what the trigger control does and either actuate it directly or mimic its effect.