Start Simple

Lance Allen says

[Ed: Rather than relays or H-Bridges or PIDs in many cases you can drive a moter with a..] N channel LOGIC LEVEL mosfet such as RFD14N05L or even up to BUZ101L ( good for 29 amps). They are cheaper than a relay [ed: or anything else] and dont need another drive transistor.

The Mosfet connects the gate directly to the [uProcessor or control curcuit], the source to 0 volts and the drain to the motor -ve, motor +ve goes to +5volts. You will need a reverse biased diode across the motor for back EMF suppression, such as 1N4004

Classic overcurrent detection scheme.

by Michael Rigby-Jones [mrjones@NORTELNETWORKS.COM] 

                R
From 12v _____/\/\/\____________ To motor
           |__              |
              \>|           |
                |--/\/\/\---+
               /|    4k7
              |
              |

            To PIC (needs dropping to 5v for PIC)

The transistor is a PNP device. Resistor R is chosen so that at the current you want to detect, the voltage drop across it is enough to switch on the transistor, say about 0.6 volts. The other resistor is to limit the current through the base/emitter junction.

So for 10 Amps this would give 0.6/10 =0.06 ohms. Not a very easilly obtainable value! But to achieve this kind of resistance you can experiment with a small coil of copper wire, you won't need much. Some commercial designs use a PCB track as the curent sense resistor.

Note that is circuit is not a precision design, the current at which it activates will vary a bit with temperature etc, but it should be OK just for detecting a stalling motor.

EMF detection

When the motor stops rotating it'll stop making back EMF and the voltage across it will drop. You can use an op amp as a comparator.

See also: