Hardware Peripheral Input/Output

Motor Over-Current detection / protection

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.

See also:

• http://romanblack.com/current.htm Motor Over-Current Sensor: Detects over-current situation with a motor or other DC device.