Hello,

I'm new to these forums (as well as circuits in general), and I just had a quick question about the following circuit:

This might be super obvious, but what purpose does the diode between the collector and ground serve?

 

The diode would normally be placed across the motor and not the transistor collector emitter.

This may help?

 

Okay, that's what I kind of figured - thank you very much!

 

OR - if you don't want the current spike to appear on the 12 V bus, or if the 12 V bus is not available if you are adding the diode after the wiring has been set, the diode can be a zener installed as shown. In this case the inductive kick is clamped at the zener voltage, not simply one Vf above the rail, and the spike current goes to GND. In a 12 V system, a 15 V or 18 V zener does a nice job. Some IC relay driver chips have the zener built-in.

ak

 

A further advantage of using a zener in the circuit shown is that the energy will dissipate much faster and the motor/coil current will thus die more quickly (10x to 20x isn't uncommon).