Two transistor questions

Thread Starter

Chris Wright

Joined Jul 26, 2006
1) when using a small NPN transistor (or PNP for that matter) as a switch, does it matter if the load is on the collector or the emitter? (If it makes any difference, the load is a 12V, 400 ohm relay coil.)

2) On the above coil would a diode be necessary to stop reverse current into the transistor from the coil? If so, would it be in series with the coil, or across the coil in parallel? It seems like I've seen them both ways.


Joined Oct 14, 2005
Yes. It will affect the saturation voltage (VCE). Normally, NPN transistors are used as low-side switches (load connected between positive supply and collector, emitter to ground) and PNP transistors as high-side switches (load connected between collector and ground, emitter to positive supply). These will ensure the lowest power dissipation due to VCE drop.

It might not matter that much if your load power is small, but it is a good practice anyway.

Yes, a diode is necessary and it is parallel with the coil. A series diode would not prevent the back emf from destroying the transistor.