Electronic – Where should I put the kickback diode in a transistor switch


When driving inductive loads with transistors, we use kickback diodes.

What I understand is a kickback diode supplies a path for the inductive charge to discharge. Also, an inductor will try to resist the change in the current, turning into something like a voltage source that will source the current in the same way it was before, in case of a break in the current (for example when the transistor turns OFF).

In the below circuits, there are two different placement of the kickback diode. D1 is placed in a logical way, so that the charge in L1 will discharge through it, protecting Q1's collector from over-voltage or breakdown.

However, the second circuit with D2 makes no sense to me. How can D2 prevent any damage when it is reverse biased? I was seeing this configuration rarely, however I saw it in a Lenze driver schematic and couldn't understand it.

How does D2 prevent any damage due to inductive kickback?

Kickback diode configurations

Best Answer

The first circuit D1 is correct in that it safely deals with the inductive kickback.

The second circuit makes little sense on its own. As Federico pointed out, D2 could provide a safe path for the kickback current it if were a zener, but it's not a shown as a zener and a 1N4001 is definitely not a zener.

D2 might make sense if L2 is more than just a inductor and could externally be driven backwards. That could be the case if it is a motor winding, for example. In that case D2 clips negative voltages before they can harm Q2, but it does nothing to safely limit inductive kickback when the transistor is turned off.