Thread Starter


Joined Mar 7, 2007
How do I figure out what Diode I need? It’s to prevent back feed to a 12v relay that is running 6 LED trailer marker lights. I don't know off hand how much power they use. I'm a total rookie in case you couldn't tell. So if you can make it as dumb as possible for me I would greatly appreciate it.


Joined Feb 24, 2006
Diode - a poem about death

I'm not sure what you mean by the term "back feed". When a switching circuit is connected to an inductive load such as a relay coil or a motor winding everything is cool until you try to turn the switch off. The voltage across the coil is proportional to the time rate of change of the current in the coil. This voltage is called flyback and may be what you are refering to. The connection of the diode depends on the configuration of the switch and the inductive load(coil). The size of the diode depends on the voltages and currents present in the circuit when the switch is turned off. What you are trying to do is clamp the junction between the coil and the switch to either the positive rail for an NPN switch or GND for a PNP switch. Things are slightly more complicated if there are negative voltages being used, but the principles are the same.

Some google terms for you:
Clamper Circuits
inductive kick
flyback voltage