Discussion in 'General Electronics Chat' started by mikey9821, Mar 7, 2007.

  1. mikey9821

    Thread Starter New Member

    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.
  2. Papabravo


    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
  3. Dave

    Retired Moderator

    Nov 17, 2003
    For inductive kick(back), could you look at Volume 3 - Chapter 3.8?

    Sadly the article for Clamper Circuits hasn't been written as of yet, and there is nothing in the on-line e-book for flyback voltage.

  4. CaliusOptimus

    Well-Known Member

    Aug 14, 2005
    a 1n4007 would be more than enough, 1amp 600v