back emf

Discussion in 'Homework Help' started by fran1942, Jul 26, 2011.

  1. fran1942

    Thread Starter Member

    Jul 26, 2010
    Hello, regarding back EMF and how a flywheel diode is used to protect the transistor, I don't understand how the transistor could be at risk.

    My thinking is (using a typical transistor - relay circuit) that current is flowing from the positive supply through the relay, then through the transistor and then to negative.
    When the relay is opened and a back EMF is suddenly generated, the back EMF generated current flows in the opposite direction ie. from the relay back into the power source, so how could it destroy the transistor which is on the other side of the relay and not in the path of the back EMF current ?

    Thanks for any help. I am getting this mixed up badly somehow.
    I have attached a pic of how I see things.
    Last edited: Jul 26, 2011
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    No, the back-emf appears across the coil, but the supply voltage normally does not change significantly, so the transistor sees the increased voltage. The emf will be in such a direction that it adds to the supply voltage, and if the current falls quickly enough the result can be many times the normal working voltage, with possibly catastrophic results.

    Edit: The protection diode shown in your circuit will in fact turn on - so that the collector voltage will rise about 0.7V above the supply at the moment it switches off. With no diode, the rise would be much greater.
    Last edited: Jul 26, 2011