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Leaner question : what is the purpose of this diode in the diagram?

  1. Mar 2, 2010 #1
    Hello experts, :!!)

    The following is a diagram posted by a learned forummer and i like to find out what function does the diode serve for the coil of the relay.


    Thanks for reading my question.

  2. jcsd
  3. Mar 2, 2010 #2
    Hello ramone,
    When the relay circuit is switched off the magnetic field collapses and the energy it stored is dumped back into the circuit.There is a momentary high voltage and this can result in a high current that will zap the transistor.The diode protects against this by providing a low resistance path for the current to flow through.I am no expert and my memories of this are vague but if I am wrong I know others will come in to better inform.
  4. Mar 2, 2010 #3
    Depending on which textbook you read, it's called a freewheeling diode or a flyback diode. It's used for electromechanical relays and also for solid state switches too. Basically, any place in your circuit which has a switch that controls a large power current should include a freewheeling diode. Like Dadface said, it gives the circuit a safe path to drain the energy from high voltage spikes that occur during switch transitions.
  5. Mar 2, 2010 #4
    Hi Dadface,
    Am i right to say that when the coil is not powered, the coil will still have a voltage that generates a current in the same direction, that is running from the collector of the transistor to the emitter? Current cannot flow from emitter to the collector for NPN right?

    Is the voltage called the back emf?

    Ramone :)
  6. Mar 2, 2010 #5
    Hi Okefenokee,
    I have a circuit using 2 dpdt relays (used in bi-direction switching) that seems like not switching properly. Should i put a diode similarly for both the relays?

    Ramone :)
  7. Mar 2, 2010 #6
    The diode and coil provide a L/C circuit with a fairly long L/R time constant for the coil to discharge when the coil current is interrupted. In some applications, this time constant may be too long. For a shorter time constant, put a resistor in series with the diode.

    Bob S
  8. Mar 7, 2010 #7
    Dear experts, I am confused by back emf in terms of the current flow direction.
    I have seen on the net that oscilloscopes show back emf is really negative voltage across the coils. At the same time, i read somewhere that the coil when its magnetism collapsed will actually sort of induce a current to force its way through the coil in the same direction as when the coil was energised.

    So, can i assume that back emf can be visualised as the coil having a voltage with reversed polarity which is why the diode is in that orientation to short-circuit (flyback) the current in the loop containing the coil and the diode preventing the transistor from being zapped by the very high voltage, back emf?
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