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Diode in Relay Circuit

Discussion in 'General Electronics Discussion' started by fscii, Apr 23, 2013.

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  1. fscii


    Nov 13, 2012
    I'm trying to understand how to protect my components from a relay. More specifically when the powered up relay closes and the field collapses so the current doesn't come back and cook my transistor. First, is this a free wheel or clamping diode setup?

    I saw this circuit diagram and the way the diode is, really confused me. Yes I am noob.

    Ok, now with electron flow we know - in a DC circuit is supplying the current/electrons. So they go up the emitter and out the collector to the coil. Simple.

    But look at the diode. Its oriented with Cathode towards positive ?!?! Moreover, its connected in parallel. Why isn't it connected in serial between the collector and relay coil with the Cathode towards positive?

    The way I'm seeing this diagram is when the coil collapses and current starts going backwards (in a surge) towards the transistor, it'll also go up the parallel branch to the diode BUT then it'd get blocked from going up the diode because the diode is pointing backwards (Cathode is towards positive?!) and come down and cook my transistor anyway.

    Again, it seems intuitive that the diode would go in series, between collector and relay coil, with cathode pointing on the negative side.

    Can someone please explain:

    1. Why the diode is in parallel instead of serial?
    2. Why the diode is seemingly reversed (cathode towards positive)?
    3. What ramifications, I assume there is, would there be if I put the diode in series (Cathode towards neg) between collector and relay coil?

    Remember I'm looking at it from an electron flow standpoint not conventional...

    Confused... but grateful for any help...

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The relay coil is an inductor. It stores energy in the magnetic field of the coil. If you turn off the transistor, the field collapses and generates a negaive voltage at the formerly positive end and a positive voltage at the formerly negative end. That is why the diode is oriented backwards and in parallel: the negative volatge from the coil is now at the cathode, the positive voltage at the anode of the diode. Therefore the diode becomes conducting and short circuits the coil. Thus the energy is disspated in the diode - not in your circuit where it would wreack havoc.

    You can also think of this as follows: When the transistor is turned off, the energy stored in the inductor tries to keep the current flowing in the same direction as before. But since there is no way for the current through the transistor (it is off), it takes the path through the diode.

    Putting the diode in series would have no positive effect at all, since the transistor still blcoks the current.

    A more detailed explanation gives Wikipedia.
  3. fscii


    Nov 13, 2012
    Harald, thanks for the great explanation. I get it now :)

    Let me ask this -- if I had gone ahead and put my diode in series w/cathode pointing to negative... When the transistor was powered up current would appropriately flow thru the diode to the relay.

    When the relay (and transistor) switched off, since the coil polarity is now reversed, the coil would be pushing + towards my transistor and go right thru the anode of my diode and zap the transistor.

    Is that correct? If so then I get why its reverse biased (diode) and in parallel.
  4. duke37


    Jan 9, 2011
    Saying the same thing as Harald.
    When the transistor is on, current passes through the coil. A diode is connected across the coil in the direction where it does not conduct.

    When the transistor turns off, the current will continue until the stored magnetic energy is dissipated. This energy may go into a spark or damage the transistor by pulling current through it when it is turned off.

    The parallel diode will allow this current to pass until the coil resistance and diode drop dissipates the energy.

    A diode in series with the transistor collector will not do anything since the current will always be in the direction that the diode will conduct.
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Think about what you're saying.

    If you connect the diode as you say, anode to the relay, cathode to the transistor: Where would any current flow if the transistor is turned off?
    It would try to continue flowing through the diode, but the transistor being off this would lead to a very high voltage and result on destruction of your circuit.
    If you connect the diode in parallel, the anode still is at the lower end of the relay coil - no difference here to your series connection. But now the current (which still will try to flow through the coil in the same direction as in the on staet of the transistor). The diference is that in the parallel connection the diode offers a closed path for the current back to the other end of the coil.
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