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How to check the condition of coil in the relay?

  1. Mar 8, 2017 #1
    I am using the relay as chatter. That is I am continuously on-off the relay to produce bursts. I am using 12V DC relay. I got voltage upto ~120V while it is switching very fast initially. Now I am not getting high voltage while switching. I am getting only ~10V. What may be the reason? Is it problem of coil inside the relay? relay.png
    I am measuring the voltage between the point of coil where +ve of battery is connected and the NO terminal.
  2. jcsd
  3. Mar 8, 2017 #2


    Staff: Mentor

    As you have drawn it, the NO terminal is not part of a closed circuit. It makes no sense to measure there. That forces the circuit to be completed through the voltmeter. That's not how voltmeters are supposed to work.

    When switching from NC to NO, current is being forced to zero. That can cause a high voltage at the switch common and, sparking between the circuit and NC terminal.

    You also don't measure the switching frequency. How many times per second is it switching?

    Frankly, given all that, I don't know what the heck you are measuring.
  4. Mar 8, 2017 #3
    Your equipment is not up to the task. The peak of that voltage is most likely far higher.

    I think the switch is landing on the 'no' terminal before the arc extinguished, so ~ the voltage peak might be measurable there (but not with a common multimeter I think)

    At that point the 12V coil has quite a voltage overload. I think some turn failures are likely to happen.

    Any method to check a transformer for turn failures would be more or less OK if the switch of the relay is temporarily fixed.
  5. Mar 8, 2017 #4
    Outside of the issues pointed out above... for a basic, unsealed relay in particular, this type of activity is very hard on the relay contacts. When they were new the contacts may have been making a very "clean" break when opening, as they have worn out, the opening of the contacts is a little slower, lowering the di/dt that creates the high voltage. - just a theory, would need to look at this on a scope and compare to a new relay.
  6. Mar 8, 2017 #5


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    Science Advisor

    When the contact was opened, the high voltage spike produced by the inductor damaged the coil insulation. There are now a number of short circuited turns in the coil. It no longer has much inductance because it has shorted turns.

    If you want to protect a coil you can use a flyback diode.
    You can limit the high voltage to about 100 volts with a gas discharge tube such as a neon lamp.
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