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Coil suppressor module. Help please.

  1. Jan 4, 2014 #1
    I am trying to make a coil suppressor to restrict any voltage above say 130v. It is a simple VDR RC circuit. I am using a resistance of 47ohm, 250v and a capacitor of .1nF 250v a VDR of 130v. But the circuit is not working as expected. Is it because of the voltage spec of resistance and capacitor? Should it not restrict any voltage above 130v. But when am testing it, voltages above 130v are passing through it.
  2. jcsd
  3. Jan 4, 2014 #2


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    Staff: Mentor

    What is a VDR? Can you attach a schematic of the circuit?
  4. Jan 4, 2014 #3
    Am very sorry, its a varistor. I am attaching a schematic. Am using it as a protection device for a type of relay or contactor to save from the over voltage during turn on and turn off. The working voltage is mainly 110v so i wanted to limit the maximum voltage to a maximum of 130v. Basically a RC Varistor type snubber. Is there a necessity of a fuse for that purpose?

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  5. Jan 4, 2014 #4


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    Your application is still not clear. Is this 110VAC? Can you show the drive circuit and coil also. Also, in what way is it not working as expected?
  6. Jan 6, 2014 #5


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    Your circuit is correct and will protect most switch or relay contacts from arc damage.

    The VDR you have specified at 130V is, I suspect, probably not operating. Unlike zener diodes, VDRs are specified not to break down at the specified voltage. It usually takes about 50% more before they breakdown and they then hold at maybe 20% above the specified voltage.

    Also, a symmetrical protection diode or VDR rated 130V will probably be rated as 130V RMS, which is 185V peak, then add the 50%. You must study the data sheet carefully when designing with VDRs to identify the tolerances.

    A ZnO varistor has a lifetime determined by the number and magnitude of breakdown events. It should not be asked to perform continuously.

    An RC snubber as you have shown will handle slightly inductive loads that switch rarely. Maybe if you are switching highly inductive loads you should consider using a Triac with an optoisolator rather than a relay.
  7. Jan 8, 2014 #6
    The Contactor will handle slightly inductive load. But i need to restrict the voltage to a maximum of 120v-130v. This is the reason i used the 130v vdr. But i didnot have the knowledge about the same. I have to use the same vdr rc citcuit for space and drawing constraints but what is the more efficient way to do this? How how much in approximation should i change the VDR rating to get better results. Please help out.
  8. Jan 9, 2014 #7


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    It is a bit difficult trying to help when I do not know what you are trying to do, or why the voltage must be clamped so tightly.

    You have referred to a contactor. Is it the contacts of that contactor that you are trying to protect ?
    Is the contactor switching AC or DC current ? What is the RMS / DC voltage ?
  9. Jan 9, 2014 #8
    I am actually trying to protect the coil here. Yes this is basically an electro magnetic contactor, and the working voltage of the coil is 110v. The contactor works in DC voltage. To protect the coil from surge i am using it as a protection device.
  10. Jan 9, 2014 #9


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    If you have a contactor coil being driven by a DC current, then the best protection of the coil and it's driver is to place a reverse biassed power diode in parallel with the coil. No VDR is needed.

    The inductive voltage kick when the coil begins to turn off is clamped by the diode which carries the coil current while the magnetic field decays. The diode must be able to handle a surge current equal to the contactor coil current. A resistor and capacitor are not usually needed in that situation, but they will reduce the voltage slew rate.


    I hope that answers your question.
    If not, ask more specific questions and/or attach a circuit diagram of that part of the circuit.
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