MOV metal oxide varistors testing

Discussion in 'General Electronics Chat' started by SamEricson, Apr 26, 2015.

  1. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    They use these MOV metal oxide varistors for light ballast circuits
    They are these very small and the color is blue

    With the circuit turned off, how do you test and check MOV varistor compoents?

    I put my ohm meter across a MOV varistor and I got no ohm resistance reading it just said O.L overload.

    How do I know if the MOV varistor is shorted or open or good?

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  2. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    The test for a metal oxide varistor is to take it above its breakdown with a low current high voltage, to see if it conducts and what the clamp voltage is.
    Treat it like two zener diodes cathode to cathode in a package with their anodes exposed.
    Not recommended for anyone that... well if you needed to ask you probably should not bother trying. High voltage for a ballast is going to be in a don't go there zone unless you are extremely certain what you are doing, need to know really badly, and take the precautions required.

    The overload test you got with your ohmmeter is the most important test and a good sign. It means they are not shorted.
     
  3. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    So how does an electronic technician test MOV varistors? to know if they are open? or working

    I would have to remove them for the circuit and do what? apply an AC or DC voltage across the MOV component leads?

    Also I tried to use a huntron to measure the MOV varistor signature and i got no component signature from it.
     
  4. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    I can't explain it more clearly.

    The only part I will repeat and clarify is that it is not safe for you to fully test the MOV in a ballast. Please don't try.
     
  5. SamEricson

    Thread Starter Member

    Apr 25, 2015
    196
    0
    yes i will remove the MOV for testing. I will test the MOV out of circuit

    It's true there is no way for an electronic technician to test MOV in circuit right? only out of circuit tests?
     
  6. devisree

    New Member

    Apr 26, 2016
    1
    0
    Varistor acts as a dielectric, the device has a capacitance that depends directly on its area and varies inversely with its thickness. Therefore, the capacitance of a Varistor is a function of its voltage and energy ratings. It is important to take care of electrical history of varistor, short both ends of varistor to discharge and measure the capacitance after removing the short.

    Many thanks,
    Theyvees
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,510
    2,369
    It is a variable RESISTOR the resistance of the device changes/decreases with increase in applied voltage the important numbers are normal operating voltage, expected peak and capacity to absorb energy in Joules.
    https://en.wikipedia.org/wiki/Varistor
    BTW, an old thread.
    Max
     
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