Testing MOV with a DMM

Discussion in 'General Electronics Chat' started by Andreas, Feb 15, 2011.

  1. Andreas

    Thread Starter Active Member

    Jan 26, 2009
    68
    4
    Hi,

    Is there a simple test one can do to check if a Metal Oxide Varistor is defective or not?

    I have a TNR7V471K which I have taken out of circuit. Physically it looks OK. Resistance and Diode tests reveal nothing. Capacitance test shows 0.3nF (non polarised).

    I have searched the Vol references as well as a forum search but to no avail.


    Thank you,
    Andreas
     
  2. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    Not doubting your info, tho' humor an old geezer

    Capacitance measured 3/10 nanofarad ?? Was this measured with an instrument,:eek: [[ I want one !! :D ]] or dedeuced mathmatically.
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    If you do not get a low reading in diode check mode than it is most of the time OK.

    I mostly check for short circuits.
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    A 300 pF measurement isn't terribly difficult. Here are some examples of meters that can do it; there are lots of others on the market (my 20+ year-old Fluke 83 DMM resolves to 10 pF). A GR650 from the 1930's could do it too.
     
    PackratKing likes this.
  5. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    No simple test to confirm correct operation as per spec. Short circuit test is obvious. Open circuit test is the main problem. However, successive operation of the MOV causes internal micro short-circuiting, so open-circuit typically happens in a rather destructive manner ;-)

    Ciao, Tim
     
  6. Andreas

    Thread Starter Active Member

    Jan 26, 2009
    68
    4
    In resistance and Diode settings the DMM shows O.C. (Open Circuit)
     
  7. Andreas

    Thread Starter Active Member

    Jan 26, 2009
    68
    4
    So it looks like I may have a MOV which is indeed OK. Hmmm, hoping it was stuffed. Thought I had the fault cracked - so back to the drawing board.
    Thanks Tim,
    Andreas
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    In normal operation the Metal Oxide Varistor has a very high resistance:

    this quote come from this wiki page:
    http://en.wikipedia.org/wiki/Varistor

    With a DMM you will read a very small capacitance or a VERY high resistance.

    Bertus
     
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  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    what are u repairing?
     
  10. tom66

    Senior Member

    May 9, 2009
    2,613
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    My cheap multimeter shows 0.02nF residual measurement on cap mode as does my expensive BK Precision.
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    You can allways connect it to a high enough voltage for it to break, through a resistor, and see if it actually does conduct at the target voltage. Ofcourse if it is meant for mains protection you will hardly get the required voltage.
     
  12. Andreas

    Thread Starter Active Member

    Jan 26, 2009
    68
    4
    A Switch Mode Power Supply with dual rail output.
     
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