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Discussion in 'Troubleshooting and Repair' started by ronnie702, Jan 11, 2014.

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  1. ronnie702

    ronnie702

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    Jan 11, 2014
    This is a circuit of a Canon printer which was plugged in to two minutesa 220V Power outlet where the printer is obviously 110v. It is a new printer and Cannon cannot identify the part because they do not know what it is something made in China only can do is replace the whole PowerBrick and this cost too much money. If anybody can help identifying objects that would be great. I need to know what it is and I can get one from because I don't want to place the whole PowerBrick. this is component VZ101
     

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  2. davenn

    davenn Moderator

    13,618
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    Sep 5, 2009
    Hi ronnie
    welcome to the forums :)

    Its a MOV (Metal Oxide Varistor)
    they are designed to protect against over voltage. remove it from the board
    and if nothing else has been damaged by the accidental 220V supplied, the board will still work without it.
    If it doesnt still work, and blows the main fuse there, then other damage has been done and its going to take some work to determine what other components have been blown

    Dave
     
    Martaine2005 likes this.
  3. ronnie702

    ronnie702

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    Jan 11, 2014
    your the man!!!!!
     
  4. benjaminalexandercai

    benjaminalexandercai

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    Jan 7, 2016
    Oh my god, this is so helpful. Thanks for it!

    By the way, should we join up the contact point after removing the damaged MOV?

    Ronnie, did you get your printer working?
     
  5. Martaine2005

    Martaine2005

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    May 12, 2015
    No. It's the MOV that shorted. Connecting the joint again will cause another direct short.
    Cut the component leads low to the board so they cannot be bent over to touch each other.
    It's also a good idea to replace the MOV if you want to protect the circuit in the future.

    Martin
     
  6. benjaminalexandercai

    benjaminalexandercai

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    Jan 7, 2016
    Thanks for the help Martin.

    Just for educational purpose, if we compare the MOV to a pressure valve, am I right to say that once the MOV have failed, the valve will be permanently opened (closed circuit), meaning that it will direct current away away from the main circuit board regardless of whether the voltage is right? Will there be any case that the damaged MOV will not direct current and remain permanently closed (open circuit) since I understand that it is connected by some ceramic materials; if the materials chipped off from the explosion then wouldn't it be an open circuit, hence directing current to the main circuit board?

    The reason why I am asking is because if the damaged MOV becomes permanently open circuit (closed valve), then there wouldn't be a need to remove the MOV right?

    Hope I am not confusing anyone. Thanks a lot!
     
  7. Martaine2005

    Martaine2005

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    May 12, 2015
    Hi Benjamin,
    MOV's are a strange devices. I don't have the physics or math knowledge to answer this with any real technical lingo. But somebody will surely put me right where I go wrong!:)
    Your comment on "permanently opened (closed circuit)" Can only be one or the other. But I understand what you mean.

    As far as I understand, your analogy would depend on whether it's a parallel or series connection.
    Also whether the MOV failed 'short' or 'catastrophically'. The former, in parallel would continuously blow fuses. The latter, could allow the circuit to continue working normally until the next 'spike/surge' which then causes real damage to the circuit.
    But a series 'short' would work and 'catastrophic' would 'open circuit'. Assuming catastrophic failure to mean complete separation.
    But in reality, the MOV is almost always changing it's characteristics due to voltage and temperature variations. Time being another important factor too.
    And it's job is to absorb voltage surges and dissipate heat from those surges. Most failures are due to bad design. If a higher rated MOV was used, it wouldn't have failed so quickly.
    So your analogy can't really be used unless we use 'automatic gate valves', 'temperature switches', 'diverter valves' and 'expansion vessels'.:) I think!!.
    So, the MOV has two or three ways to fail. Open circuit, Short circuit and as linear resistance.
    To keep it simple for me too, MOV's that have catastrophically failed 'open', can be left in circuit only in a parallel circuit. That being said, you wouldn't know a fault existed after replacing the fuse.
    Obviously in a series circuit the opposite is true.

    Martin
     
  8. dorke

    dorke

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    Jun 20, 2015
    No, the other way around:
    MOVs are connected in parallel to the circuit they need to protect.
    Thus an open MOV is an open parallel circuit, no current flows through it,
    and it does not "direct current away" .

    It is bad practice to leave damaged parts in a circuit as they are no longer predictable nor reliable in any way.
    Very much so for safety/protection devices(MOVs included) because if you don't replace them with the same value working part, your are left with no safety/protection (I hope you already no the value of that ;)
     
    Last edited by a moderator: Jan 8, 2016
  9. Martaine2005

    Martaine2005

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    May 12, 2015
    Hey Dorke, forgive my ignorance. But perhaps you can elaborate on that statement?
    I was under the impression that MOV's are either connected in parallel OR series..
    If a MOV is in parallel to both phases and fails 'open', no current can flow. So the original circuit will work normally after the fuse has been replaced. But now, without surge protection?
    If it fails 'open' in a series circuit, there will be no current at all..Circuit is dead.
    On the other hand, if connected in parallel across the live and ground (earth), and fails 'short', the current will be diverted to earth? until the RCD or what ever kicks in?
    Just like 'cross bonding' pipework, gas, electric meters, baths and sinks?
    Or have I climbed the wrong monkey tree?

    Martin
     
  10. Minder

    Minder

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    Apr 24, 2015
    MOV's or VDR's (voltage dependant resistors) are there to suppress voltage spikes that exceed the voltage and energy level of the spike, they are rated in voltage and Joule level.
    The most often fail open when a spike that exceeds the joule rating is seen.
    If fail shorted then obviously they take a fuse out and they are detected and replaced.
    Mainly connected across a supply.
    M.
     
  11. Martaine2005

    Martaine2005

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    May 12, 2015
    Ok Minder..
    But what about failing open?. The circuit works normally without protection (parallel). Also what about diverting the current to ground when shorted?
    I need these answered so that I understand. Thanks.

    Martin
     
  12. dorke

    dorke

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    Jun 20, 2015
    Well,
    MOVs(Varistors) are connected in parallel to the circuit that needs protection from over-voltage surges.
    I guess series connection application can be found, but the overwhelming majority of their applications are parallel.
    Mostly between Live and Neutral.
    But ,Live to ground and Neutral to ground as well.
    If activated as a result of an over-voltage surge they will trip a fuse or RCD.
    They mostly fail open. If failed they should be replaced otherwise the circuit doesn't have protection anymore.
    Note:
    You can think of them as a Diac device, if it helps the understanding of the their action in the circuit.
    Below are helpful links to MOVs

    Their "cousins" the NTC and PTC Thermistors are used extensively in series connections for protection of over-current surges.

    Varistors Introduction by VISHAY

    Intro to varistor Tech by Littlfuse
     
    Last edited by a moderator: Jan 10, 2016
  13. Minder

    Minder

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    Apr 24, 2015
    Not sure what is meant by 'divert the current to ground'? in the majority of cases they are connected across the input supply etc, generally the suppression WRT ground is a low value capacitor.
    There are also suppression filters that consist of L/C arrangement often fitted to the input of processors and PC's, see Corcom for one manuf. http://www.te.com/usa-en/products/emi-filters.html
    M.
     
  14. Martaine2005

    Martaine2005

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    May 12, 2015
    So I was correct. The circuit will continue normally (after fuse replacement) if it blows?
    I was referring to the OP (re-directing). A short cannot happen unless the current is shorted (directed) at something else. Otherwise there is no circuit problem.
    The main reason for initial failure is actually a 'short'. Then due to internal temperature and high current absorption, they fail 'open'. Or physically burnt apart (separated) inside. That is how I understand it.
    I could also be very wrong!

    Martin
     
    Last edited by a moderator: Jan 10, 2016
  15. dorke

    dorke

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    Jun 20, 2015
  16. Martaine2005

    Martaine2005

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    May 12, 2015
    Thanks Dorke,
    The first page has really inspired me to carry on reading. How come you find really useful web sites?
    Never mind, really interesting. Thanks all the same.:)

    Martin
     
  17. Minder

    Minder

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    Apr 24, 2015
    Before the current tendency of manufacturers practice of putting up PDF's on their web sites, there was a practice of distributing many hard copy technical manuals, Fortunately I still have many of mine accumulated from the 80's and on, including the GE Transient Voltage Suppression Manual, which goes into depth on the issues of VDR characteristics and selection which is out there but takes some searching.
    M.
     
  18. benjaminalexandercai

    benjaminalexandercai

    3
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    Jan 7, 2016
    Thank you all. =) I have learned so much.
     
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