MOV . Overvoltage Protection

Discussion in 'General Electronics Chat' started by Sal M, Nov 30, 2014.

  1. Sal M

    Thread Starter New Member

    Nov 30, 2014
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    Hello everyone

    I have a coffee maker 127V – 550 Watt with auto shutoff (circuit board) and has an MOV in that board.

    If I install a 5Amp fuse then an MOV after the mains and before the circuit board, will the MOV that I installed will it clamp in the case of a voltage spike (220V) and the MOV in the board will not clamp or will both of them MOV's clamp?

    Note: I’m trying to make protection from a 220V.

    Nominal mains voltage in the building are: 110V / 220V

    Thanks,
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The coffee maker should already have one.

    Are you trying to replace one or re-engineer the product? If so, what is your evidence that you need to add one?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    The steps in selecting a MOV is:
    1/ Determine the neccessary steady state voltage
    2/Establish the transient energy absorbed by the Varistor (usually a rough guess)
    3/ Calculate the peak transient current through the MOV
    4/ Determine any power dissipation requirements
    5/ Select a model to provide the required voltage-limiting characteristics.

    The problem with MOV's is they usually destruct when acting on overvoltages of certain energy, if it is not obvious, the appliance could carry on working without future MOV protection.
    Why the 220v protection if the C.M. is 120v?
    If you operate on 120v with a 220v MOV it will only react to extreme overvoltages.
    Look up one of the suppliers such as Littlefuse etc.
    http://www.littelfuse.com/varistor
    Max.
     
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  4. Sal M

    Thread Starter New Member

    Nov 30, 2014
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    Thanks for your reply,

    I know the coffee maker has one (MOV) and I mentioned that in my original message. But I’m trying to install an additional (MOV) + a (fuse) between the mains and the circuit board so the (MOV) that I installed would clamp instead of the one in the circuit board. In other words I don’t want the circuit board to be burned.

    So do you think that one or both MOV’s will clamp?
     
  5. #12

    Expert

    Nov 30, 2010
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    According to the curves on the datasheets, both MOVs will conduct, but at different currents. Whichever one triggers first as the voltage rises will pass current which raises its personal voltage drop in that moment. Shortly thereafter, the second MOV will start conducting as the voltage drop across the first MOV rises and that rise exceeds the turn-on voltage of the second MOV.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    What is so special about this coffee maker or the building/country you are in that you think you need to do anything at all?
    Its just a coffee maker.. If it ever dies..just buy a new one.
    some people and their coffee.. o_O
     
  7. Sal M

    Thread Starter New Member

    Nov 30, 2014
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    Thanks much.
     
  8. Sal M

    Thread Starter New Member

    Nov 30, 2014
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    Not funny at all . . !!
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I wasn't really trying to be funny..
    I do wonder what is so special about this coffee maker that you think that you know better than the original designer and think its essential that you add additional protection over whats already incorporated..
     
  10. #12

    Expert

    Nov 30, 2010
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    He doesn't want the circuit board to get burnt in the unlikely event that he installs a 220 volt power cord on it instead of a 120 volt power cord? o_O

    That's as far as I can get, but even if this sounds stupid, it's his right to experiment with crazy ideas, as long as I don't get a sense that he's about to fry himself.
     
  11. Gdrumm

    Distinguished Member

    Aug 29, 2008
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    I believe the first one would do the job and that would kill the power coming in, so the second one would remain intact, but would be worthless, because there is no power.

    I seem to recall having had a Surge Protector that blew several (4 or 5 in tandum) same rated MOVs, (due to lightning I think), but if your first one is higher rated, and it goes, then everthing down stream should be dead, and dead, right?
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    MOVs are for dissipating spike energy, a 150V MOV will likely shatter if the appliance is accidentally plugged into a 220VRMS outlet.

    A sidac might failsafe by blowing the fuse, but for sure fire crowbar protection; use the sidac to fire the gate of a hefty triac.
     
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