The placement of thermistor and varistor,please help,thanks.

Discussion in 'The Projects Forum' started by L_jack_xing, Jan 12, 2014.

  1. L_jack_xing

    Thread Starter New Member

    Nov 15, 2013
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    I am confusing that why so many SMPS manufacturer's application notes recommend placing thermistor after fuse but before varistor.As there is a likelihood that the thermistor would explode in a surge test.Just change the placement of thermistor and varistor can solve the problem simply.If use the recommended circuit(thermistor place before varistor),there must be using higher level thermistor for take the surge test.Please give me a hand to solve my confusion.Thank you very much. :)
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    The thermistor limits the peak transient current into the varistor which also benefits the fues. While you might not see any failures during your surge tests with the thermistor before the varistor, allowing high transient currents could kill the fuse or the varistor. Check the ratings of all three parts and be careful to keep some margin.
     
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  3. L_jack_xing

    Thread Starter New Member

    Nov 15, 2013
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    Oh yeah.I think you are right.And I will test it again with a large varistor.Thank you.
     
  4. westom

    Member

    Nov 25, 2009
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    Transient testing means spikes are too short to trip a fuse. Fuses are for currents after damage has occurred. So that a fire does not threaten human life.

    Testing for transients involves currents and times that are different. What an inrush current limiter addresses is different from what a fuse or varistor address.

    Varistors are often so undersized as to fail catastrophically - in violation of varistor specifications. Something must exist to limit or cut off a 'follow through current' AFTER a varistor fails catastrophically. Failure to do this explains why many power strip protectors have created house fires.
     
  5. L_jack_xing

    Thread Starter New Member

    Nov 15, 2013
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    Thanks for help.I understand what you said in first two paragraphs.But I am so sorry, my english is not very well.And I am confusing about the last paragraph.It may have no association with my thread.What exactly you want to explain there?Thanks again.
     
  6. westom

    Member

    Nov 25, 2009
    52
    5
    Normal failure mode for an MOV is a voltage change of 10%. This results in no visual indication - no burning.

    Top of an MOV datasheet are Absolute Maximum Parameters. These must never be exceeded. If an MOV exceeds these numbers, then it can burn - obvious external damage. Also called a catastrophic failure. No MOV must ever fail this way. House fires have been created by catastrophic MOV failure.

    MOVs can fail catastrophically when too small; when it absorbs too much energy. A protector will say how many joules it can absorb. Maybe hundreds. Typically destructive surges are hundreds of thousands of joules. Notice a gross mismatch.

    So that grossly unsized power strip protector does not fail catastrophically, a thermal fuse is installed. Their tiny fuse blows as fast as possible to disconnect MOVs from a surge. And leave that surge connect to attached appliances. The fuse is only to avert catastrophic failure and potential fire.

    If that SMPS fuse does not blow fast enough, then an undersized protector may fail catastrophically. A serious threat to human life.

    A supply fuse that powers a supply is often too large to blow on tiny surges that can cause a varistor to fail catastrophically. Hopefully that fuse might blow in some conditions AFTER the MOV has failed catastrophically and shorted. Of course, that fuse must be a fast blow type. Apparently the inrush current limiter is also an attempt to make catastrophic failure less dangerous.
     
    Last edited: Jan 21, 2014
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  7. L_jack_xing

    Thread Starter New Member

    Nov 15, 2013
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    I think I have understood.Placement of fuse and NTC are all for lessen the likelihood of MOV's catastrophic failure.So the human safety .Thank you very much!
     
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