MOV's

Discussion in 'The Projects Forum' started by RodneyB, Jan 27, 2015.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Good day

    I am looking for information on MOV's.

    My Confusion came about when I asked at an electronics shop as to what size MOV to put on the Output of a 13.8Voltcharger. I said battery charger 12 Volt. They gave me a 12 Volt MOV for 13.8 Volts. I questioned them but they assured me it was correct.

    I connected the 12 Volt MOV across the 13.8 Volts and the MOV did not blow or short.

    Thus the confusion I thought that when the voltage exceeded the rated voltage the MOV shorted blowing the fuse?

    I would like to understand how to work0ut the size of the MOV for both AC and DC applications
     
  2. Rory Starkweather

    Member

    Jan 27, 2015
    41
    4
    There is always a little 'wiggle' room in electronics components. It can save, or waste your time.

    13.8 = 1.15 x 12. A 15% deviation is not a big deal. Check out a few capacitor data sheets. I think they consider 50% deviation acceptable.

    What I would suggest is a 12 volt switching regulator, or, if you go AC, a pair of 6 volt ones, if you can find them. Kind of like an LM7809/LM7909 pair.
     
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  3. DickCappels

    Moderator

    Aug 21, 2008
    2,649
    632
    The part number for TKS, EPCOS, and NTE Electornics MOVs relate to the RMS voltage rating. Don't know abuot the others, but generally they are intended for AC use and it makes sense that they would be rated for the RMS voltage similar to AC capacitors.
     
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  4. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Does this mean that MOV's are not suitable for DC surge protection
     
  5. Rory Starkweather

    Member

    Jan 27, 2015
    41
    4
    I'll have to try that and see. I've been trying a lot of things lately, but I got most of my training during the vacuum tube days so there are a lot of new ideas and components floating around.

    Are you looking for something like an ICL? (Inrush Current Limiter) I went nuts on them yesterday, and I can definitely tell you that there are a lot of options there.

    Nope. I read your message again, and that is not exactly what you are looking for.

    No matter. I think you are looking for a current limiter. I found a nice looking circuit, using two to four BJTs, on line yesterday. Or you can go cheaper, but actually better if you use an LM317 in current regulator mode. (See page 379 of 'The Art Of Electronics, 2nd Edition.) LM317s are less than a US Dollar here now. I think they top out at 1 Amp of current in normal operation, but they have some nice internal protection features. Whatever, stick a heat sink on any linear regulator. The legendary inefficiency of linear regulators means that a lot of the applied power will turn into heat. I had one go up in smoke yesterday, and it's probably harder for you to get them.
     
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  6. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    We have to import the majority of our components from South Africa they end up really expensive.

    Thanks for your help
     
  7. Rory Starkweather

    Member

    Jan 27, 2015
    41
    4
    BTW, love your avatar. :)
     
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  8. DickCappels

    Moderator

    Aug 21, 2008
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    MOVs are great for DC; not only do they work, but they are very fast. I once used them on DC power supplies to clamp "deadly" spikes that were being created by huge sub-nanosecond ground current spikes.
     
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  9. Rory Starkweather

    Member

    Jan 27, 2015
    41
    4
    Naturally, I don't know much about them. I understand the principal, I think, but have never used one.

    Are they capable of 'soft start' type activity?

    And that reminds me of something else that I need help with. I bought 50 x 1 Amp thermistors for soft start, and I got 'No Start' instead. I can hook them up between a 12 VDC source and a 1.5 kOhm load, and the resistance never drops below 10 mOhm. Am I doing something wrong?
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
    I don't know if it is still in print, but if you can locate a copy of 'Transient Voltage Suppression Manual' by GE it pretty much gives you everything.
    In the mean time.
    file:///C:/Documents%20and%20Settings/Administrator/My%20Documents/Downloads/9781402077395-c1%20(1).pdf
    http://www.littelfuse.com/~/media/e...n_devices_and_principles_application_note.pdf
    Max.
     
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  11. Rory Starkweather

    Member

    Jan 27, 2015
    41
    4
    I'm sire that I am ntr in print, but ovrweall imptrssiba

    (What's up with this is editor. I spend one minute making a post and then 10 minutes editing it.)

    Basically an MOV is a pair of back to back diodes.


    Enough of this. 45 minutes for the last sentence. Not worth ny time
     
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