Choosing a MOV for a AC transient/surge filter?

Discussion in 'General Electronics Chat' started by TheLaw, Nov 29, 2010.

  1. TheLaw

    TheLaw Thread Starter Member

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    Hey guys,

    I have a question regarding MOVs...

    So I want to put a transient filter on my power supply, but I don't know how to choose a MOV because their ratings don't make sense to me.

    I think a transient filter (surge suppressor), should look something like this:

    [​IMG]

    RV1 is the varistor (MOV). It doesn't make sense because by nature I thought the voltage rating should be around say 200V and the clamping voltage around 135V (US 120V Power).

    But it turns out that if I wanted a MOV with a clamping voltage of 135V, then it would only be rated for about 40V continuous....

    I'm utterly confused. What should the value(s) of RV1 be?

    Thanks.
  2. retched

    retched AAC Fanatic!

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  3. TheLaw

    TheLaw Thread Starter Member

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    That's an excellent link but I'm still confused. Google isn't helping me too much. I can't find too many good results...hmm...
  4. TheLaw

    TheLaw Thread Starter Member

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    Okay so I guess I figured it out. The AC value shouldnt be like 130, it should be closer to 170V which is the peak AC you can get from the wall.

    The clamping volting would be more around 400-600V not like 175V.
  5. timrobbins

    timrobbins Active Member

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    Wiring anything to the mains without fully understanding the operation and the consequences with respect to safety and damage and protection is not a good idea.

    That said, why do you want to add a filter/supressor?
  6. TheLaw

    TheLaw Thread Starter Member

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    Well if I follow the wiring diagrams for the transformer and use components with the proper ratings...what could go wrong, in theory?

    I want to add a surge supressor, just to have means of protection and to add something to it.
  7. timrobbins

    timrobbins Active Member

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    A design would normally come with specified parts, to work with a defined input supply and load. Were you planning to use common-mode chokes and line to earth caps, and do you have their specs?

    Is your mains supply prone to any particular disturbance, and is it well regulated, and is it deep in a domestic wiring environment or sitting at the incoming distribution? Do you have a special bit of equipment you want to protect, or is just to give you a good feeling? Are you going to regularly check that the MOV is still functioning, or will it be tucked away out of site only to looked at after you hear a bang or smell something burnt, or open it up in 20 years time? Are you worried about earth potential rise stresses? These are some typical issues affecting the choice of MOV.

    Ciao, Tim
  8. TheLaw

    TheLaw Thread Starter Member

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    Alright, that was fairly discouraging. But in a good way. I barely put any thought into any of those questions. But a question I do have is.. if I can use a wall transformer brick and then wire a power supply to it?

    That would hopefully eliminate or reduce the chance for catastrophic disaster.

    The power supply is intended to put out maybe 1-2A of power. Nothing big.
  9. TheLaw

    TheLaw Thread Starter Member

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    Now that I reread your post, I'm not thinking that wiring up a mains power supply could be that hard/dangerous. I suppose if I put a fork into the outlet while plugging it in, I could potentially do some damage, but I am trying to think, "What could really happen?"

    The input protection that I want to put on the supply is really for no good reason, other than to have it. Is it crucial? Not at all. I just want it to power some of my own small projects. It's not going to be powering anything more than a board with a Atmel or Microchip MCU on it.

    Checking the varistor and all the other things...Again, its not a very important supply. If something goes wrong, no one's life will be on the line (as of now).

    So, yeah....maybe I'll re-think this.
  10. timrobbins

    timrobbins Active Member

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    Sorry - I didn't mean to be discouraging - but rather to make you think through the topic and hone in what you really need. Forums are good to then zero in on a particular issue. For example, if your concern was an expensive uP circuit, or small projects powered from DC, which you didn't want to go over-voltage, then it's a good idea to firstly focus on the power supply that generates your DC and appreciate how it could go over-voltage, and what level of voltage could cause failure. It's a great topic, and makes you appreciate the energy in transients and how transients may or may not lead to a failure.
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