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AC/DC switch ratings question

  1. Jun 26, 2012 #1
    I have a motor that is running off bridge-rectified mains voltage. There are no filtering capacitors across the rectifier output.

    When figuring the ratings needed for any switches in series with the motor, would I use the AC or DC ratings of the switch?

    My understanding is the biggest difference in the ratings between AC and DC switching for contacts is that the lack of a zero voltage/current diminishes the ability of the arc to break, hence why DC switching ratings are less than the AC ratings for a given set of contacts.

    However, in this application, I still have the zero voltage/current so therefore I think I can use the AC ratings. Am I correct?
  2. jcsd
  3. Jun 26, 2012 #2
    If you intend to use it for AC purposes then use the AC switch ratings. However, it's always better to overspecify the ratings even if it means spending a little bit more money. I recommend contacting company representatives directly with questions regarding their products.
  4. Jun 26, 2012 #3

    jim hardy

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    If you place the switch in the AC line to the bridge you can use its AC rating.

    If you have it on the DC side use its DC rating. Even though your applied DC voltage voltage dips to zero twice per cycle , the motor's inductance maintains current and the bridge acts like a flywheel diode..

    Draw yourself a full wave bridge with inductive load and observe how the bridge completes a current loop for inductive "kick". But that feature really eases things for ths switch if it's on AC side of bridge.

    Better safe than sorry.
  5. Jun 27, 2012 #4
    Jim, that was awesome as always.
  6. Jun 27, 2012 #5

    jim hardy

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    Well golly, md, thanks for the kind words !

    Hope your project works out smooth .

    old jim
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