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Re: AC conversion

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  • David
    You can t, unfortunately, make a blanket judgement about a relay contacts AC and DC ratings. GENERALLY, a relay, like a switch, has a considerably lower
    Message 1 of 3 , Mar 5, 2008
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      You can't, unfortunately, make a blanket judgement about a relay
      contacts AC and DC ratings. GENERALLY, a relay, like a switch, has a
      considerably lower contact current rating for DC than AC, and the DC
      rating is at a much lower voltage. As mentioned, this has to do with
      the fact that the arc formed during the switching period between the
      contacts is fully sustained at full driving potential for the period of
      the contact closure, whereas, in AC, the full peak driving potential
      only exists for an instant in time, and is decreasing towards zero (or
      increasing, in the case of the negative part of the cycle) for the rest
      of the time. This means that the arc intensity is always going to be
      lower when averaged over a unit of time, thus less corrosive to the
      contacts. They do verious tricks to mitigate the arc problems in DC
      contacts, but the bottom line is that they are almost never (I'm sure
      there is an exception somewhere that I am not aware of) rated higher
      for DC than AC current. As a side note, you have to be careful about
      too LOW of a current as well---if you use a relay that is rated for a
      high current, and you switch a low current, you may get lower contact
      life due to the fact that the arc that forms normally has some "wiping"
      action that helps keep oxidation clear of the cotnacts. You have to
      use special relays that are rated for low current, using bifurcated
      contacts, mercury wetted relays, etc. While it isn't as in vogue
      today, due to the proliferation if increadible, sophisticated, and
      cheap semi-conductors, relays have a real science of their own, and it
      can get complex. Your best bet is to either buy a relay rated for the
      DC current you want to switch, or if you are using a surplus or "on
      hand" relay, use it as is and hope for the best. :)

      David H.
      --- In SeattleRobotics@yahoogroups.com, "thewagx99" <thewagx99@...>
      wrote:
      >
      > Hi there, this might be a stupid question, but I want to make sure, Im
      > looking at a relay that is rated at 8 Amps AC, I was wondering what
      > the equivalent DC rating is? Is there a formula, or does it just stay
      > 8 amps?
      >
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