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Relay "Ratings"

Discussion in 'General Electronics Discussion' started by cjdelphi, Apr 18, 2012.

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  1. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Never really fully understood this... a relay with specs of "7 amps @ 240volts" or... "120vac @ 12amps"


    be 1amp at 120volts (120 watts) or 240volts at 0.5amps (120 watts) that's the max the relay will allow, but higher the voltage the less current and less heat?...

    eg, imagine boiling a kettle from 6vdc how thick would the wires have to be to handle that much current, higher voltage less heat? so how come Relays don't work on the same principal ?

    120 volts, max 10 amps, 240 volts, max 10 amps, 2000 volts, max 10 amps if it's the current that causes the heat not the voltage?

    (Chances are i'm being amazingly dense right now, if so please tell me where my brain is going wrong - cheers :) )
     
  2. GreenGiant

    GreenGiant

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    Feb 9, 2012
    if you have a current limiting source that is not dependent upon voltage then you could theoretically exceed the voltage rating....

    BUT a lot of times current is determined by the voltage (like through a resistor) this being the case they have the limits so that the voltage going through the coil will not exceed the 10 amps

    so if you have 120V , 10 amp rating 1200W... the impedance is about 12 ohms, putting 240 volts straight through is would result in 20 amps across it

    at least that is my take on it...

    On solid state relays higher voltages will make the pins act like capacitors and this can lead to all sorts of problems, so that's why there are limits on those, or you can burn out the chips


    I may be wrong, but its always made sense to me
     
  3. duke37

    duke37

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    Jan 9, 2011
    The rating of a relay is based mostly on the ability of the contacts to switch off the current. The higher the voltage, the bigger the arc so the same relay may be rated at 120V, 10A and 240V 7A.

    Switching AC is easier than DC since the arc is extinguished twice per cycle. In the case of DC, the arc will continue as the contacts open.

    Switching of an inductive load is more difficult than switching a resistive load since the inductive current cannot be stopped instantly.

    Running a relay too hard will burn the contacts and they may even weld together.
     
  4. cjdelphi

    cjdelphi

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    Oct 26, 2011
    ahhh now i'm with it, i forgot about the arc/air gap/voltage ration... I was just assuming heat killed it, but of course so will the higher voltage....

    so, let's say we controlled 20,000volts with a relay that only handles 120vac, i'm right in the fact the heat wont kill it or fuse, because the voltage is high current would be low (i hope lol)...

    1. energize the relay (to bridge it, with the 20kv OFF)
    2. once contact's have been made, switch on the 20kv

    since heat's not an issue that relay will work fine.

    obviously it makes the purpose of a relay (in this example) useless simply because the point of the relay to begin with was to switch on higher voltages/currents from a small voltage/current..

    if that makes sense i got it :) (if not heat, the arc's heat will kill it)
     
  5. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Thanks :) (19 more seconds to wait before i can say thanks) ..... twiddles thumbs, looks out the window, it's dark um. time up yet? pfffft...

    Cheers :D
     
  6. duke37

    duke37

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    Jan 9, 2011
    10kV a bit much for a small relay, the gap may not be adequate.
    It is not the switching on that is the problem, here you have cold contacts coming together and the arc will only start when the contacts are very close and closing quickly.
    Switching off is a different matter, the arc starts as the contacts separate a minute amount and the arc draws out as the contacts separate.
    Power relays use various techniques to quench the arc, the relay may be immersed in a special gas or even a vacuum. Magnets are used to divert the arc into a loop to make it longer or air blast is used to blow the arc into a loop.
    In your example you were using a relay as an isolator, this is much kinder than trying to switch off a load.
     
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