Thomas McGahee wrote:
>
> Why not use a dual approach?: Have the MT1 and MT2 of your triac
> connected ALSO to the contacts of a Normally Open relay.
>
> When you want to turn on your 2.5kW load, FIRST turn on the
> triac and wait about one cycle time. THEN turn on the relay.
> Keep both activated until you want to turn the load off,
> then FIRST turn off the relay, **wait long enough for the relay
> contacts to fully open**, and then turn off the triac.
> Easy to do with a PIC.
>
> The triac can easily take the initial turn on load. Then
> when the relay kicks in the voltage across the relay contacts
> AS THEY CLOSE will be only a volt or two, since the triac
> is on. But then the relay will conduct the load for the
> several minutes the load is on. When turning off, the triac
> will ensure that there is no arcing of the relay contacts.
> Then the triac turns off.
>
> I have been using this method for cutting down on the wear
> and tear of a heavy duty relay I use to control an electric
> room heater. The triac needs only a fairly moderate
> heatsink, as the relay ends up handling the ON current
> for all but the turn-on and turn-off transition times.
>
> My unit (which is not PIC controlled, but rather controlled
> by some logic gates and timer circuits) has been running since
> 1975 without a problem. Oh, yes, I use a snubber network
> to ensure the triac does not get re-triggered at turn-off.


That's brilliant! Nil contact arcing for the cost
of a cheap triac that is hardly ever on...
I love it! :o)
-Roman

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