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Relay / Solenoid IO

A relay is a switch typically actuated by a solenoid or other source of movement; e.g. a reed relay is actuated by a magnet. A solenoid is a coil of wire which produces a magnetic field when an electrical current is applied, and a slug or other ferromagnetic material which is attracted by the field to produce mechanical force. It is the early electro-mechanical version of the transistor although much slower and noisier.


Hey, don't power it from the regulated supply that is running your processor... use the unregulated DC and connect the controller to the relay via a semiconductor like a transistor, mosfet, etc...

Relay Contact Noise and Failure

Buy the book cover "Noise Reduction Techniques in Electronic Systems" by Henry Ott.

There is a section on relay contact protection, which details the exact failure modes, the different circuit protection topologies, and even design equations to determine the right component values, like R and C in a snubber. It is a treasure on this subject! 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...

See also:




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