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Solid state 110 relays

I've used a lot of solid state relays, but never for heating/cooling equipment. Some quirks that might cause problems:

1) Heat is their enemy and the larger the load, the more heat they produce--you need a good heat sink and good natural ventilation (forced ventilation if you pack a lot of them in a small area).

2) Pay careful attention to the load rating. Resistive loads aren't as bad as inductive loads but even with resistive loads give yourself at least 25% headroom in the load rating. A filament burning out in a lightbulb will frequently take out a closely sized solid state relay.

3) They're extremely reliable--I had one set of four that probably each switched 40-50 times per hour for 4-5 hours per day for over 10 years. When they fry due to excessive load they fail open. I did however have one fail closed for a reason I never understood--this alone may be a "killer".

4) You can only test them with a load connected. You can't just use an ohm meter.

5) They're available in a number of "coil" voltages--I've only used low voltage DC but believe I've seen them for 110VAC.


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