Tuesday, November 29, 2011

Relay board

Over the years I've moved away from relays and adopted triacs for switching high voltage AC loads. Their quiet operation and compact size (heat sink excluded) are a plus. In the last few years, however, I've reverted to the electromechanical device. The prodigal son has returned (nahhh). Relays have their drawbacks including mechanical failure, contact point burning/pitting, and coil burnout, but they provide superb electrical isolation between control and power side of the circuit and negligible voltage drop across the contacts.

So here's my relay module v1.0 complete with polarity reversal protection diode, flyback diode, and LED power-on indicator. The relay in this case is a SPDT with a contact rating of 10A @240VAC/24VDC and a coil of 12VDC (measured resistance = 420 ohms). The board is designed such that the LED and its current limiting resistor need not be installed for the circuit to work--the additional current draw may be undesirable or an indicator lamp may simply be superfluous. Even the input diode in series with the positive rail isn't absolutely essential (if polarity reversal isn't an issue) and a jumper wire can be soldered in its stead. Omitting the flyback diode, on the other hand, is an experiment for foolhardy to try out.

DC coil power input is via the 2-pin connector with red going to V+ as per convention, while relay contacts are accessed via the 3-way terminal block. I'm obsessed with isolation so I've placed them on opposite ends of the board.

The paper phenolic board is 1 x 3 inches, scored and snapped off from a 4x3-inch "mother" board which contains three other clones. Yeah, I'm mass producing this.... Uhhh sort of.

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