[AMPS] SS relay on mains

[John Lyles]

Solid State relay (dual SCR or Triac unit) will work fine on the powerline
if you rate it carefully for the inrush current. And put some transorbs,
MOVs, or something on your line for transient protection.

It is very dangerous for safety however, and one should always pull the
plug or turn off a physical switch before sticking hands in, due to the
leakage through the relay semiconductor. It will charge up power supplys
very slowly, and will cause nasty shocks (mA or so on some older devices,
depending on the voltage). Can also fail shorted, with obvious results...

Also, the zero crossing feature: If the voltage and current were in phase,
as a resistive load would make,
then yes, the voltage at zero is when the I is at zero. However, with your
fans, transformers, and such, you could be lagging power factor of 80-90%.
There will be some current flowing when voltage is near zero in the SSR.
Just be sure and rate it conservatively. If you try and open a circuit with
current flowing, you can get a voltage transient. It is good practice to
put an MOV (AC line rated) across the SSR contacts too, to help prevent
overvoltage failure.

Once I had about 3 SSR's off the same 220 VAC line, turing on bias, screen,
filaments, etc in a transmitter. The plate power remained on a big
contactor as I wanted lots of safety factor. Sometimes the SSR's would get
falsed and spike on for a moment, charging up their load power supplies, if
another circuit switched on, particularly the plate contactor. Solved this
by loading the SSR load side with a shunt power resistor, that dissipated a
little AC power. It also reduced the leakage charge up phenomina. Never
quite understood it, but it worked.

John
K5PRO



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