Hello,
I have some AC relay questions. I have a generic relay driver control signal that drives a small RC to gate a triac. The triac provides a ground to a 24VAC off board relay. Here's the circuit:
24VAC | C| o---------o G C| relay coil | \\ C| 0-5V control ___ | MT1 || | o | o --- | --- 0.47 50V --- | 0.1 100V --- o | | | | o | | | o------o----------------o | GNDNow, MT2 is greater than MT1 so the "left" pointing gate will conduct when gate (G) has a negative or positive voltage. Thus, I "source" a ground to the 24VAC relay coil and can activate the relay. My questions are as follows:
1) What is the proper method to size and rate the 0.1 cap? I surmise this cap is used to absorb the energy created when the current through the inductor (relay coil) is suddenly stopped. So the energy stored is (1/2)*L*I^2. Is the 0.1 cap really needed? Won't the triac alone route any remaining sine wave from the AC signal until the current drops below the holding current thereby absorbing the energy. I ask this because I'm concerned if the 0.1 cap fails shorted (improper voltage rating) then the relay coil is always energized! Maybe the analogy is a zener/diode combination in DC circuits to speed up the energy absorption especially if the relay is driving an inductive load, is that what the 0.1 does in this case? The RC would be the coil resistance and 0.1uF so this would be a small time constant (not sure if its faster than the holding current method though!).2) I surmise the triac must be selected with heavy consideration to ensuring that the relay coil current is >> than the holding current of the triac, right?
3) I'm curious about the RC. The control signal comes from a 5891 driver. Do triac gates require slower slew rates? If not, why the RC? If so, why?Thanks!