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AC Relay Control w/ Triac



Jan 1, 1970
Hopefully this isn't considered a cross post. I'm uncertain if design or
basic is the most suitable group...

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:

o---------o G C| relay coil
| \ C|
0-5V control ___ | MT1 |<| MT2 |
signal -|___|o---o o--o|\|o---------o-------o
330 | | |>| |
o | o
--- | ---
0.47 50V --- | 0.1 100V ---
o | |
| | o
| | |

Now, 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

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

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,

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?

On second thought, after reading a bit more, I think the 0.1uF cap is to
help ensure triac turnoff. With an inductive load (ac relay coil), the
current and voltage will be out of phase so as the current falls below the
triac holding current, the voltage will be near a peak (positive or
negative) due to the inductor.

I see two issues with this. 1) If the voltage is high enough, the triac can
stay on regardless of the gate (static high forward bias scenario is the
way I'm thinking about that). In my case at 24Vac I think this is a
non-issue with a 2N6073 with a 400 peak repetitive off state voltage. 2)
More importantly, the quick stopping of current in the inductor will create
a spike in voltage that I'm guessing will have a significant slew rate to
it (???). If this slew rate exceeds dV/dt for the part the off cycle
command will fail regardless of the current dropping below the holding
current. What's odd is that I think in most cases this would be handled
with an RC not just a C!

So I'm not sure what rate to expect the voltage profile to have when the
inductor is shut off (HELP!). And once that is defined,

1) I presume you target the RC to have a time constant much slower than
dV/dt. Any guidelines? Should the RC time constant be 5x > than dV/dt,

2) I'm still uncertain how to size the cap voltage rating. If I'm on track
that this cap is for triac off state protection, then it seems the most
critical voltage is the spike during inductor current flow cut off. I'm
trying to estimate that by:

0.5*L*I^2 = 0.5*C*V^2 <<< Solve this for V
V = ((L*I^2)/C)^0.5

So I'm using V as my max voltage experienced by the cap, L as the ac relay
coil inductance and C the value of the cap. Is this the correct approach?
If so, any good estimates for L of an ac relay coil? Of course, I have I
which is the relay coil current.