So this is sort of an odd series of questions. I'm currently working on some boards that use a 555 timer running on 12Vdc to switch the ground side of a 30Vac light bulb, using a 2N6075A triac. The 12VDC is produced by running the 30Vac through a single diode to produce 30Vdc, stabilizing it with a 680uf capacitor, and then running it through a 12V regulator. The output pin of the 555 is connected to a 1k resistor, then the gate pin of the triac, then to a 10k resistor, and finally ground. The lightbulb is connected to AC supply and I believe MT2 of the triac, with MT1 grounded.
From what little I've learned about triacs (I don't see much AC), this *should* result in essentially only the positive side of the AC sine wave being passed through...and yet the whole thing seems to be passing through. If I use the 30Vac to trigger the gate pin, the brightness of the bulb doesn't change, and if I trigger it using the AC through a diode, either direction, the brightness drops. I haven't yet looked at it on a scope, but I'd say chances are good the whole wave is being passed using only DC. Is this something special about the 2N6075A? I can find nothing about DC switching capability in the datasheet, so I'm trying to understand *why* this works, when everything says it shouldn't.
One thing I am seeing is that at the gate pin of the triac, when it's driven high using DC voltage, at the gate pin I'm measuring 0.455Vdc and 0.560Vac. Is this AC voltage leakage through the gate pin, or just noise that my meter's not smart enough to filter out? If I go to the 12Vdc supply, the AC is completely gone.
But now, onto the real question. Because of the high rate of failure of the 555 timers and a few other design flaws, and also because the designer has passed away and replacement boards aren't available, I'd like to design a new board, replacing the 3 555s and a lot of the other timing components with a single PIC12F. Since these boards may end up in other locations where they're switching 12Vdc instead of 30Vac, and I'd like to make one board with a couple jumpers to cover everything, the PIC will be driving transistors anyways, probably 2N3904s. I'm wondering if I can safely switch those 2N6075A's using 12Vdc through the 2N3904's, or if the AC voltage I'm seeing at the gate will cause them to fail, and may be the reason the 555s are failing. Anyone have a guess?
From what little I've learned about triacs (I don't see much AC), this *should* result in essentially only the positive side of the AC sine wave being passed through...and yet the whole thing seems to be passing through. If I use the 30Vac to trigger the gate pin, the brightness of the bulb doesn't change, and if I trigger it using the AC through a diode, either direction, the brightness drops. I haven't yet looked at it on a scope, but I'd say chances are good the whole wave is being passed using only DC. Is this something special about the 2N6075A? I can find nothing about DC switching capability in the datasheet, so I'm trying to understand *why* this works, when everything says it shouldn't.
One thing I am seeing is that at the gate pin of the triac, when it's driven high using DC voltage, at the gate pin I'm measuring 0.455Vdc and 0.560Vac. Is this AC voltage leakage through the gate pin, or just noise that my meter's not smart enough to filter out? If I go to the 12Vdc supply, the AC is completely gone.
But now, onto the real question. Because of the high rate of failure of the 555 timers and a few other design flaws, and also because the designer has passed away and replacement boards aren't available, I'd like to design a new board, replacing the 3 555s and a lot of the other timing components with a single PIC12F. Since these boards may end up in other locations where they're switching 12Vdc instead of 30Vac, and I'd like to make one board with a couple jumpers to cover everything, the PIC will be driving transistors anyways, probably 2N3904s. I'm wondering if I can safely switch those 2N6075A's using 12Vdc through the 2N3904's, or if the AC voltage I'm seeing at the gate will cause them to fail, and may be the reason the 555s are failing. Anyone have a guess?