Hi again,
Thermal mass of the wire? What wire? Maybe a drawing of what you have already would help.
As you know, when we use a bridge rectifier with a switching device across the DC side of the bridge, any inductance of the load produces a large kickback that gets rectified with the full wave bridge and appears as a high DC voltage across the DC side of the bridge rectifier. When we do something like this in an inverter, we always have the input caps to fall back on that not only absorb the energy in the spike but also that means they are able to give that energy back when it comes time for the next pulse. But with an AC chopper circuit this is not the case so we have to provide some sort of damping, while at the same time trying to keep the efficiency up. Since the actual value of inductance is a bit of an unknown, the best bet is to do a few little experiments to see how high the spike goes with some small capacitance through a small resistance (the resistance needed so that the transistor does not have to try to dump all of the energy stored in the cap in zero time...it gets more time to do that). This also limits the minimum 'off' time otherwise the voltage in the cap would ratchet up and up and up.
If you are on the secondary side the situation is a little better. Since the output is low to start with a high spike like 100 volts will only mean we need a transistor of say 150 volts. But of course some experimentation and measurement is in order for sure. See how high it goes after adding some small capacitance and series resistance and go from there.
"Dimmer" circuits have come a long way since the diac. But amazingly even a decent one isnt that complicated. After all, all that we are trying to do for any dimmer is *time* the pulse to the triac, to get it there at the time that correlates to the desired conduction angle. To see how simple this really is, say we want to 'fire' at 1/2 of the total sine wave half-period. And say we are at 50 Hz here. 50 Hz has a period of 20 milliseconds, so half period is 10 ms. That means of course that the 90 degree point of the sine (and that is half way of half the total period) is at 5ms and at 15ms (the negative and positive excursions). So all we have to do is:
1. Detect the zero crossing of the power line sine, and
2. Start a timer that times out 5ms.
So if we look at the first cycle, it starts at t=0.000 seconds, and that is when we start a one shot. The one shot is adjusted for 5ms, so after 5ms have passed (t=0.005 seconds) the one shot fires and that is used to drive the gate of the triac. So the triac turns on at 5ms and we see a sharp rise in voltage at the load, then it starts to come down as a sine does until it crosses through zero again and that turns the triac off, but also starts the one shot all over again. Then after another 5ms the one shot fires again and that turns the triac on again (t=15ms) and that's where we get the negative part of the output.
There are a lot of ways to do this, all of which involve an adjustable one shot and possible another one shot to fire the triac with s short pulse. The adjustable one shot is made adjustable via a potentiometer that the user can adjust to get smaller or larger phase angles. The beauty here is that if the one shot can adjust down to 100us say, then we have almost full conduction, and since it can certainly adjust up to 9.9ms, we have adjustment all the way to zero output...something that a diac circuit might have trouble doing. That's why many of the older diac circuits could not dim all the way down to near zero light output with a standard bulb.
Sound simple enough?
Oh yeah, dont be afraid to switch at 1kHz or even 10kHz as the MOSFET's have no trouble with this and you'll get a more well behaved output.