There should be no problem doing phase control at 12V (after a
transformer) using triacs. The triac voltage drop is now a larger
percentage of the line voltage, so you have to keep that in mind. The
current limit resistor on the MOC triac will also probably have to be
reduced to get adequate trigger current.
        Triacs CAN drive transformer loads, it's just a little tricky. One thing
to watch out for is that the phase control is very well balanced between
positive and negative half-cycles. Any DC imbalance shows up as DC
voltage across the transformer primary, leading to core saturation and
high currents. Careful design of the phase control circuitry can minimize
this, however.
        You can also use "solid state transformers" after a phase control dimmer
to drive low voltage lamps. These are small high frequency switching
power supplies. The input AC line goes through a full wave bridge to a
power oscillator that drives a transformer. The high frequency AC drives
the lamps directly. These are available as a consumer item at pretty low
cost. Since they have no (or minimal) filter capacitor after the
rectifier, the phase control "passes through" the device.
        Another approach (we did a 72 channel DC dimmer this way) is to have a
big 12V transformer running off the line. Full wave rectify the
secondary, grounding the negative side of the bridge output. Run the
positive side through the loads to FETs to ground. The FET gates can be
directly driven with a phase control signal (from a PIC or other phase
control generation circuitry). This approach is nice in that the FETs do
not have to deal with a negative voltage on the drain (as they would if
we were to phase control the AC directly). Phase control of AC would
either require two FETs, two IGBTs, two SCRs, or a single triac.
Rectifying the power early saves us from this hassle. Also, the lack of
filtering on the output of the rectifier makes it so standard dimmer
curves still work (it's still a sine wave, but the negative half is
switched back up to the positive side). We also avoid the cost and space
requirements of a large capacitor to filter the DC to handle the lamp
loads. Finally, the RDSon of the FETS is very low, so minimal heat
sinking is required. The only real voltage drop (thus requiring heat
sinking) is in the bridge rectifier. You can reduce this drop by using a
center tapped transformer and two diodes (instead of the four). We used a
12V 50A secondary toroid transformer driving a bridge rectifier module in
our 72 channel dimmer.
        I have not done anything with reverse phase control, since most of our
circuits use triacs or SSRs. I have not seen any economical method to
move away from triacs or SSRs in higher power circuits. Using FETs or
IGBTs in AC circuits requires two per channel along diodes to steer the
current to the appropriate device. Coming up with isolated gate drive
also seems to be a problem. By the time you add the steering diodes to an
FET circuit, the voltage drop approaches that of a triac, so we don't get
any efficiency gain by going to the more complex circuitry. IGBTs have an
even higher voltage drop. I tend to look at them as like a Darlington
pair with the first transistor being a FET. Turning on the FET shorts the
base to the collector. However, since it takes 700mV to turn on the base,
the collector cannot drop below 700mV, while the saturation voltage on a
BJT is down in the 300mV area. If we just used a FET, we'd be dealing
with RDSon instead of the saturation voltage, and have higher efficiency
due to the lower voltage drop.

Harold

FCC Rules Online at http://hallikainen.com/FccRules
Lighting control for theatre and television at http://www.dovesystems.com

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