Hi Scotty,

An SCR only conducts in one direction whereas a triac will conduct in both directions which is why we might associate an SCR with DC and a triac with AC. But SCRs and triacs aren't really DC and/or AC devices per se.

That said, you can think of a triac as a current controlled switch similar to a relay. That is, when you apply current to the coils (the gate) the contact closes. When you remove current from the coils (zero gate current), the contact opens ... well ... almost. Unlike a relay, a triac will conduct once it has been triggered. And it will continue to conduct until the current through the device falls below the "holding current." So simply removing gate current is not sufficient to switch it off. It basically latches much like those old latching relays.

There are several ways a triac can be triggered. Some of them are undesirable which is where "snubber" circuits come in ... but that's another story. So when you ask, "does varying the gate voltage limit in some way how much ac is passed throug the triac" the answer is: sort-of.

When you apply a voltage to the gate you are inducing a current ... and that current has to reach a certain level in order to properly trigger the device. So in that sense, yes, applying an appropriate gate voltage will trigger the device. But once the device is triggered, the gate voltage doesn't affect current flow through the device -- at least not in the same sense as say, a series/parallel resistor. It acts like a switch, not a dimmer.

However, if you trigger the device at a certain point in the AC cycle, you can vary the power that is delivered to the load. This is how the fancier light dimmers and router speed controllers work. They don't trigger the device until the a certain point in the AC cycle ... that is, at a certain "phase" or "phase angle" in the cycle (phase being 0 - 360 degrees). During the period prior to triggering the device, it is not conducting. So it reduces the effective power that is delivered to the load. Which is pretty cool beans. The guys that figured all this stuff out really did great work!

The only down-side to using SCRs/triacs for power control is that they are not linear and won't get you all the way up to 100% once they're cut in. One of the papers I read said that somewhere in the neighborhood of 90%. That's why I started insulating my boiler if I wanted to use the RSC. Once the RSC was cut in, I couldn't "turn it up" enough to drive the column adequately for certain boiler charges. I'd love to have a nice variac, but they're too expensive.

Regards,
--JB