vectorges:
dhenry - Thanks for a nice concise explanation.CrossRoads - Thanks for nothing. When I googled, I got several seemingly conflicting pieces of information. One was that a triac is AC only. One was that a triac is isolated from the controller. One was that the triac was not isolated from the controller. ANd one was my original question. I was hoping that this group would have a nice concise answer from a group of intelligent electronics experimenters, which it did.
The problem with the internet in general is that there is a lot of misinformation out there, and if you are not an expert it may be hard to separate the wheat from the chaff. Fortunately this forum is able to do that. With a rare exception or two.
Well triacs are (almost?) always used in AC circutis. Triacs along with SCRs are members of the thyristor family of four layer diodes. A triac is just effectivly two SCRs wired in reverse parallel with their gates tied together, allowing each SCR to handle just the negative or positive portion of the AC sinewave. The main difference in that these 4 layer diodes once triggered stay 'latched on' until the external current decreases to zero, where as a transistor properly biased can operate in a linear manner and regulate the current that flows through it by varying the base current or gate voltage. So Thyristors can only act as switches, where as transistor can be operated as switches or amplifiers.
Lefty