Questions about triacs

"GiveMeL"

** Baffles lots of newcomers, for some strange reason. ** About 1.5 volts or so.

** All triac triggering is done by applying a current between the gate and MT1.

As you can see from the table above, the amount of current needed depends somewhat on what voltage is on MT2 and the polarity of the trigger voltage.

When MT2 is positive relative to MT1 and the gate voltage is also positive, triggering is best.

** No.

The common rail of such a circuit is connected to MT1.

** Yep.

** Meaningless question without a schem of what you are considering.

Keep well away from triacs until you understand what you are doing.

.... Phil

Phil, thanks for replying. Yeah, I won't try anything before I really understand what's going on =]

Heh. Well, for DC circuits I can be certain that ground == 0 volts no ? But that doesn't seem to be true for AC circuits ?

(Where I live electrical outlets usually have only 2 holes. Which means, I guess, that people here don't much care about which one is hot and which one is neutral since you can connect any appliance one way or the other by simply turning the plug around...)

Anyway, I got this schematics from

R1 100 N1 Load +----/\\/\\/---------------+----------o o-------> 120V | | Live | | MT2 o +-+ SWITCH \\ G | | TRIAC o / | | | / +-+ | | | MT1 +-----------------+ | | +-----------------------> 120V Neutral

When the switch is open, N1 follows the mains voltage ? Now, say the switch is closed when Vmains = 60V (first quarter of the cycle) ? What's the amount of current flowing through R1 into G ? I guess it's determined by the voltage G is at ? But what voltage is that ?

Charles, thanks, I'll read that.

"GiveMeL"

** That may be a while yet ......

** With AC mains power, you must always assume that either or both current carrying conductors are live. ** Correct.

** About 600 mA - for a couple of microseconds. Then the triac is triggered "on" and there is a low voltage ( say 1 volt) between MT2 and MT1.

The triac will briefly go off when the supply voltage drops to under 1 volt and be re-triggered almost immediately when it rises again in the other polarity - long as the switch stays closed.

** G is at about 1.5 to 2 volts above MT1 ( assuming MT2 is positive compared to MT2 ) at the moment of triggering ( ie for about 2 uS) then drops to the same voltage as MT2.

.... Phil

Hope this one is better ?

R1 100 N1 Load +----/\\/\\/---------------+----------o o-------> 120V | | | | Live | | MT2 o +-+ SWITCH \\ G | | TRIAC o / | | | / +-+ | | | MT1 +-----------------+ | | +----------------------->

120V neutral

Well, I think I've a clearer picture now...Thanks Phil!

So, in that schem the current for the gate comes directly from the mains and is limited by R1. If the triac was a bt137 which has a peak current of 2A, and Vmains was 220v, R1=150 (say) would limit gate current to a safe value.

And it is also possible to use a separate source of current for the gate and that's what the circuits that interface a uController to a triac do, I guess ? In that case the current comes from the uController's PS - So 5V DC can be used to trigger the triac, just like 110/220 AC.

"GiveMeL"

** There are three main categories of triggering circuit used with triacs:

1. Pulse triggering ( aka pulse firing )

1. Continuous triggering (aka hard firing )

2. DC triggering

The schem with the switch and the 100 ohm resistor is, despite appearances, a form of pulse triggering - cos a brief current pulse flows at the moment of switching and at the beginning of each AC cycle.

Continuous triggering uses a stream of 2 to 5 uS long pulses, spaced about

50uS apart, to trigger the gate - usually via a pulse transformer.

DC triggering simply applies a continuous current to the gate, sufficient to fire the particular triac in all quadrants.

BTW:

Once you have figured out how to make a triac conduct - you can start worrying about how to make sure it goes off when you want it to.

Cos that is whole nuther can of worms.

.... Phil

Hehe. Well, I'll start with a resistive load and see what happens...

Thanks a lot for your help Phil!

P.

Trigger voltage is a Vbe (near 0.6V), but beware using voltage as you can zap it with too much current. To trigger at different parts of the AC waveform, one controls the timing of the trigger. There is no such thing as a "logic" triac.

"Robert Baer"

** Try reading a triac data sheet sometime.

... Phil