Talk:TRIAC

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i'm pretty sure this article is misleading if not downright wrong

If a triac was really equivilent to two thyristors back to back then it wouldn't matter which way round you connected the two main terminals but it DOES matter. The gate voltage must be relative to the correct main terminal or the cuircuit doesn't work Plugwash 21:46, 8 May 2005 (UTC)

You're right; the SCR analogy isn't precise (but it's close enough that it's often used.)

Actually, unlike our SCRs, it doesn't matter a huge amount which way you connect the main terminals of a TRIAC. Imagine, if you will, a graph of gate voltage versus (I think) MT2 voltage. With sufficient voltage on the gate, the TRIAC will trigger in *ANY* of those four quadrants. Yes, it triggers more readily in Quadrants I and III (matching polarity on the Gate and MT2), but it will trigger in Quadrants II and IV as well. The real place the "two SCRs" analogy falls down is on turn-off. Two SCRs connected in inverse-parallel will reliably turn off for nearly any load impedance (because each SCR has an entire half-cycle to turn off), but a TRIAC has a very hard time turning off with certain loads; IIRC, inductive loads and the resulting dV/dT will keep the unitary TRIAC device in conduction right through the zero-crossing, requiring use of a snubber to assure turn-off.

Atlant 00:27, 9 May 2005 (UTC)

well i remeber building a simple triac cuircuit and getting the triac the wrong way round stopped it working. if it were the equivilent of two devices in inverse parallell with the gates connected it would make no difference which way round i connected the two MT terminals in the cuircuit Plugwash 01:23, 9 May 2005 (UTC)

Here's the first article I Googled-up with the query "Quadrant-III triggering":

http://www.du.edu/~etuttle/electron/elect5.htm

The triggering with MT2 positive, as in the figure, is called Quadrant I, while with the supply reversed, it is called Quadrant III. A triac is most sensitive in these two quadrants, but will also respond to gate currents in the other directions. Many modern triacs seem to be symmetrical with respect to MT1 and MT2. To study triggering in Quadrants II and IV, simply interchange the connections of MT1 and MT2.

There were plenty more.

You probably got caught by an older TRIAC that wasn't very symmetrical and your trigger simply wasn't providing enough "oomph" to do the job in the "even" quadrants.

Atlant 17:14, 9 May 2005 (UTC)

Contents 1 inquries 2 Photo 3 opto coupler for triac 4 12v motor control bi directional

[edit] inquries

How to check a triac using multimeter with out current. —The preceding unsigned comment was added by 219.92.48.36 (talk • contribs) .

With the exception of checking for shorts, I don't think you can check a TRIAC using just an ohmmeter. I'm pretty sure the ohmmeter should see all three terminals as open circuits to the other terminals for both polarities.

But it would be pretty easy to use a battery-operated test light and a small resistor to trigger the gate.

Otherwise, unless you have a curve tracer, then observing the TRIAC in the actual circuit using an oscilloscope would be the best test.

Atlant 19:47, 30 April 2006 (UTC)

[edit] Photo

I think I have a spare TRIAC in a parts bin in the theatre I work for, but I am not sure. I will take a photo of it and post it here and if it is a TRIAC then it could be used in the article. -JWGreen 21:40, 25 September 2006 (UTC)

TRIACs come in all sorts of packages, so this would hardly be representative, especially if it is without adequate identification! DFH 13:44, 26 September 2006 (UTC)

[edit] opto coupler for triac

to trigger the triac on AC , an ac current is required. can opto couplers be biased to enable this and provide ac isolation at the same time or is it best to use a triac which incorporates the opto isolation? 216.249.8.77 19:02, 30 October 2006 (UTC)30 Oct o6

As mentioned above, some TRIACs are perfectly happy being triggered by a pulse "of the wrong polarity". Others aren't. For those that aren't happy being so-triggered, pulse transformers driven by bridge circuits provide one alternative as does triggering the main TRIAC with a smaller opto-TRIAC. Other possibilities also exist (limited only by the creativity of the circuit designer).

Atlant 19:44, 30 October 2006 (UTC)

[edit] 12v motor control bi directional

Looking for switching for 12vdc motor, must 200amp spikes motor has. rev mot leads to switch direction, currently use automotive type starter solenoids failure rate is high, because of load. 204.61.205.150 20:02, 10 November 2006 (UTC) brian