> On 8 Aug 2003 at 9:08, zantos wrote:
>
>>I too thought the zener and the second diode should be reversed but
>>found the efficiency was better as I have it. My great problem is
>>turning the triac on and off. The triac I used is supposed to
>>interface directly to a micro (so the spec says). I will try and use a
>>opto and see if this improved matters.

Brent Brown wrote:
> If you stick to your original circuit you could loose the second
> 1N4001 as it doesn't do anything. Nothing really wrong with the
> circuit, just I would prefer not to add the extra "stress" on the
> zener of the neagative half cycle as well as the positive. 1N4001's
> are fairly rugged so help improve the safety aspect. Only advantage
> of your original circuit is that the junction of the zener and the
> first 1N4001 may be a good place to get a 100/120Hz reference pulse
> from (-0.7V to +5.1V) if you wanted one.
>
> As for why your TRIAC turns on with high or low on the gate, you need
> to consider the four modes of TRIAC triggering (back to the text
> books). From memory, (anyone feel free to correct me), there are 4
> "quadrants" of operation:-
>
> MT1 = +ve, MT2 = -ve, Ig = +ve
> MT1 = -ve, MT2 = +ve, Ig = +ve
> MT1 = +ve, MT2 = -ve, Ig = -ve
> MT1 = -ve, MT2 = +ve, Ig = -ve
>
> Any of the above can trigger the TRIAC into the ON state where it
> will stay until the current flow between MT1 and MT2 drops to zero
> (such as happens at the zero crossing point with a resistive load).
> The four modes are not symetrical in that some modes require a higher
> trigger current.
>
> So with your circuit, you have a problem. Logic high from the micro
> will turn the TRIAC on, as will logic low (which you have
> discovered). Best bet may be to use an optocoupled TRIAC instead
> (which might drive a small lamp directly anyway). Otherwise, and this
> is a bit of a guess, put a 1N4007 in series with the gate to stop
> negative gate current flowing back into your PIC. The 47k resistor
> should be way lower, say 470R like someone else suggested.

Yes, most triacs have 4 quadrants but some only have 2.
I didn't see the OP's circuit but as long as there's no
voltage between MT2 and the gate, the triac will be off.
I've driven 4 triacs directly from a 12F675 with no problems.
My applcation was 24Vac but that shouldn't make a difference.
Could you guess it was for Christmas lights? :-)
David...
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David Duffy        Audio Visual Devices P/L
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