On Fri, 26 Dec 2003, Spehro Pefhany wrote:
> At 11:42 AM 12/26/2003 +0000, you wrote:
> >TR2's data sheet says it needs a max 50mA of gate current to turn on,
>=20
> Unless you are operating in the pesky quadrant IV (in which case, 100mA=
),
> which you are not.

I believe the moc3041 has a built-in zero-crossing detector, so I guess=20
that means it can only turn on at the beginning of the 1st quadrant and=20
the end of the 2nd?


> Peak voltage is 1.41 * 240 =3D 340V, so current is 61mA at the peak
> of the AC line before the load turns on.

Errmm, what's the 1.41 all about then?

=20
> Note: You really should be using more like 100R than 5K6. You want it t=
o
> turn on near the beginning of the AC cycle, not some poorly determined
> time around 1/4-1/2 way through (getting earlier as the thyristor heats
> up), right? Note that the 100R will BURN UP if the triac does not turn
> on so you need to use a flameproof type for safety. This will also
> take out the MOC, most likely.

The max forward current of the output photo-triac is rated at 1A, so=20
surely if I use a 100R resistor in there it'll get 3.4A (using your peak=20
voltage above)? And if the gate current is spec'd as a max of 50mA at the=
=20
triac, wouldn't it break too?


> >Any ideas of what I'm doing wrong?
>=20
> I suspect you have MT1 and MT2 swapped.

I was under the impression it didn't matter which way round they were? I=20
didn't see it mentioned in the data sheet?

=20
> Also, how did you calculate the 200R? You need to guarantee 15mA (@25=B0=
C,
> MORE at lower temperatures) through the LED for this part to be guarant=
eed
> to work.

It was an incorrect, hasty decision :) ... I did 5v/200R (what I could
find in my junk box at the time) and got the 25mA, but now I realise that
the double-diode drop would make it more like 2/200, so there's only 10mA
flowing. But there's still 240vAC at the triac's gate, and I didn't see=20
any mention of coupling-transfer ratio's like in normal opto-isolators, s=
o=20
I assumed that if I can see voltage there, I'm getting all the current=20
too?

I'm starting to think I'll stick with relays instead. You know where you=20
are with a switch.

What sort of application would an optically driven triac be better than a=
=20
relay in? I can think of dimmer circuits and motor-speed controllers, but=
=20
that's all.

Thanks for the input, Spehro.

Cheers,

Tim

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