Can a TRIAC vary AC voltage?

[max666]

Note that in this case you loose the possibility to do phase control.

Phase controlling a heating element is a very bad idea anyway, at this high currents you'll create huge amounts of EMF and without filters it will all bleed back into the mains.
And it's completely useless anyway, since the heating element is slow in the order of seconds, switching at phase level is ridiculous.

[miceuz]

Note that in this case you loose the possibility to do phase control.

Phase controlling a heating element is a very bad idea anyway, at this high currents you'll create huge amounts of EMF and without filters it will all bleed back into the mains.
And it's completely useless anyway, since the heating element is slow in the order of seconds, switching at phase level is ridiculous.

I agree, but sometimes it might make sense if the heater element is more capable than your mains circuit breaker.

[ConKbot]

Note that in this case you loose the possibility to do phase control.

Phase controlling a heating element is a very bad idea anyway, at this high currents you'll create huge amounts of EMF and without filters it will all bleed back into the mains.
And it's completely useless anyway, since the heating element is slow in the order of seconds, switching at phase level is ridiculous.

I agree, but sometimes it might make sense if the heater element is more capable than your mains circuit breaker.

Use zero crossing switching for turn on and turn off, no harsh EMI from turn-on mid cycle, and you can still control the element on time with 8ms resolution.  If  youre popping breakers in under a single cycle, then getting it working on a 15A supply might be a bit on the challenging side.

[scott216]

I'm looking at the MOC3031 and MOC3032 zero crossing optocouplers.  I have a question about the gate trigger current (Igt).  For the 3031 it's 15mA and for the 3032 it's 10mA.  The only gate I know of is internal to the part and is used by the internal zero-crossing circuit.  So, should I care about what the gate current is if I'm driving a BTA26-600B triac?

[David_AVD]

I'm looking at the MOC3031 and MOC3032 zero crossing optocouplers.  I have a question about the gate trigger current (Igt).  For the 3031 it's 15mA and for the 3032 it's 10mA.

I can't see those figures (Igt) in the data sheet I have.  Maybe link to the sheet you're reading?

[hamster_nz]

I tired this setup and the toaster too 10 minutes to get to 225C, so I'm going to revert to original wiring and see if I can get the triacs to control the amps to a reasonable amount.

Wow! Is that a really big oven, or are you sensing the temperature wrong? My el-cheapo 1.5KW oven takes under 5 minutes (http://www.thewarehouse.co.nz/red/catalog/product/Necessities-Toaster-Oven-White-9L?SKU=1689799). No wiring changes were needed - just a SSR on an extension lead.

Mike

[scott216]

Wow! Is that a really big oven, or are you sensing the temperature wrong? My el-cheapo 1.5KW oven takes under 5 minutes

The reason it took 10 minutes is I rewired the heating elements so some were in series instead of all parallel.  The result was they elements didn't get as hot, but this didn't work out.  So I wired them back, but I can't have the top and bottom elements on at the same time, this draws about 30 amps (120 VAC).  My solution will be to just alternate between the top and bottom heating elements.  I think I'll switch once a second or something like that.

[scott216]

I'm looking at the MOC3031 and MOC3032 zero crossing optocouplers.  I have a question about the gate trigger current (Igt).  For the 3031 it's 15mA and for the 3032 it's 10mA.

I can't see those figures (Igt) in the data sheet I have.  Maybe link to the sheet you're reading?

I think I figured it out.  On Digikey's site, they call it Igt (see here).  But on the data sheet they call it I_FT which is the LED trigger current.  It seems to just be an error with Digikey.

[David_AVD]

I'm looking at the MOC3031 and MOC3032 zero crossing optocouplers.  I have a question about the gate trigger current (Igt).  For the 3031 it's 15mA and for the 3032 it's 10mA.

I can't see those figures (Igt) in the data sheet I have.  Maybe link to the sheet you're reading?

I think I figured it out.  On Digikey's site, they call it Igt (see here).  But on the data sheet they call it I_FT which is the LED trigger current.  It seems to just be an error with Digikey.

That's why it pays to read the actual data sheet and not rely too heavily on the supplier's page.   

[scott216]

I'm still struggling with this.  I got a MOC3032M zero-crossing opto-coupler and wired it up as shown in the attached schematic.  If I enable the Arduino output, my heater comes on, but I'm still getting too much current flowing into the triac's gate.  I measured about 220mA. I only kept the heater on for a couple seconds at a time.  The resistors between the opto-coupler and triac get warm pretty quick.  The reason I have 3 resistors like this is I just had these laying around.  I thought the opto-coupler would just turn on for short pulses when the voltage was zero and thus there wouldn't be much current flowing into the gate.  That doesn't seem to be what's happening. What am I doing wrong?

[David_AVD]

Here's something I built a while back.  Take note of where the opto output is wired to.  Hope that helps.

[scott216]

What is R6 doing in the circuit?

[David_AVD]

What is R6 doing in the circuit?

R6 reduces the gate's sensitivity to leakage in the opto and stray crap that may otherwise turn it on unexpectedly.

[scott216]

Thanks.  I'll add that to my circuit and see if it helps.

[David_AVD]

Go back and see how my circuit is different to yours.  There is one important difference to do with the opto output top connection.

[scott216]

Well, you have R5 on pin 6, where I have it on pin 4.  I guess in my circuit it doesn't matter, but when you add R6 it changes things a bit.  These resistors just limit the current going through the opto, so I don't think it matters if it's on pin 6 or pin 4, but maybe I'm wrong about that. 
The other difference is I have pin 6 connected to neutral, you've got it going to the MT2 (via R5).  So if you had the load connected to pin 1 on the connector and 120 hot to pin 2, then there would be a voltage drop across the load, so the voltage seen by pin 6 would be less in your circuit.  But wouldn't you basically have a voltage divider, something like: Neutral---Load---R5---120 VAC.  The resistance of the load is going to be pretty small (heating coils in my case), so I can see how with your circuit the voltage going to the opto is less, but it doesn't seem to me it would be a huge difference.  You also have the snubber circuit, but I have a resistive load, so I don't think I need that.   So, I see some differences, but I don't really understand to root cause of my problem - though I'm trying hard too.

[David_AVD]

The circuit I posted works.  Please try it and see how you get on.  Leave the snubber off if you like.

[scott216]

I'm going to give it a try tomorrow when I'm back at my workbench.

[scott216]

The circuit I posted works.  Please try it and see how you get on.  Leave the snubber off if you like.

I tried your circuit (without snubber) and it works.  Thanks!!  The triac gate was only drawing a couple mA.  I also tried it without R6 and it still worked.  And I tried putting R5 on pin 4 where I originally had it and it still worked. So the real difference is how pin 4 is wired.  I attached a simplified schematic of the working and non-working circuits.  In my non-working circuit, I'm assuming the problem is that the zero-crossover circuit isn't triggering at the right time, thus letting too much current and voltage through and into the triac gate.  But I don't understand why.

[David_AVD]

My circuit followed the exampled in various application notes.  Google "MOC3021 application note" and you'll get tons of hits.  They explain the operation in there.