How to switch hot side AC with Triac, using DC control circuit?

[KL27x]

**Oops, my mistake. I thought I posted this to the beginner's section. Can a Mod move this for me? **
Short version:

AC, half wave rectified to supply a DC control circuit.
DC circuit has ground referenced to one side of the AC.
The circuit switches the ground referenced side of the AC to a load via a triac, by pulling the gate to -1V in respect to MT1, seemingly. When the triac is off, the load is no longer powered. But the load is still connected to the "hot" side of the AC (hence, if it is say the heater/thermocouple combo of a Hakko T12 tip, the DC voltage produced across the load is hard to measure while the triac is off?)

Is it possible to make the triac switch the "hot" side of the AC, rather than the ground referenced side? What is the easiest way to control this with 5V DC control circuit?

[KL27x]

So I have played around a bit.

I think it makes more sense for the ground referenced side of the AC to be switched, obviously, since this makes the DC control circuit easier to do. If I am connecting MT1/A1 to the hot side of the AC, I will have some issues to control it.

After desoldering and playing with the triac (off my hakko 888 board) it appears that a TRIAC is current-controlled, the gate essentially acting like a two way diode between gate and MT1/A1. And the triac turns on when the current between gate and MT1 reaches about 20mA. So if I am to switch the hot side, MT1/A1 is going to be fluctuating between +-40V in reference to ground... so in order to control it with DC control circuit, and be able to switch it on during any given zero crossing whether on the way up or down, I would need to... switch it with an SSR?

If this is totally boneheaded, someone please advise. Perhaps there is a faster way to switch it. But this is what I got so far. It seems kinda silly to have to use an SSR to switch a TRIAC, plus I have to make sure the pulse doesn't last very long, else I risk burning up the SSR. I figure I'll use a 400R resistor, small signal SSR, and cut the SSR hopefully before the potential is higher than 10 or 20V.

Incidentally, I notice that when switched with current flowing from gate to MT1, MT1 to MT2 has zero voltage drop, and MT2 to MT1 has 0.6V drop. But then switched with current flowing from MT1 to gate,  the voltage drop between MT2 and MT1 is zero, but there is only 0.2V drop between MT1 and MT2. So it is a little more efficient in one direction than the other. 

[KL27x]

Sidenote: The hakko 888 appears to drop some signals. Despite the sensor showing heater is too cold, the micro misses an occasional "on" pulse at the zero crossing. Presumably because the 7 segment routine and/or button input scanning takes precedence over the actual function of the iron, and an ISR to service the heater would take a whole couple dozen microseconds. 

[Kean]

Typically you'd use a triac driver opto-isolator - e.g. the MOC3043 to control the triac.  Check the datasheet for that part to see the hot-line switching schematic.  I imagine it will work with low voltage AC with some adjustment to component values, but there may be a more suitable part - it is just one I've used before.

[KL27x]

 Thanks @kean.
I shoulda known to search for triac driver, duh. I am very ignorant with AC. I have a hammer, and everthing looks like a nail.

I already ordered some fast SSR. On time of 0.13mS typical, 0.5mS maximum. With load condition of 50mA. This shoupd do the job, although if if i screw up the firmware i can burn up the part.

I will try a triac driver if i have issues. Im not sure how to compare slew rate of triav driver to on time listed for SSR, though.... i dont need the zero crossing detection, but it looks like they are cheaper. What i really want is speed, though. I want to do an adc reading at the zero crossing before deciding to turn on the triac, preferably. Without losing duty cycle or spiking emi.if i cant do it fast enough, i will have to drop a half period, here and there.

Also i wonder if zero crossing detector on the driver would have already passed by then, waiting til next cycle. I imagine it would trigger on the way to zero rather than exactly zero or on the way back up. I think i can trigger triac slightly after zero crossing without issue, with voltage still under 10V? Will see. It looks on scooe that the method the 888 board uses (very high imedance potentil of only about 1v!) Doesnt turn on the triac until pretty late. Maybe 10V from zero. I think i can maybe beat that, even. But my goal is to at least match it.

[KL27x]

Btw, i am trying to mod my 888 5o be able to use either 9901 or 2027 hand pieces. With the flip of a switch.

I have read hakko reads the thermocouple within the zero crossing. If they can do, i can do it. Eventually. Or maybe ill find out that is not even oossibkex and that the hakko 951 droos some half cycles? It would be nice if someone scoped their 951. Only thing i find when i google is video of luis rossman opening the case and knocking on the way a 951 knockoff performs without even testing it.

I've already finished the modification, in my head. I just have to put together the parts and write the code.

This makes me curious to see the inside of the 951, though. The input of the opamp also needs to be protected from the full heater AC voltage! I think I might try using a fast SSR for that, too, but I have another idea.

[DerekG]

I will try a triac driver if i have issues.

A triac will automatically turn off at each zero crossing all by itself.

Only use a zero crossing triac driver if you want to turn on the triac at the zero crossing point. These are generally only installed if you want to turn the triac on & off like a switch.

If you wish to pulse width modulate (PWM) the AC, then you do not want to use a zero crossing triac driver. Instead, perhaps consider using a regular opto-isolator to do control the PWM.

[KL27x]

^I DO want to turn on the triac only at (or only slightly after) the zero crossing, for EMI and for maximum output efficiency and responsiveness. Hopefully it is possible to do an ADC reading and a compare at every single zero crossing before even deciding whether the TRIAC is going to switch on that cycle or not. I don't think a zero crossing triac driver will do that without some signal conditioning to make it detect zero "late." I could even throw in some extra reads during cycles where the heater is off, but I think 120 reads per second will be fine. Basically, the zero crossing circuit I already have will sync my micro. And I can zone in on the exact spot where the TRIAC turns itself off. At exactly this time, an SSR will finish turning on to allow read of the thermocouple thru opamp and ADC pin, turn off opamp SSR, do a compare, and then conditionally switch the TRIAC with another SSR. That should work, I think, at least until the first aberrant cycle comes along and blows everything up.

PWM? Light dimmer style? There is no reason to chop up the sine wave any more than necessary to control a soldering iron. I want to switch the AC on like a switch, only at the zero crossing, but I don't think I can use a zero crossing TRIAC driver, at least not easily.

What I want is the lowest latency between deciding to switch the triac and getting the electrons flowing. So far, I think SSR has got a shot at the title. Seems like the only way to do it from 5VDC control circuit is to use optoisolation, whether or not it is SSR, 2x optocouplers, or triac driver. But I dunno.

The other way to cut out the opto part is to switch the ground referenced end of the AC and to read the uVDC on the AC sine wave? With floating ADC reference and +-50V maximum voltage rating? I don't think that exists?

[DerekG]

You might find this webpage quite interesting:

http://bestengineeringprojects.com/ac-motor-speed-controller-circuit-using-microcontroller/

[kripton2035]

use moc3041 for zero crossing.

[KL27x]

Is there any reason to use opto triac over opto FET or opto BJT to drive a 26VAC power triac? This isnt 100s of volts. I just want to switch it FAST.

I read somewhere that power triac is 5x faster than power SSR. But as far as small signal opto triac vs opto fet or bjt.... which ones are going to switch/drive the power triac the fastest? Or does it matter?( Im considering that perhaps the driving signal might come a hair later than zero crossing. ) I have 5vdc control rail and it takes about 20-30mA to make my power triac trigger, lets say.  Why I "should" use opto triac to drive triac if i already have 0.13mS SSR in the mail? I can maybe get optotriac more tolerant/robust at higher voltage for same speed? Or i can get faster switching speed? Please, if u tell me i should use x, maybe u can say why, too? Before I burn up $90.00 in parts?