TRIAC Question - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: July 22, 2022, 02:22:36 am
content: I have a question about a TRIAC (datasheet attached).I'm measuring approximately 71 ohms between gate and A1 out of circuit. Measuring TRIACs is something I have little experience with, and, from looking online, I'm seeing more measurements between A1 and A2 rather than the gate.Should this be higher, and, more importantly, how would I know what the resistance should be by looking at the datasheet?

author: floobydust
author_rank: Super Contributor
timestamp: July 22, 2022, 03:22:28 am
content: I would say 71Ω is a fail unless you are using something older for a multimeter, what compliance voltage and current it is pushing on that ohms range?Try the measurement to MT2 as well. The gate is a double PN junction but some devices have added resistance MT1-G to lower dV/dt sensitivity and your is "snubberless" so I would expect some ohmic resistance- but 71Ω seems really too low.If you're not sure, you can test the triac with low voltage say 12VAC and a car light bulb.

author: bostonman
author_rank: Super Contributor
timestamp: July 22, 2022, 03:39:09 am
content: It's a Fluke 179.I'll try testing it, but all other pins are high impedance.

author: floobydust
author_rank: Super Contributor
timestamp: July 23, 2022, 05:36:36 pm
content: If you're finding the triac is not turning on, it further points to the gate being shorted. I think it's dead.Careful, I had some chinese triacs that failed at very light load, the gate sort of shorted yet the triac fired with really high gate drive but it smoked the potentiometer when I cranked it way up (dimmer). I'm saying you can get other circuit damage if the triac unexpectedly stays off.

author: bostonman
author_rank: Super Contributor
timestamp: July 23, 2022, 06:06:52 pm
content: The odd thing is the iron heats, so I’ve not looked at the TRIAC because I assumed it’s good. If you look at the thread I reference, you’ll see I’ve had an odd issue where the LED doesn’t blink ready. I’m thinking maybe the TRIAC isn’t conducting in one direction and the code isn’t satisfied with the iron taking too long or short to heat.Since I’m at the end of my rope, I took a last attempt to find anything, and found 75 ohms from gate to ground, so I removed a few components including the TRIAC and found it’s 75 ohms out of circuit.

author: bostonman
author_rank: Super Contributor
timestamp: July 24, 2022, 12:59:25 pm
content: I mixed up my threads and stated I referenced a thread in this one, however, I realized I didn't.Initially I had a TRIAC question that I posted, then posted a question about my broken soldering iron (the thread I'm going to reference and meant to reference), and then had another TRIAC question which is how this one started.Sorry for the mix up and I'm trying to avoid starting too many threads over one issue, but feel they are separate questions.https://www.eevblog.com/forum/repair/weller-soldering-iron-wes51/

author: floobydust
author_rank: Super Contributor
timestamp: July 25, 2022, 06:19:41 pm
content: It's Ok, I think I'm wrong anyhow with the triac ohmmeter readings. I measured some I have, ohms Gate-MT1 (low power ohms):BTA41-600B 102ΩBTA08-200B 80ΩMAC228 270-290ΩMAC223 70ΩT2500D 75ΩNTE5608 163ΩG-MT1 Diode test is anywhere from0.5-1.2V0.17-0.82V depending on the part and multimeter used.The WES-51 soldering station problem is almost crazymaking. Note it uses a capacitive-discharge technique to drive the triac, with no current-limiting resistor so the triac gets that 5V/2.2uF cap discharging into it and I thought it might stress the gate junction, the peak currents could be high.

author: bostonman
author_rank: Super Contributor
timestamp: July 26, 2022, 02:47:02 am
content: Damn, then the possibility exists measuring 75 ohms doesn't mean mine is bad.Why does a TRIAC measure different gate to MT1 versus gate to MT2?If 75 ohms does mean this TRIAC is good, then I'm out of ideas for what can be wrong with my iron (remember, I already have a thread going on the iron - so I don't want to double post). I've measured everything and this was my last hope on something odd being wrong causing the LED to not blink ready.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 26, 2022, 03:53:51 am
content: G-MT1 is a back-to-back diode. See the diagram. As mentioned, there may be load resistance, shorting out these junctions to make it less sensitive = more immune to transients.MT2 should have high resistance to either remaining terminal.Tim

author: floobydust
author_rank: Super Contributor
timestamp: July 26, 2022, 04:31:19 am
content: Although Gate-MT1 is back to back diodes, it will not always read that using multimeter diode-test due to the low shunt resistance seen across the junction. It depends on the multimeter test current, for example I can get 0.170V or 0.430V for the diode drop, either polarity, on triacs with the <100R gate resistor, on two different multimeters.So the resistance boosts triac dV/dt capability but gets in the way of readings. MT1-MT2 is an open-circuit.For your soldering iron troubleshooting, when things don't make sense it means a bad assumption has been made or something usually obvious has been overlooked. I back up or start over and revisit the problem from scratch, a little more thorough. Checking the obvious and one step at a time.

author: bostonman
author_rank: Super Contributor
timestamp: July 27, 2022, 01:10:20 am
content: Interesting. I thought because the gate controls each diode that it should have the same resistance from gate to either diode since (I thought) it was two separate diodes with one single gate.QuoteI fully agree with you and usually take the approach of clearing all debris off my bench/table, removing jumper leads so I can untangle wires, etc... and start over. Unfortunately I've done this several times. If you look at the schematic (referencing the link), this solder station doesn't have much to it. My assumption is that I shorted two pins on the PIC and it got corrupt, however, I extracted the code and posted it (reference same link).Someone went through the code and believes the PIC didn't get damaged. They removed something that allowed me to burn into a new PIC (my thought was maybe I damaged an output and it couldn't drive a circuit). Even with a new PIC, the station still acted the same. I've even built a fake thermocouple to inject a small voltage allowing me to trick the station into thinking the thermocouple has reached a specific temperature (I have a pot to make adjustments) and still nothing (I'm guessing the PIC may have something in the code that detects it has "heated" far too quickly)From what I can tell, the iron heats fine, but doesn't blink the LED to show it's ready. Technically I can use the iron as is, but something must be wrong and maybe it means the iron can't reach a steady temperature.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 27, 2022, 03:13:34 am
content: Quote from: bostonman on July 27, 2022, 01:10:20 amYou mean like this?That "equivalent" always bothered me. It's kinda sorta true, but the two halves aren't designed symmetrically and turned around. Obviously, you'd only have a -6V or so range before the one explodes. Vkg is equivalent to Veb in BJTs. There's something missing here.A more-correct diagram would show the right one as an upside-down PUT, or maybe a complementary one I guess. That is, the gate to the top of the triangle. And now the blocking voltage makes a little sense.But it's more than that; the charge carriers from the one side, diffuse over to the other as well, and trigger the whole thing. Hence 4 quadrant operation (usually with one of them performing poorly, but still possible).BTW, the correct way to wire SCRs together like this, if you should happen to need to (large TRIACs I think are less common than SCRs?), is to put a reverse shunt diode (K to G) rated for gate current, then a photoTRIAC between gates (and the specified limiting resistor, of course!). It always turns on whichever one is ready, by sinking current through one diode into the opposite gate, and the other one is always ready to block (with a whole cycle's worth of recovery time, not just the zero crossing recovery time for a TRIAC -- should be more reliable for weird loads).Anyway, the trick with TRIACs is, the upper layers are both very wide (epitaxial?), so they block voltage either way. But given some charge injection from below, the whole stack becomes conductive, and more emission (of electrons and holes) results, hence positive feedback and it latches on. There's no forced / reverse recovery as for SCRs, diodes, and BJTs at turn-off (storage): TRIACs can only turn off by waiting for recombination, which takes 10s of µs, maybe even 100. Hence why they're basically only useful for mains frequency stuff.Tim

author: bostonman
author_rank: Super Contributor
timestamp: July 28, 2022, 02:20:10 am
content: Wow, it amazes me how little I know when I get these explanations.Unfortunately it looks like I'm out a Weller solder station (about what... $600?) over something as simple like the LED not blinking ready.I was so happy to see 75 ohms on the gate to MT1 and thought maybe it wasn't conducting properly in one direction.If anything, you saved me from wasting money ordering one from DigiKey (or Mouser) as I planned to replace other components too like some capacitors.