trouble with a triac - Page 1
Forum section: electronics / beginners

author: 1makermold
author_rank: Contributor
timestamp: April 17, 2019, 12:31:21 am
content: I am having a problem with a triac BAT16, when I trigger it with an optocoulpler it turns on but the voltage is half of what it should be, Im trying to make a switch for 12 to 24 vac, I have tried a Q4006l triac but still have the same results

author: soldar
author_rank: Super Contributor
timestamp: April 17, 2019, 10:12:58 am
content: I do not understand what you are trying to do. Can you post a schematic?

author: 1makermold
author_rank: Contributor
timestamp: April 17, 2019, 12:30:33 pm
content: Here is the schematic, I figured it out last night I changed the 150 ohm resistor to 100 ohms works good now, thanks,but I was wondering if I could trigger the gate with a transistor other than a optocoupler, or would it latch and just stay on

author: soldar
author_rank: Super Contributor
timestamp: April 17, 2019, 12:55:55 pm
content: The text of your OP makes no sense with that circuit.You cannot use a bipolar transistor to control the triac because the switching voltage and current reverse direction.

author: 1makermold
author_rank: Contributor
timestamp: April 17, 2019, 01:05:57 pm
content: I know, on the schematic it says 240 volts but I'm only using 24 vac, thanks

author: soldar
author_rank: Super Contributor
timestamp: April 17, 2019, 01:11:16 pm
content: What do you mean by "but the voltage is half of what it should be"?

author: 1makermold
author_rank: Contributor
timestamp: April 17, 2019, 01:15:02 pm
content: when I would fire the triac across the 24 vac, I would measure it and it would read 12 vac, I think it was triggering on both half cycles,

author: soldar
author_rank: Super Contributor
timestamp: April 17, 2019, 01:45:25 pm
content: What is your intended load?

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2019, 01:50:21 pm
content: Quote from: 1makermold on April 17, 2019, 01:15:02 pmWhat's the opto-coupler's part number? Does it have zero crossing?If so, then I suspect that could be the problem, rather than the TRIAC. Opto-couplers with zero crossing won't fire, until the voltage is above around 30V, which is close to the peak voltage of 24VAC and would explain why you're measuring half that. Replace it with an opto-coupler which doesn't have a zero-crossing detector and if you need zero-crossing, do it separately.

author: 1makermold
author_rank: Contributor
timestamp: April 17, 2019, 02:03:16 pm
content: The optpcoupler is a moc3023, the voltage that I am looking for is 22-23 vac, I do relize that there will be a voltage drop going through the triac

author: kripton2035
author_rank: Super Contributor
timestamp: April 17, 2019, 02:11:15 pm
content: the optocoupler must be between A2 and G of the triac for the triac to operate "normally"

author: soldar
author_rank: Super Contributor
timestamp: April 17, 2019, 02:21:44 pm
content: Quote from: Zero999 on April 17, 2019, 01:50:21 pmGood thinking! I had not thought of that. But a quick look suggests the diac is indeed zero crossing. Maybe someone con confirm. A 'scope would confirm it easily.

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2019, 02:32:54 pm
content: Quote from: 1makermold on April 17, 2019, 02:03:16 pmThe Q4006l has a built-in DIAC so would be no good either, as it won't trigger until around 30V.The BAT16 and MOC3023 should be good though.How are you triggering the opto-coupler? Is the circuit isolated from the 24VAC supply?A good alternative to an opto-coupler is a pulse transformer, but it's more difficult to use.

author: 1makermold
author_rank: Contributor
timestamp: April 17, 2019, 02:38:14 pm
content: I am triggering the circuit from an isolated source

author: soldar
author_rank: Super Contributor
timestamp: April 17, 2019, 02:51:48 pm
content: Quote from: Zero999 on April 17, 2019, 02:32:54 pmI cannot see any reason to not use an opto-isolator but if there was a strong reason then a plain old relay would work. A reed relay can be tiny and do the job.

author: 1makermold
author_rank: Contributor
timestamp: April 17, 2019, 03:07:42 pm
content: I was thinking about the same thing, but I need the circuit go into a small space, and I don't thing a small reed relay could handle 5-6 amps, plus to expensive

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2019, 03:23:59 pm
content: I think the idea is to use the reed relay to trigger the TRIAC, not drive the load directly.Lots of reed relays aren't isolated enough to protect against electric shock, but that's a non-issue here, assuming the 24V or trigger circuit aren't floating at hazardous voltages i.e. are both isolated from the mains.I think an opto-coupler and TRIAC is a good idea. The only alternative, other than a relay, is a MOSFET solid state relay, consisting of two back-to-back MOSFETs and a photovoltaic opto-coupler, such as the VOM1271, but it's more expensive.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 17, 2019, 04:28:16 pm
content: Triac's are not symmetrical devices.Most can be triggered in 4 quadrants, but one of the quadrants is always worse then the other three, and you are using it in the "bad" quadrant.Combined with your low output voltage It is likely that your triac only is triggered in one direction, and this explains why you measure half the voltage.Maybe you can get it to work with lowerint ge value of the resistor, so the triac has more gate current. You could also make a tap of the 24V before the load, and use a diode and capacitor to buffer it to generate a voltage to trigger the triac. If you build this circuit in such a way that it pulls current out of the gate instead of pushing it in, you also avoid the "bad" quadrant.

author: 1makermold
author_rank: Contributor
timestamp: April 17, 2019, 04:34:02 pm
content: I did lower that resistor, but Im not sure how much lower I should go