Making a triac phase control circuit - are my values correct? - Page 1
Forum section: electronics / projects

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 03, 2020, 05:43:38 pm
content: Hi,I need to build triac phase control for industrial application. I base my circuit off an old one which I need to recreate to replace in other machines. The original features: KT728/800 triac (800V, Imax = 15A, Igmin = 100mA, Igmax = 5A), 100nF/630V cap, 100kohm pot, some rando diac.My new design:BTA16-800CWRG triac (800V, Imax = 16A, Igmax = 35mA)47Kohm 500V 2W pot6.8kohm R1 (in series with the pot to limit gate current to 35mA when pot is all the way shorted)100nF 630V cap -should i keep the original value or adjust it to the new resistor values and how?ER900 diacThis is my first real electronics project, I'm normally a hobbyist.Thanks for any help

author: floobydust
author_rank: Super Contributor
timestamp: March 03, 2020, 08:37:02 pm
content: What is your mains voltage and frequency? I think 0.1uF is a bit big and with 47k pot too small, it does not give enough delay.Roughly, you would like the potentiometer at 1/2 way to give a 1/8 cycle mains delay so the control has a good range. This is how I select the potentiometer and timing cap. About a 2-2.5msec delay to charge the timing cap to diac 32V.For 120VAC 60Hz I use 0.047uF 250k potentiometer, and 240VAC 50Hz 0.047uF 500k potentiometer. Spice simulations show DB3 diac gives >500mA peak Igt so lots.I add the extra diodes which discharge (reset) the timing capacitor every cycle and give better low end control. This is the circuit schematic that works for me.More can be found in figure AN1003.12 orTeccor App note AN1003

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 03, 2020, 08:56:47 pm
content: This circuit runs on 240ish 50Hz. It drives primary of a transformer. Thanks a lot for the schematics! I'll build it like you recommend. Can I use a low power bridge rectifier (like 100mA)?Before I put it in the machine, can I test it with like an incandescent bulb to see if it works properly? Also thanks for the delay info, I'll use that in the future!

author: floobydust
author_rank: Super Contributor
timestamp: March 03, 2020, 09:11:07 pm
content: That circuit was originally an eBay "4000W SCR" speed controller I bought, that did crappy and the pot smoked and the triac shorted so I fixed it.The 33k resistors need to be 1-2W, I did not try optimize those. I've never seen a 100mA bridge rectifier, but the diodes see maybe 10mA at peak mains voltage at least 350V.Yes use a light bulb to try it out. I was surprised a smaller timing capacitor 33-47nF did better.

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 04, 2020, 04:00:53 pm
content: So I got some more specific requirements. The circuit needs to be as simple and as compact as possible. The triac you recommend is massive overkill, the 16A one is enough (and costs third as much). I will also omit the FBR discharge circuit. I understand that it can significantly improve the performance, but we are talking +-1A control (it drives transformer that provides power to heat up hoses for polyurethane foam components) and the original schematic doesn't have it.I tried simulating the circuit but got this mess, any tips? I have been off electronics projects for 6 months, so most of my knowledge is a bit rusty, sorry about thatShould I adjust the resistors for the BTA16-800CWRG? I wanna use this triac since it's in stock at my local store for few pennies.Probably a noobie question, but why is there that complicated resistor network for the gate current? Why does there have to be another 470K resistor in parallel with the 470K pot? And the 3.6k resistor in series with that? Most schematics I've seen had only the pot.Thanks and sorry for the nooby questionsEdit: simulation errors solved, now it's working fine

author: floobydust
author_rank: Super Contributor
timestamp: March 04, 2020, 06:59:18 pm
content: You'll have to see how it works with the inductive load. No difference between the small and med size triacs because the phase-control is just a time delay which is the same for any triac, and the gate trigger pulse is enough.The chinese 2W WTH118 potentiometers have a low quality carbon track and the resistance (end-end) can be 20% out and they are not linear. The extra 470k resistor across the pot was to get rid of the dead spot, where nothing happens and then a sudden jump at say 10'o clock.The 3.6k I added to limit max. current through the potentiometer after one started sparking.

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 04, 2020, 08:26:16 pm
content: I am planning on using this pothttps://www.tme.eu/en/details/sp1.2-470k-a/carbon-single-turn-potentiometers/telpod/sp-1-2-470k-a-20p3/I'll build it like you have (just with the different triac).What heatsink do you recommend for the triac? Is small one (like a tiny TO220 one) enough or do I need a large one? The original control block had no heatsink at all.By the way, is it possible for a triac to die if the snubber network fails and it switches an inductive load?

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2020, 01:44:31 am
content: Quote from: ExtraThiccBoi on March 04, 2020, 08:26:16 pmYep, certainly.You said the phase control is for a transformer primary, what is the load on the secondary because this really matters ?

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 05, 2020, 06:13:53 am
content: You can see here. The secondary drives heaters.In one of our machines, this triac circuit keeps failing. I had no idea why could it keep failing, but now I think it's possible the snubber network failed. I think it's a separate component, like a giant blue dipped mica cap, so i could replace it or just trash it and use the snubber in the phase control design suggested by floobydust

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2020, 07:32:47 am
content: Quote from: ExtraThiccBoi on March 05, 2020, 06:13:53 amOK the secondary is a purely resistive load, much easier to protect the Triac.Maybe your snubber is a MOV, I dunno unless we identify it. Or you could replace it with the RC snubber from floobydust's circuit.One last question, how often is the mains supply switched and are there other transformers on it ?

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 05, 2020, 07:51:39 am
content: wdym switched? The 240V rail comes from a main 3 phase 400V transformer somewhere

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2020, 07:53:27 am
content: Quote from: ExtraThiccBoi on March 05, 2020, 07:51:39 amOK, so it's ON 24/365 ?

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 05, 2020, 08:02:29 am
content: nono, it's turned on when it's needed, like let's say works for 10hrs of foam spraying, then switched off

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2020, 08:16:30 am
content: Quote from: ExtraThiccBoi on March 05, 2020, 08:02:29 amOK, good. No further special requirements other than checking and/or replacing the snubber and installing a new Triac of very similar spec to the original.I don't particularly like these simple Triac control circuits but if everything is right them seem to give reasonable service whereas I prefer a Triac driver/phase control IC but for your case the additional component count may not be suitable as a replacement.

author: Zero999
author_rank: Super Contributor
timestamp: March 05, 2020, 09:54:32 am
content: Quote from: ExtraThiccBoi on March 05, 2020, 06:13:53 amIf all you're doing is controlling heaters, then it's quite likely you don't need phase control. Burst control will be much easier on the TRIAC, but the control circuitry is a bit more complex.

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 06, 2020, 09:52:32 am
content: I mean sure, but i need it to be as simple as possible and in operation next week.

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 08, 2020, 10:59:22 am
content: I should get the parts this week, so I'll update this post whether my house burned down or not

author: Zero999
author_rank: Super Contributor
timestamp: March 08, 2020, 03:02:24 pm
content: Quote from: ExtraThiccBoi on March 06, 2020, 09:52:32 amIt depends on what skills/parts you have at your disposal. Burst control can be implemented with a microcontroller, but if you have a PLC then it's just a programming exercise.

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 08, 2020, 04:10:23 pm
content: That is way above the budget, requirements and my skills :/

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 09, 2020, 05:54:25 pm
content: Quote from: floobydust on March 04, 2020, 06:59:18 pmSoo built the circuit, but I must have gotten something wrong because i got a bunch of flashes (with 75W bulb) and then the triac died. I didn't include the FBR, the two resistors and the 470k across my pot (also 470k). I used 3.3k resistor in series with the pot, 0.047u cap as you recommended. Idk what happenedThe circuit was wired according to the schematic. The triac turned into a 60 ohm resistor.I guess the gate current was too high when I turned the pot all the way?Edit: my parts tester (the cheap atmel based chinese one) reads both of my triacs (good one and bad one) as resistor between MT1 and G. Does that mean both of them are broken?

author: floobydust
author_rank: Super Contributor
timestamp: March 09, 2020, 08:20:24 pm
content: Well, you seem to have the curse of the triac.The whole circuit works on the premise that the triac turns on - if it doesn't, then the load gets power through the potentiometer/diac and it smokes.Check the triac pinout you don't have MT1 and MT2 flipped, and that you are not using counterfeit triacs. There are many dud triacs out of china that are fakes. You could make a test circuit keeping the 70W light bulb and just a resistor from gate to MT2.The only thing I could be wrong about is the delay/trigger cap is too small for low-sensitivity triacs and 0.1uF is better, but there is well over 50mA gate-trigger pulses from a 0.047uF at 32V. Check the diac should do nothing until around 32V when it turns on, if you have a shorted diac then there would be no gate current pulses.Between MT1 and G you should read a diode junction I think, no resistance.

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 09, 2020, 08:33:15 pm
content: Quote from: floobydust on March 09, 2020, 08:20:24 pmI thought MT1/MT2 can be flipped since the current runs both directions? Well anyways, I tried to do the math and me dumbass didn't take the resistance change after removing the 470k into account. This means the timing was kinda f'ed. I rebuilt it with the 470k, will check the mt1/mt2 and try tomorrow bulb test again. It's most likely not counterfeit. I buy my parts from a local store and never had an issue with fakes, they do look very real with all the respectable STM markings (both diacs and triacs are STM).

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 10, 2020, 07:18:22 pm
content: Quote from: floobydust on March 09, 2020, 08:20:24 pmSo yeah, the first triac is dead, coz i rewired the circuit and it didn't work. Ill build it with the second one. Hopefully it will work this time.

author: floobydust
author_rank: Super Contributor
timestamp: March 11, 2020, 10:15:37 am
content: If nothing happens at 1/2 way on the potentiometer, don't crank it up until you find out what is wrong.

author: ExtraThiccBoi
author_rank: Contributor
timestamp: March 11, 2020, 10:19:36 am
content: Learned the hard way :DD Yeah I always put it to max resistance before plugging it in.

author: Zero999
author_rank: Super Contributor
timestamp: March 11, 2020, 11:04:21 am
content: Quote from: ExtraThiccBoi on March 09, 2020, 08:33:15 pmNo, the gate drive always have to be relative to MT1. Most TRIACs will trigger on all four quadrants i.e. the gate current can be positive or negative, with respect to the voltage on M2, but not all quadrants have equal sensitivity. Some TRIACs will not trigger on all quadrants. Check the data sheet.