TRIAC surprise - Page 1
Forum section: electronics / projects

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 24, 2024, 02:14:28 pm
content: Hi, friends!I am trying to make a simple TRIAC control of a 3-phase load with reverse.Shortened scheme:There was a problem with the TRIAC opening spontaneously at the time of power on.I know, but not very well, the vagaries of TRIAC in theory. But I can't figure out what the problem is.At some time, a short circuit occurs when switching on. If it was possible to turn on without a short circuit, everything works steadily.For verification, I have deattach cross triacs.For direct TRIACs, the control pins are raised from the board and a locking resistor is connected to them only. There should be no interference with the control pin.At the moment of switching on, there is some kind of transition process that gives a brief opening of the TRIAC.The waveform one phase to neutral:What could it be and how to deal with it? Please help me.4 Quadrant Triac are used:https://datasheet.lcsc.com/lcsc/2105241606_KY-BT137S-800E_C2831693.pdfFull schematic:

author: KerimF
author_rank: Frequent Contributor
timestamp: January 24, 2024, 04:46:53 pm
content: Decreasing the resistance between G and T1 increases the triac limit of dv/dt though it increases the driving current which in turn increases the phase at which the triac is on (after zero crossing).On your schematic, I read 620. If it means 620R (620 Ohm), it is relatively high (it has no significant effect).The internal G-T1 is somehow like two reversed diodes connected in parallel (this can be checked using a multimeter).So, the excess driving current for the 620R is 1/0.620 (assuming Vd=1) ~=1.6 mA onlyA value of 33R (instead of 620R) may be needed here. The excess driving current for the 33R is 1/0.033 ~= 30mA. If the delayed phase is acceptable in your application, MOC can deliver this added triggering current.For instance, the resistance, 620R, between a phase and MOC could also be reduced a bit if necessary (lower limit if 240Vrms is 450R with a margin around 25%).One more point, here R8 is connected to U4 and U5. I personally prefer driving a MOC couple, as U4 & U5 for example on your schematic, by two separate resistors instead of one.Hope this helps.Kerim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 24, 2024, 06:06:20 pm
content: Triac's have parasitic parts too.A part of those is capacitive coupling between the gate, and the other terminals.As a result, fast voltage transients over the other terminals can couple enough current into the gate to trigger the triac.Common remedies are:1. Use a less sensitive triac.2. Add a snubber.3. Use a "snubberless" triack (High dV/dt), low capacitive coupling).4. Short the gate in an attempt to re-direct this current (I'm not sure if this works).

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 25, 2024, 09:05:21 am
content: Quote from: KerimF on January 24, 2024, 04:46:53 pmThanks for the help!I'm radically reduced the locking resistor to 22 Ohms. The situation has improved, but, nevertheless, a short circuit sometimes occurs when switching on. Very rarely, but it appears.Maybe I chose the wrong TRIAC for such purposes? I'm thinking of replacing it with the STM T1635-800G. Or use MOSFETs.Quote from: KerimF on January 24, 2024, 04:46:53 pmU4 and U5 will never turn on together, I think one resistor is enough for them.

author: strawberry
author_rank: Super Contributor
timestamp: January 25, 2024, 10:47:32 am
content: back in the day ,I put 600..800V MKP10 capacitor across 240VAC . After week or so it went shorted.input line filter or snubber could help maybe reduce fast/HF signals across triac or/and loadfor 3 phase 240V systems usually use 1.2kV deviceshttps://en.wikipedia.org/wiki/Rectifier~560Vpk

author: jonpaul
author_rank: Super Contributor
timestamp: January 26, 2024, 10:50:06 am
content: inductive load transients and arcs in loadUse a properly rated snubber.I would start with 5..10 nF 1000V VDE X in series with flameproof 100 Ohm across the triacsj

author: Kean
author_rank: Unknown
timestamp: January 26, 2024, 02:16:38 pm
content: Yep, add a snubber across each triac. For guidance, see ST Micro AN437 or Onsemi AN1048/D.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 27, 2024, 02:37:29 pm
content: Quote from: Kean on January 26, 2024, 02:16:38 pmAn interesting circuit design in an1048-d.pdf (see arrows)

author: Kean
author_rank: Unknown
timestamp: January 27, 2024, 04:57:25 pm
content: Quote from: S. Petrukhin on January 27, 2024, 02:37:29 pmInteresting!