I have a thyristor with the connection as in this thyristor (fig 1) and i am trying to use it as a traic. THyristor

Edit: I am using 3020 optoisolator and resistor value as shown in fig.2 Supply is 220V. Can I try this?connection

  • \$\begingroup\$ Show oscillograms. \$\endgroup\$ – winny Apr 6 '18 at 8:08
  • \$\begingroup\$ i haven't use oscilloscope since the bulb is glowing with out triggering a gate (As i mentioned phase indication is present in terminal 4). I expect no supply present in gate termainal 4. Is there something wrong in my connection? Edit: I have my connection as in my first picture. \$\endgroup\$ – Ryadiff Apr 6 '18 at 8:17
  • \$\begingroup\$ Can you fix the formatting and indentation on your code? \$\endgroup\$ – Transistor Apr 9 '18 at 18:06
  • \$\begingroup\$ Sorry, it was so hard to put the code in a new line. Finally i learned. I made a another question for this new problem. I hope for your help. thanks. \$\endgroup\$ – Ryadiff Apr 9 '18 at 18:37
  • \$\begingroup\$ @Transistor there was something from my above comment. I forgot to mention you here. \$\endgroup\$ – Ryadiff Apr 10 '18 at 1:49

I have seen those &&&$ diagrams in texts too, many years ago. You cannot make a triac by connecting two SCRs like that- think about it- how is the "top" one triggered? It's not an equivalent to the triac structure at all, unless the gates are ignored. See the gate structure below:

enter image description here

In your circuit, the gates will break down with reverse voltage (maybe -10V) then the opposite SCR turns on.

You can connect an opto-triac between the two gates (and diodes across each gate to cathode to conduct when reverse gate voltage is applied) and turn them on like that, or use a triple coil pulse transformer (two secondaries) to trigger both gates at once.


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ alternatively: use a bridge rectifier and use one thyristor to switch both half-cycles. \$\endgroup\$ – Curd Apr 6 '18 at 10:17
  • \$\begingroup\$ @Curd The 200A diodes might be more expensive but I like this idea simplicity on lower power SCR's. But coils are so easy to make/buy with 5kV insulation with direct ZCS detection pulse or phase controlled with analog 'HC14 delay. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 6 '18 at 12:49
  • \$\begingroup\$ @Curd Valid option, especially at relatively low currents, but the voltage drop (and therefore heat dissipation) is worse with the bridge +SCR and he already has the two SCRs he needs right in that one module. It will pass 235A RMS so you'd need a module with four 120A diodes for a matching bridge, and a double rated thyristor since paralleling them is not the best. \$\endgroup\$ – Spehro Pefhany Apr 6 '18 at 13:39
  • \$\begingroup\$ I have edited the post to include your suggestion. Can i use MOC3020 as an optocoupler as you are suggesting? Can i try this new connection? Thanks again. \$\endgroup\$ – Ryadiff Apr 7 '18 at 6:04
  • \$\begingroup\$ Yes, you can. A lower value of resistor is more appropriate- a few hundred ohms that has the ability to withstand a power surge. I would also use a bit beefier diodes eg. 1N5819 or 1N400x since the peak current could be (eg. for 200 ohms) 240*1.4/200 = 1.7A if the turn-on is unluckily right at the peak voltage. The resistor sees almost 600W briefly until the SCR turns on. \$\endgroup\$ – Spehro Pefhany Apr 7 '18 at 9:05

And yet another way to do it, which I recognized on the "cited site".

enter image description here


Around 1977 a professional rock musician buddy asked me if I could make a chaser. 2.4 kW per channel , bidirectional scan, 8 channel chaser with ZCS control, variable sequence speed , direction and drum beat sync sound activated. It took me a day on the weekend and I used discrete CMOS detected ZCS and drove the Triacs with hand wound mini coils. The little board was just wire-wrapped with 8x 25A triacs. It all worked fine until their tech decided to flash 1kW floods at the optimum rate so they drew 5kW pulses. Hey Tony, the darn Tech burnout the chaser... 5:1 repetitive surge currents.

  • \$\begingroup\$ Thanks. I cannot figure out the input of MOC3031 and will this value of resistor works for 220V supply. \$\endgroup\$ – Ryadiff Apr 7 '18 at 6:29

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