HELP with Super triac made with 2 SCR(BT151-500) - Page 1
Forum section: electronics / beginners

author: Ele
author_rank: Newbie
timestamp: July 02, 2023, 02:28:23 am
content: I want to make a super triac with 2 scr(BT151-500) in antiparallel, my question is if connecting both gates to the optotriac (moc3041), would it work well?Should I add something else? I read out there that resistors and also diodes, but it was not clear to me.PS: I'm thinking of joining both scr with jumpers (wire 2.5mm).Edit: Errors in the schematic and I corrected it in the last image (D2)

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 02, 2023, 02:38:21 am
content: I'm pretty sure you need to place your SCRs in parallel. It looks like you've got them wired in series? I assume U2 and U4 are the SCRs?

author: Ele
author_rank: Newbie
timestamp: July 02, 2023, 02:56:12 am
content: Thanks for your answer, as I did before going to sleep I thought I had done something, and the schematic is wrong, the u4 leg should go after the fuse, however the pcb is fineu2 and u4 are the scr

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 02, 2023, 03:07:56 am
content: While I'm aware that they show a TRIAC as two SCRs connected back to back with their gates tied together, I don't think that really works.Did a quick search and found this circuit which makes more sense to me:(The circuit is also in theOPTO datasheet.)

author: Ele
author_rank: Newbie
timestamp: July 02, 2023, 03:27:54 am
content: It also makes sense to me, although I don't understand the reason for the diodes, I'll focus on back to back scrthanks for your contribution

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 02, 2023, 03:54:47 am
content: The diodes prevent the gate on the SCRs from being reversed biased, but more importantly, supply a current path for the alternate SCR's gate.ie: For example, on the positive cycle ("115VAC" is positive and "LOAD" side is negative) when the MOC303x fires, current flows through D1, the MOC, the 180R, and triggers the right side SCR.On the negative cycle ("115VAC" is negative and "LOAD" side is positive) when the MOC303x fires, current flows through D2, the 180R, the MOC, and triggers the left side SCR.While R1 and R2 do provide a current path, the diodes allow a higher trigger current at a lower AC input voltage.

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 02, 2023, 03:58:20 am
content: Lots of confusing terminology usage here....Back-to-backis two devices connected inanti-series, each facing different directionsAnti-parallelis two devices connectedside-to-side, each facing different directions

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 02, 2023, 04:32:25 am
content: Quote from: ArdWar on July 02, 2023, 03:58:20 amYup... It's why I prefer to post a schematic. Looked at page 8 on the Fairchild datasheet that I posted once more. They covered all their bases:

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 02, 2023, 05:11:01 pm
content: Whenever I got a thyristor question, I refer to the Thyristor “Bible”: General Electric’s SCR Manual, 5th edition.Here it shows the exact same arrangement, but with a reed relay instead of the photo coupler.