Pulse transformer or optocoupler to drive SCR? - Page 1
Forum section: electronics / projects

author: shamooooot
author_rank: Regular Contributor
timestamp: November 29, 2022, 07:25:10 am
content: HelloAfter many attempts and after collecting information from wonderful forum members, about driving a 3-phase full wave controlled rectifier, with ability to control phase angle (Alpha):https://www.eevblog.com/forum/projects/would-this-signal-be-sufficient-to-control-thyristors-gate/I am now in the process of redesigning the board and I again need some help..- The pulse transformer I used in the old design (77202C)https://www.mouser.com/datasheet/2/281/kmp_772-41255.pdfis hard to find, expensive and just adds more complexity and troubles to the design.. I was wondering if I can replace it with an optocoupler.- I am planning to again use TCA785 phase control IC which is obsolete and I have to source from other sources, what would you guys recommend as a good alternative for it?

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2022, 01:21:05 pm
content: A TRIAC output opto-coupler, such as the MOC3071?https://www.mouser.com/datasheet/2/149/MOC3071M-1082601.pdf

author: RoGeorge
author_rank: Super Contributor
timestamp: November 29, 2022, 01:38:28 pm
content: The optocoupler diac, like the ones from the MOC series, can have zero crossing detection embedded in the chip, if needed. I like the the optocouplers more than the transformers.

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2022, 01:50:48 pm
content: Quote from: RoGeorge on November 29, 2022, 01:38:28 pmSome do, others don't. The MOC3071 doesn't.If the description says, random phase, that means it doesn't have zero crossing detection.Whether the original poster requires it or not, is another matter.

author: jonpaul
author_rank: Super Contributor
timestamp: November 29, 2022, 03:07:42 pm
content: those transformers are common, made in millionsCoilcraft, pulse engineering, Renco, Halo all make themJon

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 29, 2022, 03:26:23 pm
content: If you really want, per your o/p, a 3 phase full bridge with scr phase control, gates will need either 6 transformers or 3 isolated negative side drivers, and 3 drivers with common supply on the positive side.Use of SCRs for this in your rating range are long obsolete.Can you consider using IGBT or Mosfets?That change will allow use of a single IC to drive all 6 devices.It is done by using floating bootstrap capacitors to store charge obtained from VCC when the (+) side devices conduct. Transformers and optocouplers are not requiredHere is Infineon selector guide:https://www.infineon.com/cms/en/product/power/gate-driver-ics/6ed2230s12t/#!documentsPage 4 of this datasheet has a good block diagram:6ED2230S12TIf you have some reason to use scrs,( historical or project, don't worry, I still fiddle with tubes) I suggest you read one of the old scr manuals published by GE and IR.I read you previous thread and the Teccor S8006 data sheets. Designing an scr gate driver even a small one has some reqirements not covered in that data sheet. The gate current initially has a fast risetime to get switch on, then a back porch to hold it on during commutation. The gate driver depends a bit on the snubbers, and type of load on the rectifier. The driver logic has to double pulse the scrs when phased back below 120 degrees because the scr current becomes non continuous.These problems mostly solved if you can select Mosfets with a monolithic driver for the bridge.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 29, 2022, 03:30:19 pm
content: Transformers are generally less fuss and more robust.

author: jonpaul
author_rank: Super Contributor
timestamp: November 29, 2022, 08:13:36 pm
content: check opto safety rating, and pin spacing.Depending on the mains voltage and transients a 1500...2500 isolation may be required for complice.Most pulse transformers are specified for such useJon