Help with understanding this SCR - Page 1
Forum section: electronics / beginners

author: jpadie
author_rank: Newbie
timestamp: June 02, 2022, 12:44:51 pm
content: HiI am testing a simple SCR circuit to understand how it works. I'm using a dual secondary toroidal transformer in a centre tap configuration (serial).At the moment I am connecting a +ve leg of the transformer to the anode of the SCR the cathode goes to a DC motor and then to the common terminal of the transformer. Attaching an oscilloscope probe to the Cathode and the grounding clip of the probe to the common of the transformer.below is a simple schematic.with the gate floating the motor does not turn. with the gate bridged to the anode there is motion. this is as expected.However I expected to see a +ve only curve on the oscilloscope when the gate is charged. However I am getting an approximate sine wave with a negative component.Ultimately i am trying to build a reversible salt water chlorinator. I've seen designs that use two pairs of SCRs with a centre tapped transformer and that struck me as an interesting idea.I'd very much appreciate some guidance in explaining why this negative component is present on the oscilloscope whereas it does not seem to impact the turning of the DC motor.Also any hints on using four SCRs with tapped/split supply to create a reversible voltage would be helpful. The couple of tries I've done have led to the release of magic smoke and chucking the SCRs.thanks!

author: ledtester
author_rank: Super Contributor
timestamp: June 03, 2022, 12:21:13 pm
content: Your schematic is not showing up -- it's just all black.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: June 03, 2022, 02:27:04 pm
content: A true black box.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 04, 2022, 11:46:20 am
content: The load needs to go between the transformer active/ live and the anode. If not, the gate is not referenced to the motor GND or circuit common. This will be a problem later on when you connect a gnd referenced drive circuit. You have to considier carefully when and how SCR commutation will be controlled. Or lots of magic smoke will occur.Floating the gate will cause erratic results. A full brige system is required for reversing. You will need isolated gate drive- trigger transformers/ optos. eg MOC3081D1 and D2 can be SCRS if you want speed control.A relay or two with or without speed a controller may be simpler solution.SCRs are fun to play with but tricky to apply without major design effort.

author: jpadie
author_rank: Newbie
timestamp: June 04, 2022, 12:04:04 pm
content: that's annoying.hopefully this one is better

author: jpadie
author_rank: Newbie
timestamp: June 04, 2022, 12:30:42 pm
content: thanks Terry but I don't think that is the way they are implemented in salt water chlorine generators.the thyristors are used as rectifiers. in the above circuit they are switching DC which means they would't switch off I think.in the box that I had a quick look in there were two pairs of back to back power thyristors (Cathode connected to anode and vice versa), one K-A pair connected to one side of the +ve output from the transformer, the other K-A pair connected to the same side of the SWCG cell.the gates were controlled by directly pulses directly from a PIC. the PIC got its power from rectified power from the same transformer and used an optocoupler to determine zero crossing (I suspect). Something like the circuit attached; but when I model this it does not function.