TRAIC switch OFF at opposite Polaity

i am trying to switch off a triac at the opposite polarity after it switches on. The project of for a dual pulse spot welder. The first pulse which is about 50ms seems to be sitching off the opposit polarity but the some second pulse are not. As in the attached.

Is it the Time delay after the ZC that needs to be adjusted ? My Mains are 240VAC @ 50Hz and the delay i’v set after zc is 5630.

Any help would be appreciated.
Switch off at same polarity

Siwtch OFF at opposite Polarty


A 50mSec pulse will shut of after 2 1/2 cycles, because each cycle is 20mSec long at 50Hz. This will; start on positive and finish on the end of a positive cycle. or start on a negative and finish on the end of a negative cycle.

You need a pulse of 60mSec to get 3 full cycles, or 40mSec for 2 full cycles. This will; start on a positive cycle and finish on the end of negative cycle. or start on a negative cycle and finish on the end of a positive cycle

Tom..... :)

Thanks TOM.

The first pulse, being a pre-weld is alway constant at 50ms - that seemed ok all the time. i will change it to 60ms if i happen to see a change.

The second pulse, being the actual weld is varies from 100ms to 450ms and the peak delay in Microseconds used is 5630. So to understand the logic of the AC cycle, does it have to be 3 ? or can be just 1 cycle ?

Hi, Give it a try, you have the test gear to monitor the pulses, see what you get.

Tom.. :)

Hmmm good idea :)

try using an SCR instead of a triac or a few scrs

Yes that is also in mind, waiting for the components to arrive.

The TRIAC switches off after 6.800ms when the gate pulse is cut off. Hence a THYRISTOR would be a better option moreover the switching off at the Reverse Polarity would also be solved here with a THYRISTOR as well.