Thyristor Firing Failed

[CataM]

Hello everyone,

I was practising with a semicontrolled full wave rectifier as the one below:



The load was a resitive one at first (100 ohms) and then changed to resistive-inductive load (100 ohms in series with 200 mH).

I had no problem with the resistive load but with the RL load.

The firing circuit (**broken link removed**) and the complete schematic is the following:



The problem with the RL load was that the 2nd Thyristor was not able to fire at 90º or above (only below 70º or so). Pictures below shows this.

Firing at 30º (works well)


Firing at 60º(aprox) (works well)


Firing at 90º (FAILED TO FIRE THE 2nd Thyristor)


Could anyone explain what could be the reason behind the failed firing of the 2nd thyristor ?

 

[schmitt trigger]

The second SCR is failing to latch.

Swap the position of both SCRs to determine if the failure follows the SCR or remains with the triggering circuit.

 

[CataM]

I have replaced both SCRs with new ones and still the same. Then the triggering circuit must be the problem ?

 

[Easy peasy]

SCR's require a minimum trigger current that goes up as the temp goes down, read the data sheets carefully...!

- - - Updated - - -

p.s. you only need one gate drive ckt as the kathodes are connected together and the wrong one will not fire under reverse bias...