Why your SCR (Thyristor) is not latching?

One of our readers has asked us about thyristor (scr) not being latched properly. Here is the circuit configuration of thyristor under experiment.

thyristor not latching

This test circuit is simulated in Proteus 8. Thyristor (SCR) is given a continuous gate pulse, so when a positive cycle comes the thyristor (SCR) should be triggered and the output voltage should be same as input and during negative cycle the thyristor(SCR) remains reverse biased so it remains off.

Ideally all those things should happen but what happens when we simulate is quite different as shown in below figure..

thyristor not latching 1
Thyristor is not latching: output voltage and current remains zero.

In above figure as we can see that thyristor(SCR) is not turning on and the current from circuit is also zero.
Ideally this should not happen, we should get output voltages like this

Load voltage regulated by thyristor phase cont...

but why is it not happening? Why your thyristor(SCR) is not being triggered ?Why is your thyristor(SCR) not being latched?
The simple answer could be “latching current” and “Holding current”, huh did anything g strike in your mind now? if yes your problem is solved else continue reading.
Latching current: A certain minimum amount of load current is required to latch the SCR latched in the “on” state. This minimum current level is called the latching current.
Holding current: A certain minimum amount of load current is required to hold the SCR latched in the “on” state. This minimum current level is called the holding current.
Now your answer :
If your load current is less than Latching current it will not latch and operate only when gate pulse is applied.
If your load current is less than Holding current it will not hold in “ON” condition.
now we try decreasing the load resistance in our test circuit and let us see what happens.
thyristor latched
Tada!!!
all things are working fine, you just needed to allow proper latching and holding current to your device by decreasing load resistance.
But do remember the current carrying capacity of your load and source when applying this example to real life otherwise something may blow up.
If you have any question in this do ask us by leaving the comment below, don’t forget to share your views.

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