Hi,
To my Knowledge one the SCR is on, it will remain on till the next zero cross.
only partly correct. More precise is:
* it will remain ON as slong as the load current is > then the
hold current of the SCR (read datasheet)
And exactly this is the problem.
In case you just have
one short ignition pulse then the load current may not reach the trigger current before the ignition pulse goes low --> the SCR stays OFF
This especially applies to inductive loads.
In case you have
one long ignition pulse until the voltage zero cross then the load current may not reach the trigger current before the ignition transformer gets into saturation. --> the SCR stays OFF
This especially applies to inductive loads.
To avoid ignition transformer saturation usually one uses pulse trains. Mind to allow fast decay.
The next is that the load current may include some ringing, the load current may temprarily drop below the hold current and the SCR may release (before the zero cross)
Additionally - especially with inductive loads - the load current is phase shifted. The SCR doesn´t release at zero cross of voltage but at zero cross of current.
Thus the SCR may be ON a dedicated time after voltage zero cross. During this time you can´t trigger the SCR for the next half wave.
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With your application I recommend to use pulse train. Stop the pulse train before the voltage zero cross.
Instead of ignition transformers you may consider to use MOCs to trigger the SCR. Read application notes.
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The SCR trigger problem is not a new one, thus there are many discussions, solutions, schematics in the internet.
I recommend to look at the semiconductor manufacturers for application notes.
Klaus