Schmitt-Trigger output signal too long

[Boncuk]

Hi,

I am working at a project for an MCU phase control circuit.

For a precise ignition from 0 to 100% an extremely short pulse of the Schmitt-Trigger is necessary to cover the entire range.

The attached circuit produces an output signal of approximately 200µs length at zero crossing of mains voltage which is still 0.2% which can't be controlled.

Changing the values of R2 (75K) and/or C1 (22nF) for higher values the output signal collapses almost completely.

If I could achieve a pulse length of 100µs that solution would almost be perfect.

What measures can be taken for shorter pulses?

Regards

Boncuk

 

[duffy]

What you're trying to do there is kind of tricky - the opto is triggering off the top of a sine wave that is fifty times wider than the 100µS pulse you want to get. Imagine zooming in on the peak of the sine wave 50x - it's going to be pretty flat.

I would suggest triggering a one-shot (555) to give you the fixed pulse width.

 

[Boncuk]

Hi duffy,

it's not tricky at all if you crack the shell. With a timed output of an NE555 there will be synchronization problems at zero crossing. The pulse must go high exactly at zero crossing without any delay.

Thank you anyway. I've found a solution myself getting the pulse length down to 80µs, which is reasonably accurate for any load to be controlled.

Boncuk