Dear Reza,

A scope is certainly the best tool for diagnosis of problem as steve said. I have also seen flickering due insufficient holding current, Your thyristors need a minimum current for remaining on called holding current. In order to avoid this problem use higher power lamps. With 100Watt+ Lamps I have not seen problems but with 60Watt or lower I have seen flickering.

I use the following setup for diagnosis of thyristor related problems. It is probably the simplest and very effective setup. It contains three lamps each 100W 230V & two diodes 1N4007 It can be directly applied onto systems using upto 440V RMS AC. It can detect half firing assymetric firing and missing firing pulses.

Center bulb [ Shown Left in schema ] - Both postive and negative half cyclces pass.
Left Bulb [ Shown at Top in Schema ] - Only positive half cycle pass. Thus it shows problems with postive cycle firing.
Right Bulb [Shown at Bottom in Schema ] - Only Negative half cycle pass. It shows problems with negative cycles.




Firing OK - All three bulbs glow center lamp double the glow of Right and left. Right and left bulbs glow with same intensity and no flickering.

No Firing - All three bulbs remain off.

Half firing - Center bulb and either left or right bulb glows.

Assymetric Firing - All three bulbs glow. Left and Right bulbs glow with diffrent intensities.

Missing firing pulses - If firing in positive half cycle is missing left bulb shows flickering. If firing in negative half cycle is missing right bulb shows flickering. Remember human eye does detect even a single half cycle missed out.

Best Regards,
Prahlad Purohit