Random Phase SCR Gate Triggering Driver

[Electronika.design]

Hi everyone,

I am using a PIC18f45k22 chip for Variable Phase angle firing of SCRs.
I am not using a TRIAC but instead connecting 2 SCRs back to back for commutation and other reasons.
I tried doing this with an Random Phase Opto Isolator MOC3021.
But later got to know that MOC only drives TRIACs...
Could any be kind enough in suggesting some other isolation driver for my SCRs???
I am using 230V 50Hz supply...
If anyone needs any more details plz leave a reply...

Thank You.

 

[ericgibbs]

hi E,
Look at page #4 of this pdf, for using the MOC3031 as as SCR drive
E

 

[Electronika.design]

Hi E,
Thanx for the attachment... but iv already tried the same circuit...
I tried MOC3021 bcz I wanted variable phase driving, while MOC3031 is zero crossing driver(cant use).
It is not giving me an output...
any other ideas you can come up with??

Thanx...

 

[ericgibbs]

hi E,
This MOC is non zero crossing version.
E

 

[Electronika.design]

Hi E,
MOC3021 & MOC3020 are the same, just some ampere and voltage changes...
I tried doing this...No output
Thanks E.

 

[ericgibbs]

Deleted.
E

 

[spec]

Hi E,
Thanx for the attachment... but iv already tried the same circuit...
I tried MOC3021 bcz I wanted variable phase driving, while MOC3031 is zero crossing driver(cant use).
It is not giving me an output...
any other ideas you can come up with??

Thanx...

Hi ED,

Be wary of some of the circuits for gate SCR/TRIAC drivers in application reports, on the net, and in books. The one that shows a 180 Ohm resistor going to the low side of the load in particular. That is only suitable for zero switching TRIAC drivers and not random drivers.

If used with random drivers that circuit will blow the SCR/TIAC trigger junctions and the optocoupler TRIAC.

Jut a general observation from past experience, many SCR/TTRIAC circuits/experiments result in graveyards of dead SCRs/TRIACs and the fatality is always blamed on over-stressing the main current path, over current or over voltage or both, but in many cases it is the gate path that has been destroyed.

Another point is that you will often see circuits where the gate is open circuit or high impedance for some part of the input sine wave. This is unsatisfactory as it can lead to false triggering, especially at high temperatures, which is normal for SCRs/TRIACs.

So the best advice is to post any circuit you are proposing before turning on, so that we can check it against the SCR/TRIAC data sheet- you are not building a welder are you?

spe

 

[ericgibbs]

hi Elect'.
This version with a MOC3023 works OK in simulation.
It shows the firing phase angle being stepped in 1mS steps, from 1mS thru 9mS of a 50Hz, half cycle

E