Hi,

I have implemented the ckt as attached in 1.jpg file. As shown in ckt, the d.c supply which gives the voltage to gate terminal is kept at 0 v. But when i am connecting the supply terminal to gate terminal its gate voltage is increase from 0 volt to 3.6 volt ( 5 volt(voltage devider) - IR(1kohm) ). So now if i am increase the gate voltage to fire SCR, voltage across anode to cathod will not decrease abruptly as per the characteristic of SCR but it will decrease slowly as we increase the gate voltage and reach to 0.7 volt when gate voltage is around 10.7 volt. Can i say SCR is fired?
Now i have implement the ckt as attached in 2.jpg file. At that time when i am connecting the supply terminal to gate terminal its gate voltage remains at 0 volt and when increase gate voltage voltage across anode to cathod will decrease abruptly as per the SCR characteristic.

Can any one tell me what is the reason?

 

Quick assumption, not really did a thoro analysis, just a quick guess.

In the second schem. you have a steady 5v. source at the cathode.

In the first schem. you are using a variable voltage divider, of 5v.

Variable because when the SCR begins to conduct, it drops the divider voltage, so a constant 5v. is no longer at that terminal?

But I think it has a lot to do with a voltage divider being used at the cathode terminal, some how.

It could also be just the simulator sees it that way.

It would have to be actually built for further analysis. and more logical assumptions to be made.