design firing circuit for thyristor

[hasan081043]

The supply is 220v 50Hz ac.I need 60V DC output voltage .For this i need to fire the thyristor at 80

degree.The main problem is to fire the thyristor at 80 degree .please sugest me the firing circuit.
for half wave controlled rectifier
Vdc=Vm(1+cost)/2*pi
here t=firing angle
Vm=maximum ac voltage
Vdc=output DC voltage

So for Vdc=60V ,Vm=1.414*220=311
t=?
the required firing angle would be 80 degree.
so please help me how i can fire the scr at an angle 80 degree.

 

[chuckey]

For 360 degs, T = 20mS, therefore for 80 degs T = 20 X 80/360 ~4.4 mS .
I would re-arrange the circuit, so the load is attached to the live AC, the thyristor is connected to the "bottom" end of the load and connected to the neutral line. This makes your control/trigger circuits attached to neutral and not floating at the live and of the load.
Feed the AC live via a 10K resistor to the gate of the SCR, put a diode cathode to gate, anode to neutral. This is to stop the negative AC blowing up the SCR. Now as soon as the incoming AC exceeds 10V or so there will be enough current to fire the SCR, so it stays on during the whole half cycle - too much DC out! . Putting a capacitor between the gate and neutral, now delays the voltage on the gate so it occurs later so you get less DC out. It also reduces the level of the voltage on the gate which also means more AC is required to get your DC output.
The 10 K sets the gate current, make it as high as possible, consistent with reliable triggering, it might be sufficient just to change its value and not use any capacitor at all.
Frank