Microcontroller-based thyristor firing circuit

A microcontroller-based firing circuit detects the zero crossing of the supply, waits for a time corresponding to the firing angle $\alpha$, and then sends a gate pulse to the thyristor through an opto-coupler that isolates the low-voltage controller from the power circuit.

 230 V ~ --+------------------------- [SCR] -- load --+
           |                           G  K           |
     step-down tr.                     ^  ^           |
           |                           |  |           |
   +---------------+   +-----------+  +--------+      |
   | zero-crossing |-->| micro-    |->| opto-  |      |
   | detector      |INT| controller| P| coupler|------+
   +---------------+   | (timer)   |  | + R's  |
                       +-----------+  +--------+
                         ^ pot / keypad (alpha)

Working

1. Zero-crossing detection: a step-down transformer and comparator (or opto-isolated ZCD) give a pulse at every zero crossing of the supply. It is connected to an external interrupt of the microcontroller.
2. Firing angle setting: the desired $\alpha$ is read from a potentiometer through the ADC or from a keypad. It is converted to a delay time $t_d = \frac{\alpha}{360^\circ} \times 20$ ms (for 50 Hz).
3. Delay: on each interrupt the microcontroller starts a timer loaded with $t_d$.
4. Pulse output: when the timer overflows, a port pin gives a pulse (or a pulse train) of about 10 to 100 µs.
5. Isolation and drive: the pin drives the LED of an opto-coupler (for example an opto-triac or opto-transistor). The output side, powered from the gate circuit supply, delivers gate current to the SCR through a series resistor; a gate-cathode resistor prevents noise triggering.
6. For full converters, pulses for both half cycles are generated with $180^\circ$ shift.

Advantages

• Accurate, stable firing angle and easy closed-loop control (speed, voltage).
• Flexible: soft start, protection and display can be added in software.
• Complete electrical isolation by the opto-coupler.