--- title: "Microcontroller-based thyristor firing circuit" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:microcontroller-firing-opto" source: https://www.bctengineer.com/q/semester_7/power_electronics/microcontroller-firing-opto site: BCT Engineering --- # Microcontroller-based thyristor firing circuit Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2070 Asar, 6 marks - Video: [Micro Controller Code Logic For controlling Thyristor Firing for 3 Phase Full Controller Rectifier](https://www.youtube.com/watch?v=bwVjme1w5hI) — Hussein Muhammad - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/microcontroller-firing-opto - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Describe a micro-controller based firing circuit for a thyristor with opto-coupler as isolation circuit. ## Short answer A zero-crossing detector interrupts the microcontroller, which delays for td = (alpha/360°)*20 ms and outputs a gate pulse. The pulse drives an opto-coupler LED; its output-side phototransistor supplies gate current via a resistor, isolating control and power circuits. This gives accurate, stable firing and closed-loop control. ## Answer 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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the zero-crossing detector's role in the microcontroller firing circuit? A. It amplifies the gate pulse B. It supplies the gate-circuit voltage C. It converts alpha to a time delay D. It gives an interrupt at each supply zero crossing as the timing reference Answer: D. The pulse at each zero crossing interrupts the microcontroller, which then starts counting the delay to alpha. (Key point: Role of zero-crossing detector) 2. For 50 Hz, how is the delay time computed from the firing angle? A. t_d = (alpha/360) x 20 ms B. t_d = (alpha/180) x 10 ms x 2 pi C. t_d = alpha x 50 D. t_d = (360/alpha) x 20 ms Answer: A. One cycle is 20 ms, so the delay is the fraction alpha/360 of 20 ms; a timer loaded with t_d fires the pulse. (Key point: Delay for firing angle) 3. How does the circuit isolate the microcontroller from the SCR? A. The port pin is connected directly to the gate through a resistor B. The port pin drives the LED of an opto-coupler whose output side is powered from the gate circuit supply C. A diode between the port pin and gate blocks the supply D. The step-down transformer carries the gate pulse Answer: B. Only light couples the controller to the output side, which delivers gate current, so there is full electrical isolation. 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