SCR control with Arduino – Half-wave controlled rectifier

This small topic shows how to build a simple single-phase half-wave controlled rectifier using an Arduino board and an SCR (Silicon Controlled Rectifier) or let’s say thyristor.

The SCR (thyristor) is a three-terminal device (Anode, Cathode and Gate) with four layers of alternating p- and n-type material. The gate terminal is used to control the SCR, the anode (A) and cathode (K) are connected in series with the load. The SCR is just a controlled diode.

In single-phase half-wave rectifier, only one thyristor is used to control the load voltage. The thyristor will conduct (ON state) when the voltage Vt is positive (Vt > 0) and a firing current pulse Ig is applied to the gate terminal. Delaying the firing pulse by an angle ‘alpha’ does the control of the load voltage. In the figure below the angle ‘alpha’ is measured from the zero crossing point of the supply voltage Vs . The load is resistive and therefore current id has the same waveform as the load voltage. The thyristor goes to the nonconducting condition (OFF state) when the load voltage and, consequently, the current try to reach a negative value.

Components Required:

• Arduino UNO, NANO …
• SCR. I used TYN1225 – datasheet
• 2 x optocoupler. I used PC817 – datasheet
• 2 x IN4007 diode
• 10k ohm resistor
• 10k ohm potentiometer
• 2 x 1k ohm resistor
• Resistive load. I used 100 ohm resistor
• 220 ohm resistor
• 12V AC source (alternating current)
• Breadboard
• Jumper wires

SCR control with Arduino circuit:
Example circuit schematic diagram is shown below.

All grounded terminals are connected together.

In the circuit there are two optocouplers, I used optocoupler 1 to detect the zero crossing of the AC voltage signal and optocoupler 2 for firing the SCR. The optocouplers are used to isolate the Arduino (control circuit) from the power circuit.
Diodes D1 and D2 are with the same type (1N4007), D1 is used to protect optocoupler 1 from reverse voltage and D2 to feed the SCR gate with positive current.

Optocoupler 1 collector pin is connected to Arduino pin 2 which is external interrupt pin, so optocoupler 1 interrupts the Arduino when there is a zero crossing event (when the AC signal goes from positive to negative). When the AC voltage is positive, optocoupler 1 is ON and therefore the collector pin is connected to ground. I used the 10k ohm resistor as pull-up resistor for Arduino pin 2 because we’ve an open collector output (optocoupler 1).

The 10k ohm potentiometer is used to control firing angle.

I got the 12V AC (50Hz) using a step down transformer (220V to 12V).

SCR control with Arduino code:
Arduino code is the one below.

The following video shows output waveform and the variation of the firing angle with an oscilloscope:

Reference:
Power Electronics Handbook – MUHAMMAD H. RASHID