Remote Control Circuit Breaker Using a Microcontroller

A PC can send a command to a microcontroller, which then drives a relay in a circuit-breaker’s control circuit. The lab board below is a Remote Control Circuit Breaker Using a Microcontroller. A power plant uses dedicated close and trip coils with interlocks. It does not use this GPIO-to-relay example.

The demonstration uses these parts:

1. Microcontroller (such as an Arduino)
2. Transistor
3. Diode
4. Resistors
5. Relay
6. LED
7. PC (Personal Computer)

Microcontroller

A microcontroller is an IC that can take commands from a PC. Typical links are a serial port, Ethernet or CAN (Controller Area Network).

A microcontroller also has GPIO pins, an ADC, timers, UART and often Ethernet, so it can talk to the outside world.
A digital output from a microcontroller is a low-current signal.

When you set a pin HIGH it typically sits at a voltage of +3.3 V or +5 V. A typical pin can source a few milliamperes to about 20 mA. That will light a small LED. The 30 mA figure sometimes quoted sits above the usual ATmega328 design current (about 20 mA recommended, 40 mA absolute maximum).

To control circuit breaker by microcontroller pin you need a driver that can source the current the coil needs. A GPIO pin cannot feed a 12 V relay coil. Relays and transistors are used as that driver in this example.

Transistor

The Transistor in this board is a BJT driver. In saturation it feeds the relay coil. The parts list links a JFET page; this schematic uses an NPN bipolar transistor.

Resistor

Resistors limit current in the LED and in the transistor base.

LED

The Light emitting diode shows whether the GPIO command is high or low.

Relay

A relay is an electrically operated contact. In this schematic it switches the circuit-breaker’s low-voltage control path, not the power poles. A plant close or trip coil is often 110 V or 220 V DC at several amperes and needs interlocks that this board does not provide.

Working Principle of Remote Controlling of Circuit Breaker by Microcontroller

When a command tells the microcontroller to close the load, the pin is set to 3.3 V in the circuit above. That turns on the NPN transistor. Collector-to-emitter current then energises the relay coil. The relay contact can feed the breaker’s control circuit. It does not itself carry the AC load current of the breaker poles.

An LED follows the GPIO pin. When the pin is high, the LED is on. When the pin is low, the transistor is off, no current flows in the relay coil, and the LED is off. That LED shows the command, not the true position of a plant breaker.

Protection Diode

When the relay coil is switched off, a back e.m.f. appears. If that spike exceeds the transistor VCEO rating it can destroy the transistor and the GPIO. A diode across the coil conducts that decaying current. That diode is the freewheeling diode.

Designing

Assume the pin is 3.3 V when high and 0 V when low. If the relay coil is 12 V and 360 ohm, the coil current is

This example coil then draws about 33 mA.

If the LED forward voltage is 1.2 V and you want about 20 mA, the series resistance R_LED is

The formula gives about 105 ohm at 3.3 V. The R_LED value of 500 Ω used in the original note drops the LED current to about 4 mA, which still lights many LEDs.

R_B can be chosen as 4 kΩ in this example to give enough base current. Graphical user interface:

A PC window can be written in a high-level language such as C# that talks UDP (User Datagram Protocol) to the microcontroller. Use that window only as a lab demo of toggling the digital output over UDP, not as a plant close or trip scheme.