Protecting circuits against overcurrent is an important aspect of circuit design. The cause of overcurrent can be an intolerable voltage fluctuation in the power supply, short circuit, failure of a device or component, and overload. Typically, to protect circuits against overcurrent and therefore damage due to overheating of components, electronic fuses are used in their power supply sections. Electronic fuses are thin metal wires that melt when passing a limit current. In the design of power circuits, in addition to electronic fuses, circuit breakers are also used to protect the circuit. Any circuit breaker operates as a relay that has the ability to detect a threshold level of current and disconnect the rest of the circuit by turning off the sources.
Although fuses need to be replaced whenever one blows, circuit breakers are an economical alternative that remain part of the power supply without the need for any replacement.
Also in this project, an electronic circuit breaker is designed using a relay and switching transistors. Switching transistors help detect a predetermined current level and drive the relay to disconnect the rest of the circuit. The switching transistor used is PN2907A. The circuit breaker designed in this project disconnects the circuit from the sources when the current exceeds 0.6 A. The main source is considered a DC source from a battery and the circuit designed in this project avoids drawing excessive current from it, functioning as a protective shield between the circuit load and DC voltage source (any battery).
Fig. 1: Electronic Circuit Breaker Prototype
Once the circuit is disconnected on relay tripping, supplies can be resumed by toggling a DPDT switch of the electronic circuit breaker designed here. The threshold current level can also be increased or decreased in the following circuit using another set of switching transistors.
Required components –
Fig. 2: List of components required for the circuit breaker
Circuit Connections –
In the circuit, a 24V relay is used to disconnect the load from the sources. The relay coil terminals are connected to the emitter pins of the switching transistors Q1 and Q2. The collector pins of the transistors are connected to ground. The base of transistor Q1 is connected to the collector of transistor Q2. The battery's anode is connected to the NO terminal of the relay, while its COM terminal is connected to the emitter pin of transistor Q2 and a bias resistor R3. A DPDT switch is connected between the battery anode terminal and the output load. The DPDT switch can be toggled to resume power by disconnecting the relay connector back to the NO position. An LED in series with a pull-up resistor is connected to the base of transistor Q2 for a visual cue of power continuity. The output voltage is consumed between the DPDT switch and common ground. The battery used in the project as a DC voltage source is a normal 12V battery.
How the circuit works –
The 12V DC is supplied at the circuit input terminals by a battery. The supplied voltage is blocked by the relay. When pressing the DPDT switch (SW1), which is initially in its normal position, the circuit receives power. When the circuit is turned on, it also activates the relay, so the DPDT switch must be returned to its normal position. From here, the load receives power through the relay and the relay itself is powered by transistor Q1.
The base of transistor Q1 is biased by resistor R1. Transistor Q2 is biased by resistor R3. Under normal conditions, when there is no short circuit or overload, the voltage across R3 remains below 0.6 volts. The threshold voltage of PN2907A transistors is between 0.6V to 1.3V according to its datasheet. Therefore, when the base of Q2 reaches its threshold voltage, only it starts to conduct. Therefore, transistor Q2 is in the OFF or non-conducting state until the voltage across R3 is lower than its threshold voltage. The current limit can be decided by resistor R3. The voltage across R3 is the product of the current and the resistor value. The threshold voltage of Q2 is 0.6V, so to decide the current limit, the resistor value can be calculated as follows –
Considering the desired current limit, I = 0.6 A
Threshold voltage of Q2, V = 0.6 V
R3 = V/I
R3 = 0.6/0.6
R3 = 1 ohm
Therefore, to set a current limit of 0.6, a resistor R3 must have a resistance of 1 ohm. A variable resistor can also be used in place of R3 for manual adjustment of the current limit.
The load at the output is powered by resistor R3. Therefore, the current consumed by the load can be measured by the measuring current in R3. When the voltage across R3 reaches 0.6 V, transistor Q2 begins to conduct.
As transistor Q2 begins to conduct, all current passes through it. Therefore, no current remains in the base of transistor Q1. This turns off transistor Q1. Transistor Q1 in this circuit energizes the relay so that when Q2 starts conducting current, transistor Q1 goes into the OFF state, so the relay is deactivated by also turning off transistor Q2. As a result, the load receiving power from the relay is disconnected from the input power source. This saves the load from a current greater than 0.6 A.
The LED at the output indicates the disconnection of the load from the input power by going to the OFF state. When the cause of the overcurrent is discovered and corrected in the load circuit, the DPDT switch needs to be reset for the first time and returned to its normal position to resume supplying power to the load circuit.
Tests and precautions –
While designing an electronic circuit breaker like this, the following precautions should be taken –
• After resetting the DPDT switch, it must be returned to its normal position. Otherwise, the input power will not be blocked by the relay, but by the DPDT switch.
• The selection of resistor R3 must consider its power depending on the desired current limit of the circuit, otherwise a low power resistor may be damaged.
Once the circuit is assembled, it can be tested by supplying power from a 12V battery to a load connected to the circuit. Use a multimeter to measure the current through resistor R3. Replace the load with another high-current load to verify that the circuit functions as a resettable electronic fuse. Try measuring the current level above which the circuit disconnects power from the battery and compare it with the theoretical conclusions derived above.
The circuit designed in this project is low cost and can be easily assembled. It is an economical alternative to conventional electronic fuses and circuit breakers. It can also be designed with a variable resistor in place of resistor R3 so that it can have adjustable current limiting. The circuit is intended to be used as a protective shield for current-sensitive loads and load circuits with transformers or high-current semiconductor components used in them.
Circuit diagrams
| Circuit-Breaker-Electronic Diagram |  |