Designing an Electronic Circuit Breaker: Topology, Sizing, and Testing

An electronic circuit breaker uses active semiconductor switching and current-sense feedback to interrupt fault currents in microseconds, drastically outperforming the milliseconds required by a mechanical thermal-magnetic breaker. For DC loads, robotics, and sensitive embedded systems, a solid-state breaker eliminates mechanical wear, prevents contact arcing, and allows for precise, adjustable trip thresholds. The direct answer for a robust, breadboardable DC breaker topology is a high-side P-channel MOSFET driven by a single-supply op-amp monitoring a low-side shunt resistor.

Topology Map: The Op-Amp and P-Channel MOSFET Breaker

The most reliable topology for a 12V to 24V DC electronic circuit breaker uses a P-channel MOSFET as the series pass element, an N-channel small-signal MOSFET or NPN transistor for gate pulling, and an operational amplifier for precision current sensing. We use this topology over a simple series-pass BJT because a MOSFET offers near-zero voltage drop (R_DS(on)) in the on-state, minimizing heat dissipation. We use an op-amp over a simple comparator IC like the LM393 because single-supply op-amps like the LM358 can sense voltages all the way down to the ground rail without needing a negative voltage supply.

Node Labels and Signal Path

• V_IN: Unregulated DC supply (e.g., 12V from a battery or bench supply).
• V_OUT: Switched output feeding the load.
• V_SENSE: The node between the load return and the shunt resistor. This voltage rises proportionally with load current.
• V_REF: A stable reference voltage derived from a divider, representing the exact trip threshold.
• V_GATE: The gate of the P-channel MOSFET. Pulled to V_IN to turn off, pulled low to turn on.
• GND: System common ground.

Current flows from V_IN, through the P-channel MOSFET (Source to Drain), out to V_OUT, through the load, returns to V_SENSE, passes through the shunt resistor to GND. The op-amp continuously compares V_SENSE to V_REF. If V_SENSE exceeds V_REF, the op-amp output swings high, turning off the gate drive and opening the circuit.

Design Walkthrough: Sizing for a 1.5A Trip Point

Let us design a breaker that trips exactly at 1.5A on a 12V DC bus. We need to select real, off-the-shelf components and calculate the passive values.

1. The Shunt Resistor (Current Sensing)

We want a measurable voltage drop at 1.5A, but we must minimize power loss. A 0.1Ω shunt resistor is the sweet spot. At 1.5A, Ohm’s law dictates a voltage drop of V = 1.5A × 0.1Ω = 0.15V. Power dissipation is P = I²R = 1.5² × 0.1 = 0.225W. We will specify a 0.1Ω, 1W metal film resistor (e.g., Vishay Dale) to provide a 4x safety margin and prevent thermal drift.

2. The Reference Voltage (V_REF)

We need a stable 0.15V reference. Feeding a voltage divider directly from the 12V V_IN is risky because if V_IN sags during a fault, V_REF sags, altering the trip point. Instead, we use a 5V LDO (like the 78L05) powered from V_IN. We then use a resistor divider from the 5V rail: R1 = 33kΩ and R2 = 1kΩ. This yields V_REF = 5V × (1k / 34k) = 0.147V, which is within 2% of our 0.15V target.

3. The Switching Element

We select the IRF4905 P-channel MOSFET. It has a V_DS rating of -55V (plenty of headroom for 12V) and an exceptionally low R_DS(on) of 0.02Ω. At 1.5A, it will dissipate less than 50mW, requiring no heatsink. To turn it on, the gate must be pulled at least 4V below the source. We use a 2N7000 N-channel small-signal MOSFET to pull the IRF4905 gate to ground when the op-amp commands it.

4. The Hysteresis Latch

Without hysteresis, the breaker will oscillate rapidly when tripping (chatter). We add a 1MΩ feedback resistor from the op-amp output back to the non-inverting input. When the op-amp trips high, this resistor slightly raises the V_SENSE threshold, latching the breaker in the OFF state until V_IN is physically cycled.

Behavior Matrix and Extreme Failure Modes

Understanding how the circuit reacts to component drift and catastrophic failure is what separates a hobby project from a reliable protection circuit. Below is the behavior matrix detailing system states.

What Breaks at the Extremes?

Extreme 1: The Sense Wire Opens. If the physical breadboard wire connecting V_SENSE to the op-amp’s non-inverting input breaks or pulls out, the high-impedance input floats. In a noisy environment, it may float high, causing a nuisance trip (safe). However, if a pull-down resistor is not present, it could theoretically float low, blinding the breaker to a fault. Fix: Always add a 100kΩ pull-down resistor from the op-amp non-inverting input to GND.

Extreme 2: The P-Channel MOSFET Shorts (Drain-to-Source). This is the catastrophic failure mode of any solid-state breaker. If the IRF4905 suffers a die-level short due to an over-voltage spike exceeding its 55V V_DS rating, it becomes a permanent wire. The electronic breaker will fail to trip, and the let-through current will be limited only by the upstream power supply. Fix: An electronic circuit breaker must always be backed up by a properly sized upstream thermal-magnetic breaker or physical fuse (e.g., a 2A automotive blade fuse) to handle I²t thermal let-through in the event of semiconductor failure.

Breadboard Testing Protocol

Do not just wire this up and plug in a 12V battery. Follow this step-by-step verification sequence to ensure the trip threshold and latching behavior function correctly before connecting your actual load.

1. Verify Dead & Check Shorts: Before applying power, use a multimeter in continuity mode. Check for shorts between V_IN and GND, and V_OUT and GND. Ensure the IRF4905 gate is not shorted to its source.
2. Apply Quiescent Power: Connect a bench power supply set to 12.0V with a current limit of 0.5A. Measure the 5V rail at the 78L05 output. Measure V_REF at the op-amp inverting pin (should read ~0.147V).
3. Verify On-State: Connect a dummy load (e.g., a 12Ω, 5W power resistor) to V_OUT. The load should draw exactly 1.0A. Measure V_SENSE; it should read 0.10V. Measure the voltage drop across the IRF4905; it should be less than 0.05V.
4. Force a Fault: Replace the 12Ω resistor with a 2Ω resistor. The theoretical draw is 6A, but the bench supply will limit it. As current attempts to cross 1.5A, V_SENSE will hit 0.15V. The op-amp will trip, the gate will pull high, and V_OUT will drop to 0V.
5. Verify Latching: Remove the 2Ω fault resistor and reconnect the 12Ω normal load. The breaker should remain off. This confirms the 1MΩ hysteresis resistor is latching the op-amp in the tripped state.
6. Test Reset: Turn off the bench power supply, wait 5 seconds for capacitors to discharge, and turn it back on. The breaker should reset and power the 12Ω load normally.

For deeper integration into modern power architectures, manufacturers like Texas Instruments offer dedicated eFuse ICs that integrate the shunt, op-amp, and MOSFET into a single package with built-in inrush current limiting, which is the logical next step if you move this design from the breadboard to a production PCB.