Designing an Electronic MCB Circuit Breaker: Topology, Trip Curves, and Bench Testing

A standard MCB circuit breaker (Miniature Circuit Breaker) protects electrical systems against overloads via a bimetallic thermal strip and short circuits via a magnetic solenoid. While electromechanical MCBs like the Schneider iC60 or ABB S200 series are cheap and robust for mains AC, designing an electronic MCB—often called a Solid-State Circuit Breaker (SSCB)—allows you to program custom trip curves, eliminate contact arcing, and achieve microsecond response times for sensitive DC loads.

Below, we break down the internal topology of a traditional MCB, contrast it with an SSCB design, and walk through a complete breadboard build for a 12V/10A electronic MCB circuit breaker mimicking a standard "C-curve" trip profile.

Internal Topology: Electromechanical vs. Solid-State MCB

To design an electronic equivalent, we first map the physical nodes of a standard electromechanical MCB circuit breaker:

• Node A (Line In): Mains or DC source entry.
• Node B (Bimetallic Strip): Thermal delay element for overloads (e.g., 1.13x to 1.45x rated current).
• Node C (Solenoid Coil): Magnetic instant-trip element for short circuits (e.g., 5x to 10x rated current).
• Node D (Mechanical Contacts): The physical switch that opens to clear the fault.
• Node E (Load Out): Connection to the downstream circuit.

Why choose a solid-state topology over the electromechanical alternative? Electromechanical MCBs suffer from contact bounce, arc flash degradation, and fixed, factory-calibrated trip curves. By replacing Nodes B, C, and D with a shunt sensor, comparator logic, and a power MOSFET, an SSCB provides zero-bounce switching, infinite mechanical life, and adjustable trip thresholds. However, SSCBs introduce conduction losses (heat from Rds(on)) and lack the inherent galvanic isolation of a physical air gap, which is why they are primarily used in low-voltage DC systems (12V–48V) rather than mains AC panels.

MCB Trip Curve Behavior Matrix

Before selecting components, we must define the trip behavior. Standard MCB circuit breakers are categorized by their magnetic instant-trip thresholds relative to their nominal current (In). Here is the behavior matrix for a 10A rated breaker across different curve types:

For our breadboard design, we will target a 10A C-Curve. This means the circuit must tolerate a continuous 10A, trip thermally within an hour at 14.5A, and trip magnetically (instantly) at 50A.

Design Walkthrough: Component Selection for a 10A C-Curve SSCB

To replicate the C-curve behavior electronically, we map the physical nodes to a solid-state topology using real, off-the-shelf components.

1. The Sense Network (Replaces Node B & C input)

We use a 5mΩ shunt resistor (e.g., Bourns CSS 4-terminal Kelvin shunt) in series with the load. At 10A, the voltage drop is 50mV. To amplify this to a usable logic level, we use the Texas Instruments INA180A1 current sense amplifier, which has a fixed gain of 20 V/V.
Math: 50mV (at 10A) × 20 = 1.0V output. At a 50A short circuit, the shunt drops 250mV, yielding a 5.0V output.

2. The Logic & Delay Network (Replaces Node B & C logic)

We use an LM393 dual comparator.

• Channel 1 (Magnetic/Instant Trip): The non-inverting input reads the INA180 output. The inverting input is tied to a 5.0V precision reference (using an LM4040). When current hits 50A, the comparator output pulls low, instantly killing the gate drive.
• Channel 2 (Thermal/Delayed Trip): To simulate the thermal mass of a bimetallic strip, we place an RC low-pass filter (10kΩ resistor and 100µF capacitor) on the non-inverting input, and set the inverting reference to 1.45V (representing 14.5A). The RC network delays the voltage rise, mimicking the I²t thermal heating curve of a physical MCB.

3. The Power Switch (Replaces Node D)

We use an IRFB4110 N-channel MOSFET. It handles 100V and 120A continuous, with an Rds(on) of just 4.5mΩ at Vgs = 10V. At 10A, it dissipates only 0.45W (P = I²R), requiring no heatsink for continuous operation.

SSCB Node Mapping Summary

• Node 1 (V_SOURCE): 12V DC Input
• Node 2 (SHUNT_HIGH): 5mΩ Kelvin Sense Resistor
• Node 3 (SHUNT_LOW): To Load Positive
• Node 4 (AMP_OUT): INA180A1 Output to LM393 Inputs
• Node 5 (COMP_THRESH): Voltage Dividers setting 1.45V and 5.0V references
• Node 6 (GATE_DRIVE): LM393 Open-Collector outputs pulled up to 10V via 1kΩ resistor to drive MOSFET gate
• Node 7 (LOAD_OUT): MOSFET Drain to Load Negative

Failure Modes: What Breaks at the Extremes?

When designing protective circuits, understanding series and parallel failure modes is critical. If a single element fails open or short, the protection topology changes drastically. Here is the failure behavior matrix for our SSCB:

Notice the critical flaw in the solid-state topology: if the MOSFET fails short, the breaker is defeated. Electromechanical MCBs do not suffer from this; their contacts physically separate, creating an air gap. Always place a fast-acting ceramic fuse in series with Node 1 of an SSCB to catch MOSFET short-circuit failures.

Step-by-Step Breadboard Testing Procedure

Testing an MCB circuit breaker equivalent requires verifying both the instantaneous magnetic trip and the delayed thermal trip. Do not use a dead short (like a screwdriver across the terminals) to test the magnetic trip on a breadboard; the parasitic inductance and breadboard trace resistance will limit the current and mask the trip time. Use an active electronic load.

1. Build the Sense Network: Solder the 5mΩ Kelvin shunt to a small breakout board (do not use breadboard spring contacts for the high-current path, as contact resistance will ruin the 5mΩ measurement). Wire the INA180A1 to the shunt sense pins using twisted pair wire to reject common-mode noise.
2. Set Comparator Thresholds: Power the LM393 with 5V. Use precision 10kΩ/1kΩ voltage dividers to set Pin 4 (Inverting, Ch 1) to exactly 5.00V (50A magnetic trip) and Pin 6 (Inverting, Ch 2) to 1.45V (14.5A thermal trip). Verify with a multimeter.
3. Wire the Gate Drive: Connect the LM393 open-collector outputs together (wired-OR logic) and pull them up to a 10V rail via a 1kΩ resistor. Add a 10kΩ pull-down resistor from the MOSFET gate to ground to ensure it turns off if the comparator loses power.
4. Simulate Inrush (Magnetic Test): Connect an electronic load (e.g., Rigol DL3021) set to constant current mode. Step the load from 0A to 55A in 1ms. Probe the MOSFET gate (Node 6) and the load voltage with an oscilloscope. The gate voltage should collapse from 10V to 0V in under 5µs, confirming the C-curve magnetic threshold.
5. Simulate Overload (Thermal Test): Set the electronic load to 14.5A. Monitor the voltage across the RC delay capacitor on Channel 2 of the LM393. It should rise exponentially and cross the 1.45V threshold, tripping the breaker in approximately 30 to 60 seconds, perfectly mimicking the I²t curve of a physical bimetallic strip.

By mapping the physical physics of an electromechanical MCB circuit breaker to analog RC networks and precision current sensing, you gain total control over the protection profile. For further reading on standard breaker classifications, refer to the Electrical Technology MCB guide, and for current sense amplifier design, consult the Texas Instruments INA180 datasheet. Always validate your breadboard prototype with a calibrated oscilloscope before committing the design to a permanent PCB.