The internal architecture of a standard thermal-magnetic breaker is not a single monolithic switch, but a strict series circuit composed of distinct circuit breaker components. Current flows sequentially through the line terminal, a thermal bimetallic strip, a magnetic solenoid coil, and finally the main mechanical contacts before reaching the load. Understanding this series topology—and how to replicate it with solid-state equivalents on a breadboard—is essential for diagnosing panel faults, designing custom DC protection, and grasping why breakers trip under specific time-current curves.

Internal Topology and Component Specifications

To analyze the breaker as a circuit, we map the internal path using five distinct nodes. In a standard 120V AC, 15A single-pole breaker (like the widely used Square D HOM115 or Eaton BR115), the current path is strictly series. This ensures 100% of the load current passes through both the thermal and magnetic sensing elements.

  • Node 1 (Line In): The bus bar stab or screw terminal receiving ungrounded (hot) conductor current.
  • Node 2 (Post-Bimetallic): The junction between the thermal sensor and the magnetic coil.
  • Node 3 (Post-Solenoid): The junction between the magnetic coil and the moving contact arm.
  • Node 4 (Moving Contact): The spring-loaded physical switch mechanism.
  • Node 5 (Load Out): The stationary contact and load terminal screw.

Below is the spec-sheet data for the physical circuit breaker components inside a typical 15A residential branch breaker. These values dictate the time-current trip curve mandated by UL 489 and the National Electrical Code (NEC).

Table 1: Physical Component Specifications (15A / 120V AC Breaker)
Component Nodes DC Resistance (Nominal) Trip Threshold & Timing Primary Failure Mode
Bimetallic Strip (Thermal) Node 1 to 2 ~1.5 mΩ 15A @ 1 hr (100%); 30A @ 15s (200%) Metal fatigue (nuisance tripping)
Solenoid Coil (Magnetic) Node 2 to 3 ~0.5 mΩ 150A instantaneous (10x In) Shorted turns (failure to trip)
Main Contacts (Silver-CdO) Node 4 to 5 <0.1 mΩ (closed) N/A (Mechanical execution) Pitting / Carbon tracking
Arc Chute (9 Steel Plates) Parallel to Node 4-5 High (Air gap) Extinguishes 10kA IC in <1 cycle Soot buildup (restrike)

Behavior Matrix: Extremes and Failure Modes

Why use a series topology instead of a parallel sensing topology? If the thermal and magnetic sensors were in parallel, a fault in one branch would bypass the sensor, rendering the breaker blind to overloads. The series topology guarantees that every electron passing to the load must first do work across the bimetallic strip and generate a magnetic field in the solenoid.

However, this series dependency means a single component failure compromises the entire protection scheme. The table below contrasts what happens at the extremes when individual circuit breaker components open or short.

Table 2: Element Change Behavior and Extreme Faults
Component If Element Opens (High Impedance) If Element Shorts (Low Impedance)
Bimetallic Strip Load loses power immediately. Breaker acts as a blown fuse. No thermal protection needed as circuit is dead. Impossible physically, but if bypassed, breaker loses overload (thermal) protection. Will only trip on dead shorts.
Solenoid Coil Load loses power. Circuit is open. Critical Hazard: Magnetic field collapses. Breaker will not trip instantaneously on a 500A dead short, relying solely on the slow thermal strip. Fire risk.
Main Contacts Normal operation (breaker is OFF or tripped). Catastrophic: Contacts weld shut due to high fault current. Breaker cannot be manually switched off. Requires upstream main breaker to clear.
Bench Tip: You can test the health of the solenoid coil on a salvaged breaker using a micro-ohmmeter across Node 1 and Node 3 (with the breaker ON). If the resistance reads significantly lower than the manufacturer's baseline (e.g., <0.2 mΩ instead of 2.0 mΩ total), the solenoid has shorted internal turns and the magnetic trip is compromised.

Design Walkthrough: Breadboarding a Solid-State Equivalent

You cannot safely breadboard a 120V AC arc chute or push 150A through a physical solenoid on a lab bench. To study the behavior of these circuit breaker components safely, we design a 12V DC electronic breaker (e-breaker). This solid-state topology mimics the thermal-magnetic series path using discrete silicon components, allowing you to map trip curves with a standard multimeter and power supply.

We will replace the physical components with their electronic analogs:

  • Bimetallic/Solenoid Equivalent: A 0.01Ω, 5W precision shunt resistor. This develops a voltage drop proportional to load current (V = I × R), mimicking the thermal heat and magnetic field generation.
  • Trip Lever Equivalent: An LM358 dual op-amp configured as a comparator. It monitors the shunt voltage and triggers when it exceeds a reference threshold.
  • Main Contacts Equivalent: An IRLZ44N N-channel logic-level MOSFET. When the op-amp output goes high, it pulls the MOSFET gate low, opening the circuit.

Component Selection and Values

For a bench-safe 1.5A trip threshold (representing a 10:1 scale model of a 15A breaker):

  • Shunt Resistor (R_shunt): 0.01Ω. At 1.5A, voltage drop is 15mV.
  • Reference Voltage (V_ref): Set via a 10kΩ trimpot connected to a 5V voltage divider, tuned to exactly 15mV.
  • Gate Pull-up (R_gate): 10kΩ to 12V VCC to keep the MOSFET normally ON (closed contacts) until a fault is detected.

Step-by-Step Breadboard Testing

Follow this sequence to build and verify the trip curve of your solid-state breaker. Ensure your DC power supply has its own current limit set to 3A as a backup safety measure.

  1. Wire the Shunt and Load Path: Connect the positive terminal of your 12V supply to one end of the 0.01Ω shunt. Connect the other end of the shunt to the Drain pin of the IRLZ44N MOSFET. Connect the Source pin to your test load (e.g., a 12V automotive bulb) and back to the supply ground.
  2. Configure the Comparator: Wire the LM358 Pin 8 to 12V VCC and Pin 4 to Ground. Connect the non-inverting input (Pin 3) to the load-side of the shunt resistor. Connect the inverting input (Pin 2) to the wiper of your 10kΩ trimpot.
  3. Set the Trip Threshold: With no load connected, use your multimeter to adjust the trimpot until Pin 2 reads exactly 15mV. This is your 1.5A trip threshold.
  4. Wire the Trip Mechanism: Connect the LM358 output (Pin 1) to the Gate of the MOSFET through a 1kΩ current-limiting resistor. Add the 10kΩ pull-up resistor from the Gate to 12V VCC. In this state, the op-amp output is low, and the pull-up keeps the MOSFET gate high (contacts closed).
  5. Execute the Overload Test: Power the circuit. The bulb should illuminate. Slowly decrease the resistance of your load (or add parallel resistors) while monitoring the shunt voltage on an oscilloscope or fast multimeter. When the current hits 1.5A, the shunt voltage exceeds 15mV, the op-amp output swings to 12V, overpowering the pull-up, and the MOSFET snaps off.

Edge Cases: Where Solid-State Differs from Mechanical

While this breadboard model perfectly demonstrates the sensing topology of circuit breaker components, it highlights the exact reason we still use mechanical air-gaps in residential panels. If the IRLZ44N MOSFET experiences a drain-to-source short circuit failure (a common failure mode for power semiconductors under extreme thermal stress), the breaker remains closed during a fault.

Mechanical breakers avoid this by using physical separation. When the solenoid pulls the trip lever, the contacts physically separate, and the arc chute stretches and cools the plasma arc until it extinguishes. A solid-state breaker has no physical air gap; it relies entirely on the silicon junction holding off the voltage. This is why modern hybrid breakers use a solid-state sensor to trigger a mechanical relay, combining the speed of silicon with the fail-safe isolation of an air gap.