The circuit diagram of a circuit breaker changes fundamentally depending on whether you are analyzing a traditional thermal-magnetic AC breaker or a modern solid-state DC electronic breaker (e-breaker). While a thermal-magnetic diagram maps mechanical linkages, bimetallic strips, and solenoid coils, a solid-state circuit diagram maps a current-sense shunt, an amplifier, a voltage comparator, and a power MOSFET. For bench design, low-voltage DC applications, and rapid prototyping, the solid-state topology offers microsecond trip times, precise adjustable thresholds, and zero moving parts. Below, we break down the exact topology, component values, and breadboard testing procedure for a 12V/10A solid-state breaker.
Topology Breakdown: Node Labels and Architecture
To understand the circuit diagram of a circuit breaker in a solid-state context, we must define the electrical nodes. Unlike a mechanical breaker that simply places a switch in series with the line, an electronic breaker uses a low-side switching topology with continuous current monitoring.
- $V_{IN}$ (Source Node): The positive DC supply (e.g., 12V) feeding the load.
- $V_{LOAD}$ (Drain Node): The connection point between the load's ground return and the MOSFET's drain.
- $V_{SENSE+}$ / $V_{SENSE-}$ (Shunt Nodes): The Kelvin connections across the current-sense resistor, feeding the differential amplifier.
- $V_{REF}$ (Threshold Node): A voltage divider setting the exact trip threshold for the comparator.
- $V_{GATE}$ (Control Node): The MOSFET gate, pulled high to conduct, and pulled low by the comparator to interrupt the circuit.
Why this topology over the alternative? The primary alternative is a high-side P-channel MOSFET switch. However, P-channel MOSFETs with equivalent current ratings have higher $R_{DS(on)}$ (on-resistance) and cost more. By using a low-side N-channel topology, we leverage cheaper, highly efficient silicon (like the IRFZ44N) while keeping the shunt resistor referenced to ground, which simplifies the op-amp common-mode voltage requirements.
Component Spec Sheet: 12V / 10A Solid-State Breaker
Here is the exact bill of materials and design walkthrough for a 10A trip point. The math dictates our component selection: at 10A, a 5m$\Omega$ shunt drops 50mV. We amplify this by 50V/V to yield 2.5V, which perfectly matches our comparator reference threshold.
| Component Role | Part Number / Value | Key Specification | Design Rationale |
|---|---|---|---|
| Current Shunt | Bourns CRE2512-FZ-R005E-1 | 5m$\Omega$, 1%, 2W | Low resistance minimizes voltage drop and heat; 2W rating survives brief overcurrent spikes before the FET trips. |
| Current Sense Amp | Texas Instruments INA180A2 | Gain: 50V/V, 26V Common-Mode | Provides precise amplification of the 50mV shunt signal up to a 2.5V logic-level threshold. |
| Voltage Comparator | LM393 (Dual Differential) | Open-Collector Output, 36V Max | Open-collector allows direct interfacing with the MOSFET gate pull-up network without complex logic gates. |
| Power Switch (MOSFET) | Infineon IRFZ44N (N-Channel) | 55V $V_{DS}$, 49A $I_D$, 17m$\Omega$ $R_{DS(on)}$ | Massive current overhead ensures the FET stays in the linear/ohmic region and doesn't overheat during normal 10A operation. |
| Gate Pull-Up | 10k$\Omega$ Resistor (1/4W) | Connects $V_{GATE}$ to $V_{IN}$ | Keeps the N-channel FET fully enhanced ($V_{GS}$ = 12V) during normal operation. |
| Threshold Divider | 10k$\Omega$ / 10k$\Omega$ (1%) | Sets $V_{REF}$ to exactly 2.5V (from 5V rail) | Creates the precise trip reference for the LM393 non-inverting input. |
For deeper reading on current-sense amplifier selection, refer to the Texas Instruments Current Sense Amplifier Guide.
Behavior Matrix: How the Topology Reacts to Faults
A robust circuit diagram of a circuit breaker must account for dynamic changes in the load and component drift. The table below maps the behavioral response of the solid-state topology when specific elements change.
| Event / Element Change | Node Voltage Shift | System Response | Time to Trip |
|---|---|---|---|
| Load Resistance Drops (Short Circuit) | $V_{SENSE}$ spikes > 2.5V | LM393 output pulls low; $V_{GATE}$ drops to 0V; FET turns off. | < 5 $\mu$s (Microseconds) |
| Ambient Temp Rises (Shunt Drift) | $V_{SENSE}$ increases slightly (Copper tempco) | Trip threshold lowers slightly; breaker becomes more sensitive. | N/A (Threshold shift) |
| $V_{IN}$ Sags (Brownout) | $V_{GATE}$ pull-up voltage drops | If $V_{IN}$ < 4V, FET enters linear region, heats up, and may fail to interrupt. | Thermal failure risk |
What Breaks at the Extremes?
Every topology has fatal failure modes. In this circuit diagram of a circuit breaker, you must design for the extremes:
- Open Shunt Resistor: If the solder joint on the 5m$\Omega$ shunt cracks and opens, $V_{SENSE}$ drops to 0V. The comparator never trips, and the breaker fails to protect the load during a dead short. Mitigation: Use a 4-terminal Kelvin shunt and inspect solder joints under magnification.
- Shorted MOSFET: If a massive voltage spike exceeds the IRFZ44N's 55V $V_{DS}$ rating, the silicon avalanches and the drain-source channel permanently shorts. The breaker is now "welded" closed. Mitigation: Place a 15V Zener diode across Drain and Source to clamp inductive kickback.
Failure Mode Contrast: Series Shunt vs. Parallel $R_{DS(on)}$ Sensing
When designing the circuit diagram of a circuit breaker, engineers often debate whether to use a dedicated series shunt resistor or to measure the voltage drop directly across the MOSFET's $R_{DS(on)}$ (parallel sensing). Here is why series shunt sensing wins for reliable protection.
Parallel Sensing (Measuring $R_{DS(on)}$): This method eliminates the shunt resistor, saving board space and cost. However, a MOSFET's on-resistance has a strong positive temperature coefficient. As the IRFZ44N heats up from 25°C to 100°C during normal operation, its $R_{DS(on)}$ nearly doubles. The breaker will nuisance-trip at 5A when hot, but fail to trip at 15A when cold. Furthermore, if the gate drive voltage sags, $R_{DS(on)}$ skyrockets, completely invalidating the trip threshold.
Series Shunt Sensing (Our Topology): The Bourns CRE2512 shunt has a very low temperature coefficient (typically < 50 ppm/°C). The voltage drop remains strictly proportional to the current regardless of ambient heat or gate drive variations. While it costs an extra $0.50 in BOM cost and wastes 500mW at 10A, the predictability is mandatory for a functional breaker.
Step-by-Step Breadboard Test Procedure
To validate your circuit diagram of a circuit breaker, you must bench-test it before committing to a PCB. Follow these steps to verify the 10A trip point safely.
- Prepare the Power Rails: Set a bench power supply to 12.0V with a current limit of 15A. Connect this to your breadboard's positive rail. Connect a separate 5V supply (or a 7805 regulator) to power the LM393 and the $V_{REF}$ divider.
- Wire the Sense Path: Solder heavy 12 AWG pigtails to the INA180A2 breakout board. Connect the shunt resistor in series with the load return path. Ensure the Kelvin sense traces connect directly to the shunt pads, not the breadboard springs (which add unpredictable milliohms of resistance).
- Verify the Threshold: Before connecting the load, use a multimeter to probe the $V_{REF}$ node (LM393 Pin 3). It must read exactly 2.50V $\pm$ 10mV. Adjust the voltage divider if necessary.
- Apply a Nominal Load: Connect a 12V automotive headlight bulb (approx. 5A draw). Power the circuit. The bulb should illuminate instantly. Probe $V_{GATE}$ (LM393 Pin 1 / MOSFET Gate); it should read ~11.5V, confirming the FET is fully enhanced.
- Simulate an Overcurrent Fault: Replace the bulb with a high-power electronic load or a bank of 1$\Omega$ 50W power resistors. Slowly increase the load current. Watch the multimeter on the INA180 output (Pin 6).
- Observe the Trip: As current approaches 9.8A - 10.2A, the INA180 output will hit 2.5V. The LM393 will snap its output low. The MOSFET gate will drop to 0V, and the current will instantly cease. Note: If using a bench supply, the supply's own OCP might trip first; ensure the supply limit is set higher than 12A.
- Test the Reset: Remove the short/overload. Cycle the power off and on. The breaker should automatically reset and conduct current again, proving the latch-free design works as intended.
By mapping out the exact nodes, selecting precision components, and understanding the thermal and electrical extremes, you transition from a theoretical schematic to a reliable, real-world protective device. For more on solid-state protection topologies, the All About Circuits solid-state breaker guide provides excellent supplementary schematic variations.
