To understand how modern AC circuit breakers interrupt fault currents without risking mains electrocution on the bench, we design a low-voltage (12V AC) solid-state circuit breaker (SSCB) proxy. This topology uses a shunt resistor, an LM358 operational amplifier, an LM393 comparator, and a 5V electromechanical relay to replicate the overcurrent trip logic of a standard 15A or 20A residential thermal-magnetic breaker. By scaling the voltage down to a safe 12V AC bench supply, you can probe the exact moment the trip threshold is crossed and observe the interruption mechanics in real-time.
Topology & Node Architecture of the SSCB Proxy
A functional AC circuit breaker requires two distinct subsystems: a sensing topology to measure current, and an interruption topology to break the circuit. In our low-voltage proxy, we isolate the 12V AC power path from the 5V DC logic path using an opto-isolated or standard relay module.
Here is the node-by-node signal flow for the series AC path and the parallel DC sensing path:
- V_AC_LINE: The 12V AC RMS source hot leg.
- NODE_SHUNT_A: Connection between the AC source and the high-side of the shunt resistor.
- NODE_SHUNT_B: The low-side of the shunt resistor, feeding the load.
- NODE_RELAY_NC: The Normally Closed contact of the relay, wired in series between the shunt and the load.
- V_AC_LOAD: The final return path to the AC neutral.
- NODE_AMP_OUT: The amplified analog voltage representing the AC current (DC offset for the comparator).
- NODE_COMP_IN+: The non-inverting input of the LM393 comparator receiving the amplified shunt signal.
- NODE_COMP_IN-: The inverting input receiving the adjustable trip reference voltage.
- NODE_RELAY_COIL: The digital HIGH/LOW output from the comparator driving the relay coil.
Component Selection & Design Walkthrough
To make this proxy mimic a real residential breaker, we need to pick component values that trigger at a precise, measurable current. Let's design for a 1.0A RMS trip threshold.
The Sensing Stage
We place a 0.1Ω 5W power resistor in series with the AC load. At 1.0A RMS, Ohm's law dictates a voltage drop of 100mV RMS (approx. 141mV peak). Because 141mV is too small for a standard 5V logic comparator to read reliably, we use an LM358 dual op-amp configured as a non-inverting amplifier with a gain of 20 (using a 19kΩ feedback resistor and a 1kΩ ground resistor). This scales our 141mV peak signal up to a robust 2.82V peak at NODE_AMP_OUT.
The Decision Stage
The amplified signal feeds into the NODE_COMP_IN+ of an LM393 dual comparator. The NODE_COMP_IN- is tied to a 10kΩ trimpot voltage divider across the 5V rail, allowing us to dial in a reference voltage of exactly 2.82V. When the load current exceeds 1.0A, the amplified shunt voltage exceeds the reference, and the LM393 output pulls LOW (it is an open-collector output, so we use a 10kΩ pull-up resistor to 5V, meaning the logic inverts: we configure the relay module to trigger on the voltage drop, or use a transistor to invert it back to HIGH to energize the relay).
The Interruption Stage
The comparator drives a standard Songle SRD-05VDC-SL-C relay module. The AC load is wired through the relay's Normally Closed (NC) and Common (COM) terminals. Under normal conditions, the relay is unenergized, and AC flows freely. When a fault occurs, the relay coil energizes, snapping the NC contacts open and interrupting the AC path.
Behavior Matrix & Failure Extremes
Understanding what happens when components fail is just as critical as knowing how they work under normal conditions. The shunt and relay must be wired in series with the load. If you mistakenly wire the relay contacts in parallel across the load, a trip event would create a dead short across the AC source, blowing the upstream fuse rather than protecting the load—a catastrophic failure mode contrast that highlights why series interruption is mandatory for AC circuit breakers.
| Element Changed / Fault Condition | Circuit Behavior | Resulting State |
|---|---|---|
| Load < 1.0A (Normal) | Shunt drop < 100mV. Amp out < 2.82V. Comparator output remains HIGH (relay off). | AC flows (NC closed). |
| Load > 1.0A (Overload) | Shunt drop > 100mV. Amp out > 2.82V. Comparator pulls LOW, transistor fires relay. | AC stops (NC opens). |
| Shunt Resistor Opens (Extreme) | Infinite resistance. Op-amp input floats or saturates to rail. Comparator trips immediately. | Fail-Safe: AC stops. |
| Shunt Resistor Shorts (Extreme) | 0Ω resistance. Amp reads 0V regardless of load current. Comparator never trips. | Fail-Dangerous: AC never stops. |
| Relay Coil Opens (Extreme) | Relay cannot energize. Mechanical contacts remain in default NC state. | Fail-Dangerous: AC never stops. |
Why Electronic Sensing Over Thermal-Magnetic?
Standard residential breakers rely on a bimetallic strip for thermal overload protection and a solenoid for magnetic short-circuit protection. Why build an electronic proxy instead of just studying a physical breaker?
- Adjustability: A thermal-magnetic breaker's trip curve is fixed at the factory. Our LM393 proxy allows you to adjust the trip threshold from 0.1A to 2.0A on the fly by turning the reference trimpot, simulating different breaker sizes (15A, 20A, 30A) without swapping hardware.
- Speed: Thermal strips require seconds to minutes to heat up and bend. Electronic sensing trips in microseconds, mimicking the magnetic instantaneous trip zone of a real breaker but with precision.
- Visibility: You cannot probe the internal voltage of a sealed molded-case breaker. The proxy exposes
NODE_AMP_OUT, allowing you to hook up an oscilloscope and watch the exact 60Hz sine wave clip at the moment of interruption.
Step-by-Step Breadboard Testing (12V AC Safe Zone)
Follow this sequence to build and verify the proxy. Ensure your 12V AC source is isolated from earth ground (a standard 120V-to-12V step-down wall transformer provides this isolation).
- Wire the DC Logic Rail: Connect a 5V DC USB power supply to the breadboard's power rails. This will power the LM358, LM393, and the relay module's optocoupler/LED indicators.
- Build the Amplifier: Place the LM358 on the board. Wire the non-inverting input to your shunt resistor's high side. Install the 19kΩ feedback and 1kΩ ground resistors to set the gain to 20.
- Configure the Comparator: Wire the LM393. Connect the LM358 output to
NODE_COMP_IN+. Wire the 10kΩ trimpot between 5V and GND, with the wiper feedingNODE_COMP_IN-. Set the wiper to exactly 2.82V using your multimeter. - Integrate the Relay: Connect the relay module's control pin to the LM393 output (via a 2N2222 NPN transistor to handle the coil current and invert the open-collector logic). Connect the relay's COM and NC terminals to your AC terminal blocks.
- Wire the AC Path: Connect the 12V AC transformer hot leg to the 0.1Ω shunt. Wire the other side of the shunt to the relay COM. Wire the relay NC to your test load (a 12V 10W incandescent bulb, which draws ~0.83A).
- Verify Normal Operation: Power on both the 5V DC and 12V AC supplies. The bulb should illuminate. Probe
NODE_AMP_OUTwith your multimeter; it should read roughly 2.35V DC (the rectified/averaged equivalent of the 1.66V peak signal, depending on your exact filtering capacitor). - Induce a Fault: Swap the 10W bulb for a 20W bulb (draws ~1.66A). The shunt voltage will exceed the 2.82V threshold. You will hear the relay click, and the bulb will instantly extinguish. The breaker has tripped.
AC Circuit Breakers FAQ
Why do AC circuit breakers have a lower interrupting rating for DC?
AC circuit breakers rely on the natural zero-crossing of the alternating current waveform to extinguish the electrical arc that forms when the contacts separate. At 60Hz, the current hits zero 120 times per second, giving the breaker's internal arc chute a chance to cool and de-ionize the air gap. DC current has no zero-crossing; once an arc strikes, it sustains continuously, melting the contacts and potentially causing a fire. This is why you must never use a standard AC-only breaker on a DC battery bank or solar array.
How does the magnetic trip in standard AC circuit breakers differ from this solid-state proxy?
In a physical NEC-compliant thermal-magnetic breaker, the magnetic trip uses a small solenoid coil in series with the load. During a massive short circuit (e.g., 500A), the magnetic field becomes strong enough to physically pull a steel latch, releasing the spring-loaded contacts in under 10 milliseconds. Our proxy uses a shunt resistor and an op-amp to measure the voltage drop electronically. While the proxy is faster and more precise at low currents, physical solenoids are preferred in residential panels because they are completely passive, immune to EMP, and require no external logic power to operate.
Can I use standard AC circuit breakers to protect a low-voltage DC solar array?
No. As noted by Fluke's electrical testing guidelines and the NEC, interrupting DC requires specialized DC-rated breakers (often with magnetic blowouts or extended arc chutes). Even at low voltages like 12V or 24V DC, a short circuit can sustain an arc across the breaker contacts. Furthermore, AC breakers are calibrated for 60Hz AC RMS heating; DC current causes different thermal dissipation profiles in the bimetallic strip, meaning a 20A AC breaker might trip prematurely at 15A DC, or fail to trip during a slow overload. Always use breakers explicitly marked with a DC voltage and current rating for solar and battery systems.
