An overload circuit detects when current exceeds a safe threshold and actively restricts power to the load, preventing thermal damage, wire melting, or source collapse. Unlike a passive fuse that destroys itself to break a circuit, an active overload circuit uses a sense element, a comparator, and a pass transistor to interrupt or limit current in microseconds. Once the fault clears, the circuit can auto-reset or remain latched until manually cycled, making it the superior choice for modern DC power distribution, motor drives, and battery management systems.
The Anatomy of an Active Overload Protection Topology
To understand what an overload circuit is in practice, we need to look at the most robust and common configuration for DC systems: the low-side active current limiter. This topology places the switching element and the sense resistor between the load and ground. This keeps the high-side voltage stable and allows the control circuitry to reference a common ground.
The circuit relies on five critical nodes:
- V_IN: The unswitched positive supply rail feeding the load.
- V_LOAD: The connection point between the load's ground terminal and the MOSFET drain.
- V_SENSE: The node between the MOSFET source and the shunt resistor, feeding the comparator's inverting input.
- V_REF: The precision reference voltage feeding the comparator's non-inverting input, dictating the trip threshold.
- V_GATE: The output node from the comparator (pulled up) that drives the MOSFET gate.
Below is the specification sheet for the core components we will use to build a 2.0A overload protection circuit. These are standard, easily sourced parts that provide a balance of speed, cost, and reliability.
| Component | Part Number / Value | Critical Parameter | Role in Topology |
|---|---|---|---|
| Comparator | TI LM393 (Dual) | Open-collector output, 2mV max offset | Compares V_SENSE to V_REF to trigger fault |
| Pass Element | IRLZ44N (N-Ch MOSFET) | Rds(on) = 22mΩ @ Vgs=5V, Id=47A | Switches load current; low Rds(on) minimizes heat |
| Shunt Resistor | 0.1Ω, 2W Metal Film | 1% tolerance, 50ppm/°C drift | Converts load current to a measurable voltage |
| Gate Pull-Up | 10kΩ, 1/4W Carbon Film | Standard tolerance | Pulls V_GATE high to enable MOSFET (LM393 is open-collector) |
| Reference Divider | R1=24kΩ, R2=1kΩ | Yields ~0.20V from a 5.0V rail | Sets the exact V_REF trip threshold |
Component Selection and Design Walkthrough
Designing an overload circuit requires working backward from your maximum safe continuous current. Let us design for a strict 2.0A trip point on a 12V DC system.
1. Sizing the Shunt Resistor
We need a voltage drop that is large enough to overcome the comparator's input offset voltage (up to 5mV on a cheap LM393) but small enough to avoid wasting power and starving the load of voltage. A 0.1Ω shunt is the sweet spot. At 2.0A, Ohm's law (V = I × R) dictates a voltage drop of exactly 0.2V. The power dissipated by the shunt is P = I² × R, which equals 0.4W. We select a 2W rated resistor to keep it running cool, preventing thermal drift from altering our trip point.
2. Setting the Reference Voltage (V_REF)
The comparator needs a reference of exactly 0.2V to match the shunt drop at 2.0A. Assuming we have a clean 5.0V logic rail available, a simple voltage divider using a 24kΩ (R1) and 1kΩ (R2) resistor yields 0.20V. For higher precision applications where a 5% resistor tolerance could shift your trip point by 100mA, swap the passive divider for a TL431 programmable shunt regulator or a dedicated low-dropout reference IC.
3. Driving the Gate
The LM393 features an open-collector output. This means it can pull the output pin to ground, but it cannot source current to drive it high. We must add a 10kΩ pull-up resistor from the comparator output (V_GATE) to the 5V or 12V rail. When the circuit is in a normal state, the pull-up holds V_GATE high, turning the IRLZ44N fully on. When an overload occurs, the LM393 sinks the gate to ground, turning the MOSFET off.
Behavior Matrix and Failure Mode Extremes
Why choose this active topology over a simple cartridge fuse or a PTC polyfuse? The answer lies in reaction time, reset behavior, and precision. A standard glass fuse takes milliseconds to seconds to blow, allowing massive let-through current that can fry sensitive silicon before the filament melts. A PTC polyfuse suffers from high hold-to-trip hysteresis and takes seconds to heat up and trip. An active current sensing circuit reacts in microseconds and can be designed to auto-reset or latch off.
| Protection Method | Reaction Time | Reset Behavior | Precision (Trip Point) | Best Use Case |
|---|---|---|---|---|
| Active Overload (MOSFET) | Microseconds (µs) | Auto-reset or Latched | ±2% (with 1% shunt) | Silicon protection, motor stalls, battery BMS |
| Cartridge Fuse (Fast-Blow) | Milliseconds (ms) | Replace component | ±20% tolerance | Catastrophic short-circuit protection, mains AC |
| PTC Polyfuse | Seconds (thermal) | Auto-reset (slow cool) | ±50% or worse | USB ports, low-cost consumer toys |
Understanding what breaks at the extremes is critical for robust design. Here is the behavior matrix detailing how the topology responds when individual elements fail or shift.
| Element / Condition | What Changes in the Circuit | System Result |
|---|---|---|
| Load Resistance Drops (Overload) | Current rises, V_SENSE exceeds V_REF (0.2V) | Comparator pulls V_GATE low; MOSFET turns off. Current ceases. |
| Shunt Resistor Opens (Fails) | V_SENSE is pulled to GND via comparator input impedance | V_SENSE drops below V_REF. MOSFET stays ON. Protection is lost. |
| Shunt Resistor Shorts (Solder bridge) | V_SENSE is forced to 0V regardless of load current | MOSFET stays ON. Protection is lost. (Always use Kelvin connections on shunts). |
| MOSFET Gate Pull-Up Opens | V_GATE floats; gate capacitance slowly drains | MOSFET enters linear region, overheats, and fails short or open. |
Breadboard Testing and Verification Protocol
Before soldering this topology to a perfboard or ordering a PCB, you must validate the trip point and reaction behavior on a breadboard. Follow this exact sequence to avoid blowing your power supply or the LM393.
- Verify the Reference Voltage First: With the power supply disconnected, wire up the LM393 VCC and GND pins, and build the 24kΩ/1kΩ voltage divider. Connect your multimeter to the non-inverting input (Pin 3 on the LM393). Power the logic rail and verify you read exactly 0.20V (±0.01V). If it reads 0V, check your breadboard ground continuity.
- Wire the Pass Element and Shunt: Insert the IRLZ44N. Connect the Drain to your load's ground return. Connect the Source to one end of the 0.1Ω shunt, and the other end of the shunt to system GND. Connect the inverting input (Pin 2) to the node between the MOSFET Source and the shunt.
- Install the Gate Pull-Up: Connect a 10kΩ resistor between the LM393 output (Pin 1) and the 5V logic rail. Wire Pin 1 directly to the MOSFET Gate.
- The Dummy Load Test: Do not connect your expensive microcontroller or motor yet. Use a high-wattage power resistor (e.g., a 10Ω, 10W ceramic resistor) as a dummy load. Connect it between your 12V supply and the MOSFET Drain.
- Measure Normal State: Power the 12V supply. The load should draw approximately 1.2A (12V / 10Ω). Measure the voltage across the shunt resistor; it should read ~0.12V. The MOSFET gate should read high (near 5V or 12V, depending on your pull-up rail).
- Force the Overload: To simulate a fault, swap the 10Ω dummy load for a 2Ω dummy load (or parallel two 5Ω resistors). The current will attempt to spike to 6A. As it crosses 2.0A, the voltage across the shunt will hit 0.2V. The LM393 will instantly pull the gate to ground. Measure the shunt voltage again—it should drop to 0V, and the load current should read 0A on your multimeter.
- Test Auto-Reset: Disconnect the 2Ω load and reconnect the 10Ω load. Because the LM393 inverting input now sees 0V (which is less than the 0.2V reference), the open-collector output will release, the pull-up will drive the gate high, and the circuit will automatically re-enable.
By building and testing this topology, you move beyond the limitations of passive fuses. You gain precise, microsecond-level control over your DC power distribution, ensuring that a stalled motor or a shorted sensor cable results in a graceful system interrupt rather than a melted wire harness.
