An adjustable overload current protection circuit built with an op-amp and a logic-level MOSFET provides precise, resettable overcurrent protection for DC loads. Unlike a standard fuse that blows permanently or a polyfuse that suffers from thermal drift, this active topology trips exactly at your configured threshold (e.g., 2.00A) and enters a safe, constant-current limiting state until the fault clears. For a standard 12V DC bench supply or battery-powered project, the most reliable and cost-effective configuration is a low-side N-channel MOSFET limiter using an LM358 dual op-amp and a 0.1Ω shunt resistor.
Why an Active Op-Amp Topology Beats a Polyfuse
When protecting a sensitive microcontroller or a DC motor driver, hobbyists often default to a PTC resettable fuse (polyfuse). While cheap, polyfuses are fundamentally thermal devices, meaning their trip characteristics are sloppy and temperature-dependent. An active LM358-based current limiter uses electronic feedback to react in microseconds rather than seconds.
| Criteria | PTC Polyfuse | Active Op-Amp + MOSFET Limiter |
|---|---|---|
| Trip Speed | Slow (100ms to several seconds) | Fast (Microseconds, limited by op-amp slew rate) |
| Threshold Precision | ±20% to ±50% variance | ±2% (dictated by shunt tolerance and reference) |
| Post-Trip State | High resistance (leaks current, stays hot) | Constant-current mode (holds load safely at limit) |
| Voltage Drop (Normal) | 0.1V to 0.5V depending on I²R heating | ~0.02V (MOSFET Rds(on)) + Shunt drop |
Topology Breakdown: Low-Side MOSFET Limiter
In a low-side configuration, the load is permanently connected to the positive supply rail, and the protection circuit switches the ground return path. This is ideal for DC systems where the load does not require a hard earth-ground reference.
Node Labels and Signal Path
- V_IN: Main positive supply (e.g., 12V).
- LOAD+: Connected directly to V_IN.
- LOAD- (Drain): The ground return from the load, connected to the MOSFET Drain.
- SOURCE: MOSFET Source pin, connected to the top of the shunt resistor.
- V_SENSE (Inverting Input): The voltage drop across the shunt, fed to the op-amp's inverting (-) input.
- V_REF (Non-Inverting Input): The adjustable threshold voltage, fed to the op-amp's non-inverting (+) input.
- GATE: Driven by the op-amp output to regulate MOSFET conduction.
- GND: System ground, connected to the bottom of the shunt resistor.
Working Principle: The op-amp acts as a high-gain error amplifier in a negative feedback loop. It constantly compares V_SENSE to V_REF. If the load draws more current and V_SENSE exceeds V_REF, the op-amp reduces its output voltage, partially closing the MOSFET channel. This increases the MOSFET's drain-to-source resistance, choking off the excess current and clamping it exactly at the threshold.
Design Walkthrough: Sizing for a 2A, 12V Load
Let’s design a circuit that limits a 12V DC gear motor to exactly 2.0A. We need to select the shunt resistor, the reference voltage divider, and the MOSFET.
1. The Shunt Resistor
We want a voltage drop that is high enough for the LM358 to read accurately, but low enough to avoid wasting power. A 0.1Ω shunt yields 0.2V at 2.0A.
Power dissipation: P = I² × R = (2.0)² × 0.1 = 0.4W.
However, motors have stall currents. If the motor stalls and the circuit limits at 2A continuously, the resistor dissipates 0.4W. To prevent thermal drift, use a 0.1Ω, 2W metal film or ceramic shunt resistor.
2. The Reference Voltage (V_REF)
We need a stable 0.2V reference. Powering a voltage divider directly from the 12V motor supply is a bad idea; if the supply sags under load, your trip point shifts. Instead, use an LM7805 to create a stable 5V rail. Connect a 10kΩ multi-turn trimpot across the 5V and GND. The wiper goes to V_REF. This allows you to dial in anywhere from 0V to 5V, giving you an adjustable limit from 0A up to 50A (theoretically, though the MOSFET will bottleneck long before that).
3. The MOSFET
For a 12V system, a logic-level N-channel MOSFET like the IRLZ44N is ideal. It has a low gate threshold voltage (Vgs(th) ~ 1.0V to 2.0V), meaning the LM358 (which can swing its output close to ground but struggles to reach the positive rail) can easily drive it into the linear region without needing a separate gate driver IC.
Behavior Matrix: What Changes When Elements Shift
Understanding how component tolerances and failures affect the loop is critical for debugging. Here is how the circuit reacts to physical changes.
| Element Changed | Direction of Change | Circuit Response |
|---|---|---|
| Shunt Resistor (R_shunt) | Increases (heats up) | Trip point lowers (trips at a lower current). |
| Reference Voltage (V_REF) | Increases | Trip point raises (allows more current before limiting). |
| Load Resistance | Decreases (short circuit) | MOSFET enters deep linear region, dropping almost all V_IN across its Drain-Source junction. Heatsink required. |
| Gate Pull-down Resistor | Removed | Circuit may latch in the "ON" state during power-up transients before the op-amp stabilizes. |
Failure Extremes: What Breaks at the Limits?
If the Shunt Opens: The physical circuit is broken, no current flows. Safe.
If the Sense Wire to the Inverting Input Opens: This is the catastrophic failure mode. The op-amp's inverting input floats or reads 0V. The op-amp assumes current is zero, rails its output high, and turns the MOSFET on completely hard. The load receives unregulated, unlimited current.
The Fix: Always place a 100kΩ pull-down resistor on the MOSFET gate to GND, and a 1kΩ series resistor between the op-amp output and the gate. If the op-amp fails or the sense wire breaks, the gate pull-down ensures the MOSFET defaults to the OFF state.
Step-by-Step Breadboard Testing Procedure
Do not connect your expensive microcontroller or final load until you have verified the trip point with a dummy load. Follow this exact sequence:
- Pre-set the Reference: Before applying power, use your multimeter to measure the wiper of the 10kΩ trimpot. Adjust it down to 0.00V. This ensures the circuit powers up in the "OFF" (limiting) state.
- Power the Control Logic: Apply 12V to the main rail and ensure the LM7805 is outputting a clean 5.0V to the trimpot.
- Connect a Dummy Load: Use a 12V automotive incandescent bulb (e.g., an 1156 brake light bulb, which draws about 2.1A at 12V). This acts as a perfect, self-regulating dummy load that will safely test the 2A threshold.
- Monitor the Shunt: Set your DMM to the millivolt range and connect the probes directly across the shunt resistor legs (Kelvin sense).
- Dial in the Threshold: Slowly turn the trimpot clockwise. Watch the DMM. As the voltage across the shunt approaches 200mV (0.2V), the bulb will stop getting brighter. The MOSFET is now actively regulating the current.
- Verify the Limit: To prove it works, intentionally short the dummy load with a piece of thick wire. The voltage across the shunt should remain pinned at exactly 200mV, and the MOSFET will get warm as it dissipates the short-circuit power. Remove the short, and the bulb will instantly return to full brightness.
Overload Current Protection Circuit FAQ
Can I use this overload current protection circuit for AC mains voltage?
No. This topology is strictly for low-voltage DC applications (typically under 48V DC). The low-side MOSFET configuration ties the load's ground reference to a switching element, which is dangerous and non-compliant for AC mains. Furthermore, the LM358 cannot handle the common-mode voltages present in 120V/230V AC systems. For AC mains overload protection, you must use an isolated current sense transformer, a dedicated AC protection IC, or a standard thermal-magnetic circuit breaker.
Why does my overload current protection circuit oscillate or ring when it trips?
Oscillation (often heard as a high-pitched squeal from the load or seen as ringing on an oscilloscope) happens because of parasitic inductance in the breadboard traces and the high gain of the op-amp. When the MOSFET throttles the current, the magnetic field in the load (especially if it's a motor or relay coil) collapses, inducing a voltage spike that confuses the op-amp's feedback loop. To fix this, add a 100Ω resistor in series with the op-amp output and the MOSFET gate to slow down the switching edge, and place a 10nF to 100nF ceramic compensation capacitor directly between the op-amp's output and its inverting input to roll off the high-frequency gain.
How do I calculate the power dissipation for the shunt resistor during a sustained fault?
During a hard short circuit, the load resistance drops to near zero. The circuit will clamp the current to your set limit (e.g., 2A). The power dissipated by the shunt remains constant at I_limit² × R_shunt (e.g., 2² × 0.1 = 0.4W). However, the MOSFET will absorb the brunt of the fault energy. If your supply is 12V and the load is shorted, the MOSFET will drop nearly 12V at 2A, dissipating 24W. You must mount the MOSFET to an adequate heatsink, or the silicon will reach its 175°C thermal shutdown limit and fail in seconds. For high-voltage or high-current faults, consider adding a secondary PTC or a fast-blow fuse to handle sustained dead-shorts while the active circuit handles brief overloads.
