Overload Voltage Explained: Thresholds, Sensing, and Protection

Overload voltage is the specific threshold voltage measured across a current-sensing element or the voltage drop across a load that indicates a circuit is drawing sustained power beyond its safe thermal rating, triggering protective shutdown. If you are building power supplies, sizing motor drives, or debugging a battery management system (BMS), you need to understand that an overload is fundamentally a thermal problem, and the voltage signal is just the messenger. People commonly confuse it with overvoltage (a spike in supply voltage from the grid or inductive kickback) and short-circuit current (an instantaneous, near-infinite current draw). Overload is slow, thermal, and relies on precise voltage sensing to prevent your components from melting.

What Overload Voltage Actually Means in a Circuit

In power electronics, we rarely measure the actual temperature of a copper trace or a MOSFET die in real-time to detect an overload. Instead, we measure current. But microcontrollers and analog comparators cannot read current directly; they read voltage. Therefore, an 'overload' is detected by looking for a specific overload voltage across a known resistance.

When a load pulls more current than its continuous rating, the voltage drop across the sensing resistor increases. Once that voltage drop crosses a predefined threshold—the overload voltage—the control circuitry intervenes. This intervention might look like foldback current limiting in a bench power supply, a hiccup-mode shutdown in a switching regulator, or an electronic thermal trip in a Variable Frequency Drive (VFD). What it changes in a real circuit is the state of the gate drivers: the protection circuit pulls the PWM enable pin low or physically opens a solid-state relay, starving the load of power before the I²R heating destroys the silicon.

The Physics: How Current Overload Creates Voltage Signals

To see how this works on the bench, let us look at a worked numeric example using a standard linear power supply design.

If you are designing this yourself, selecting the right shunt is critical. A 5mΩ resistor at 12A dissipates 0.72W (P = I²R). You must use a 2W or 3W Kelvin-package shunt resistor to prevent the sensing element itself from drifting in value due to self-heating, which would falsely alter your overload voltage threshold. For deeper design insights on selecting these components, Texas Instruments' guide on current sense amplifiers provides excellent reference schematics for high-side vs. low-side sensing.

Where You Meet This in Practice

You will encounter overload voltage thresholds in almost every piece of modern power conversion equipment. Here is where they matter most:

• Bench Power Supplies: Adjustable linear and switching supplies use overload voltage sensing to implement constant current (CC) mode. When the load resistance drops and tries to pull more current, the supply transitions from constant voltage (CV) to CC mode, clamping the output by monitoring the shunt voltage.
• Battery Management Systems (BMS): A 4S LiFePO4 BMS uses a bank of parallel MOSFETs for discharge control. The BMS IC monitors the voltage drop across the MOSFETs' Rds(on) or an external shunt. If the voltage exceeds the discharge overcurrent threshold (often around 80mV to 100mV) for a sustained period, the BMS turns off the gate drive to prevent thermal runaway.
• Variable Frequency Drives (VFDs): VFDs do not just look at instantaneous current; they calculate an electronic thermal model of the motor. The internal DC bus shunt generates a voltage signal that the microcontroller integrates over time (I²t algorithm) to trip the drive before the motor windings melt.

Real-World Scenario: When a 24V PSU Foldback Fails

Theory is clean; the workbench is not. Here is a walkthrough of a real-world failure involving an adjustable switching power supply where the overload voltage threshold was improperly modified.

1. Setup: A DIY CNC router builder uses a 24V, 15A switching power supply (similar to a Mean Well SE-350-24) to drive three NEMA 23 stepper motors. The supply features an internal overload protection circuit set to trigger in 'hiccup mode' if the load exceeds 110% to 150% of the rated output.
2. Numbers: Inside the supply, the current limit is set by a trimpot that adjusts the reference voltage fed to a comparator. Factory calibration sets the overload voltage threshold to 75mV across an internal 5mΩ shunt, meaning the supply will shut down at 15A. The builder, wanting more torque from the Z-axis motor, uses a small flathead screwdriver to turn the current-limit trimpot, raising the overload voltage threshold to 100mV (effectively telling the supply to allow 20A continuous draw).
3. Outcome: During a rapid Z-axis plunge, the stepper motor binds in the collet. The motor stalls, dropping its back-EMF to zero and acting like a dead short. The power supply attempts to push 20A. The internal shunt hits 100mV, but the primary switching MOSFETs are only thermally rated for 15A continuous with the factory heatsink. The MOSFETs overheat, fail short-circuit, and the 120V AC mains voltage is dumped directly onto the 24V DC bus, frying the CNC controller board.
4. What Went Wrong: The builder treated the overload voltage threshold as a simple 'performance dial' without understanding the thermal limits of the physical components. By raising the trip voltage, they bypassed the thermal protection margin. The protection circuit did exactly what it was told to do—it waited for 100mV—but the silicon melted at 85mV. Never adjust internal current-limit trimpots on enclosed power supplies without verifying the thermal capacity of the internal pass elements.

Overload Voltage vs. Overvoltage vs. Short Circuit

Because the terminology overlaps, it is vital to separate these three distinct electrical faults. The following matrix clarifies how protection circuits handle each condition. For a broader look at how these faults are managed in AC wiring, refer to the Electronics Tutorials breakdown of overcurrent protection.

Condition	Primary Cause	Timeframe	What the Voltage Does	Protective Device Action
Overload	Mechanical binding, undersized wire, excessive continuous draw	Slow (Seconds to Minutes)	Sensing voltage slowly rises past the thermal threshold	Foldback, hiccup mode, or thermal relay trip
Short Circuit	Phase-to-phase or phase-to-ground dead short	Instantaneous (Microseconds)	Sensing voltage spikes instantly to maximum ADC range	Magnetic breaker trip, fast-blow fuse, desaturation protection
Overvoltage	Lightning surge, inductive kickback, grid switching	Transient (Nanoseconds to Milliseconds)	Supply voltage exceeds the dielectric rating of components	TVS diode clamping, MOV shunting, OVP crowbar circuit

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