Overvoltage crowbar protection is a protective circuit that rapidly forces a power rail into a safe state when voltage rises above a defined threshold. Unlike “clamp” protection that limits voltage by absorbing energy, a crowbar typically creates a deliberate low-impedance path (effectively a controlled short) that triggers upstream protection—such as a fuse or circuit breaker—to disconnect power and prevent damage to sensitive electronics.
Crowbar protection is commonly implemented using devices such as:
– SCR (thyristor) crowbars
– TRIAC-based crowbars (primarily AC applications)
– Trigger circuits (zener/reference + comparator) that detect the overvoltage event
Why Crowbar Protection Matters in EV Chargers
EV chargers contain sensitive electronics in their power supplies, control boards, communications modules, metering, and safety circuits. Crowbar protection can help:
– Protect low-voltage rails (e.g., 5 V, 12 V, 24 V) from regulator failures
– Prevent catastrophic damage from PSU faults or miswiring
– Reduce fire risk from overheated components during sustained overvoltage
– Improve robustness against abnormal conditions in industrial environments
– Support compliance and reliability targets by limiting fault propagation
How Overvoltage Crowbar Protection Works
A typical crowbar protection sequence is:
– The circuit monitors a DC rail voltage continuously
– If voltage exceeds a threshold (e.g., due to a failed regulator or incorrect supply), a trigger fires
– An SCR (or similar device) latches on and shunts the rail to ground
– The sudden high current causes a fuse to blow or a current limit to trip, removing power
– The crowbar remains latched until power is removed and the fault is cleared
Because the crowbar acts quickly and decisively, it is especially effective against sustained overvoltage events.
Crowbar vs Voltage Clamping
Crowbar protection is often contrasted with clamping devices such as TVS diodes or MOVs:
– Crowbar: triggers a short and forces power to disconnect (latching, “hard stop”)
– Clamp: limits peak voltage by absorbing/transferring energy (non-latching, “soft limit”)
In practice, systems may use both: clamping for short transients and crowbar for sustained faults.
Where Crowbar Protection Is Used in Chargers
In EVSE and charging power electronics, crowbars are most commonly found on:
– DC auxiliary power supplies feeding logic and communications
– Critical control rails for safety functions (contactors, monitoring circuits)
– Gate-drive or control rails where overvoltage could cause uncontrolled switching
It is less common as the primary protection method on high-energy AC mains inputs, where other approaches (SPDs, fusing, isolation, and power-stage protection) dominate.
Key Benefits
– Very fast response and strong protection against sustained overvoltage
– Simple, proven design approach for protecting sensitive rails
– Forces a clear fault condition that is easy to detect (fuse blown / supply shut down)
– Prevents downstream component overstress and cascading failures
Limitations and Design Considerations
– Requires correct coordination with fuses, current limits, and wiring to avoid unsafe heating
– Can cause abrupt shutdown and service interruption (by design)
– Must be rated to handle the surge current until protection opens
– Not ideal for frequent nuisance trips; thresholds and filtering must be well designed
– Latching behavior means the system will not recover until power is cycled and the fault is removed
Related Glossary Terms
Surge Protection Device (SPD)
Transient Voltage Suppressor (TVS) Diode
Metal Oxide Varistor (MOV)
Overvoltage Protection (OVP)
Fuse Coordination
Power Supply Unit (PSU)
Fault Detection
Firmware Fault Handling
IEC 61851
IEC 62196