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Solid state relays (SSRs) are widely used in industrial and instrumentation applications due to their long operational life, silent switching, and high reliability. When SSRs are used to control inductive loads such as motors, solenoids, transformers, or coils, they can be exposed to overvoltage transients generated during switching events. If these transient voltages exceed the maximum DC or peak AC rating of the relay, permanent damage or early-life failure can occur.
A new application note from Coto Technology examines how these transient conditions arise and outlines practical protection methods to improve system robustness and long-term reliability.
Understanding the Transient Risk
Inductive loads store energy in a magnetic field. When current is interrupted, this stored energy must be released, often resulting in a high-voltage spike across the switching device. In SSR-based designs, these spikes can exceed the relay’s breakdown voltage in a very short time, stressing internal semiconductor structures. Transients may also be introduced from external sources such as power supply disturbances or nearby switching events.
Suppression Device Options
There are several common transient suppression technologies, each with distinct electrical and mechanical characteristics:
- Zener diodes (typically used in back-to-back configurations for bi-directional protection) provide fast response and low voltage overshoot in compact, surface-mount packages.
- Metal oxide varistors (MOVs) offer high surge current capability at relatively low cost, with higher capacitance and leakage current as design considerations.
- Gas discharge tubes (GDTs) deliver very high surge handling with low leakage and capacitance, commonly used where isolation and open-circuit failure modes are preferred.
- Semiconductor crowbar devices combine fast clamping with a crowbar action, effectively diverting excess energy once a threshold is reached.
No single device suits all applications, and selection depends on operating voltage, allowable leakage current, capacitance limits, surge energy levels, and acceptable failure modes.
Importance of Suppression Device Placement
The placement of the suppression device is as important as which device is chosen. Recommended approaches include protection across the SSR output, across the inductive load, or a combination of both. Dual placement can be beneficial in systems exposed to both load-generated transients and external supply surges, helping to protect both the relay and the load itself.
Rhopoint Components – Design‑Led Coto Technology Distributor in the UK
For engineering teams, effective transient protection enables the use of SSRs closer to their optimal operating range while maintaining reliability. Proper suppression reduces field failures, improves system uptime, and avoids unnecessary overspecification of switching components.
As a design‑led enigneering distributor, Rhopoint Components supports UK engineers in specifying and designing‑in Coto Technology SSRs. Our engineering team helps with part selection, optimisation, and guidance regarding transient suppression strategies for demanding industrial and instrumentation applications.
Technical guidance is available to help align protection methods with electrical, mechanical, and environmental requirements.
