A transient-voltage-suppression (TVS) diode, also transil or thyrector, is an electronic component used to protect electronics from voltage spikes induced on connected wires.

The device operates by shunting excess current when the induced voltage exceeds the avalanche breakdown potential. It is a clamping device, suppressing all overvoltages above its breakdown voltage. It automatically resets when the overvoltage goes away, but absorbs much more of the transient energy internally than a similarly rated crowbar device.

A transient-voltage-suppression diode may be either unidirectional or bidirectional. A unidirectional device operates as a rectifier in the forward direction like any other avalanche diode, but is made and tested to handle very large peak currents. The 1.5KE series allows 1500 W of peak power, for a short time.

A bidirectional transient-voltage-suppression diode can be represented by two mutually opposing avalanche diodes in series with one another and connected in parallel with the circuit to be protected. While this representation is schematically accurate, physically the devices are now manufactured as a single component.

A transient-voltage-suppression diode can respond to over-voltages faster than other common over-voltage protection components such as varistors or gas discharge tubes (GDT). The actual clamping occurs in roughly one picosecond, but in a practical circuit the inductance of the wires leading to the device imposes a higher limit. This makes transient-voltage-suppression diodes useful for protection against very fast and often damaging voltage transients. These fast over-voltage transients are present on all distribution networks and can be caused by either internal or external events, such as lightning or motor arcing.

TVS diode as array

Transient voltage suppressors will fail if they are subjected to voltages or conditions beyond those that the particular product was designed to accommodate. There are three key modes in which the TVS will fail: short, open, and degraded device.