EMC Question of the Week: August 21, 2023

VI curve for transient protection device exhibiting snap-back

Which two-terminal transient protection device has a V-I curve similar to the one shown in the figure?

  1. Avalanche diode
  2. TVS diode
  3. Varistor
  4. Thyristor


The best answer is “d.” Two-terminal thyristors have a very high impedance until the voltage across them exceeds a given threshold. When the threshold is exceeded, they "turn on" allowing a large amount of current to flow with very little voltage drop. They remain "on" as long as there is a sustaining current flowing through them. When the current drops below the thyristor's holding level, the device turns off again and returns to its high-impedance state.

Thyristors and gas discharge tubes are often referred to as "crowbar" devices. When their threshold voltage is exceeded, they essentially form a short circuit (much like dropping a crowbar across the terminals). Diodes and varistors are voltage-clamping devices. When their threshold voltage is exceeded, they conduct just enough current to hold the voltage across the terminals to the threshold value.

Crowbar devices tend to be able to handle higher-energy transients. This is because the energy converted to heat in the device is the product of the voltage across the device and the current through the device integrated over time. In a crowbar device the voltage is very low when it is "on". This keeps the energy dissipated low in the presence of a large current.

Clamping devices have the advantage of being able to react to transient voltages more quickly. They are typically used to protect against very fast, low-energy transients (e.g., ESD transients). Other factors (besides speed and energy) that can influence the optimum choice of a transient protection device include terminal capacitance, size, cost, reliability and life expectancy. 

Note: The term "avalanche diode" describes a diode with a V-I curve emulating an ideal diode. The term "TVS diode" refers to any diode optimized for transient voltage suppression. TVS diodes are typically avalanche diodes (or possibly an array of avalanche diodes in a single package).

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