6.8.2Technologies#
Fundamental characteristics of circuit components, including filtering and Surge Suppression common to transient voltage surge suppression device technologies, are summarised as follows:
6.8.2.1Gas discharge tubes#
These devices consist of a glass or ceramic tube filled with inert gas and sealed at each end with a metal electrode. Breakdown voltage is in the range 70 V to 1 kV. Typical surge current ratings are 5 kA, 10 kA and 20 kA and can be as high as 100 kA. Conduction voltage is much less than the firing voltage. GDTs are widely used in signal line protection and with MOVs in power circuits.
Key features are:
- response is somewhat inconsistent and a bit non-linear
- speed is slow
- let-through can be high
- capacitance is negligible
- energy capability and dissipation is high
- follow-on current is high; requires method to switch off
- leakage current is negligible, and
- the GDT generally fails to open.
6.8.2.2Spark gap#
These are similar to GDTs but incorporate air instead of the inert gas. They are rugged and can handle high surge energy but have high firing voltage. They require a mechanism to extinguish the arc and switch off. Spark gaps are used in extremely high lightning risk areas as the primary protection in surge suppression devices.
Key features are:
- unpredictable turn-on and response characteristics
- very slow to fire or ‘spark over’
- low capacitance
- high energy capability
- extremely high energy dissipation
- design is required to prevent follow-on current, and
- low leakage.
6.8.2.3Metal oxide varistor#
A varistor is a voltage-dependent resistor made of metal oxide particles (usually zinc) compressed together. The resistance drops significantly when the voltage exceeds a limit and the voltage is clamped near that limit. They can handle surges in the range of three to 100 kA and respond in tens of nanoseconds. These are the most commonly used device for power surge protection.
Key features are:
- high device capacity
- response is fast but non-linear
- high power handling capability
- most transient energy is dissipated as heat
- follow-on current is low except when the device fails, then quite high
- leakage is high
- MOVs’ performance degrades with exposure to transients, and
- MOVs’ fails short when overstressed, then follow-on current normally causes catastrophic rupture and an open circuit.
6.8.2.4Silicon avalanche diode#
This is a specialised semiconductor device that acts like a zener diode in turn on and current avalanche mode. However, the silicon avalanche diode uses a very large silicon chip sandwiched between large metal pellets, giving it thousands of times more current carrying capability than a zener. Transient voltage suppression diodes are often used in high speed but low power circuits, such as data communications.
Key features are:
- fastest turn-on of any device available
- response is essentially linear
- capacitance is low
- energy capability and dissipation are low
- leakage is extremely low
- follow-on current is nil except, should the device fail, and
- silicon avalanche diode devices fail short.
6.8.2.5Thyristors#
These thyristor-family devices can be viewed as having characteristics similar to a spark gap or a GDT, but can operate much faster. They are related to transient voltage suppression diodes, but can ‘breakover’ to a low clamping voltage analogous to an ionised and conducting spark gap. After triggering, the low clamping voltage allows large current surges to flow while limiting heat dissipation in the device. They are used in high power devices.
Key features are:
- response is sharp, predictable turn-on and linear within specified power limits
- speed is fast
- low capacitance
- high-energy capability
- low energy dissipation due to ‘crowbar’ effect and very low device resistance after turn-on
- follow-on current is high until device is turned off
- leakage is low, and
- failure mode is a short circuit.
