Crowbar transient voltage suppression Devices
Spark gaps, gas discharge tubes (GDTs), thyristors and thyristor surge protective devices (TSPDs) are TVS devices that are capable of attenuating very large surge currents.
When these devices are switched “ON”, the protected circuit is connected to ground through a very low impedance switch.
Crowbars are difficult to turn “OFF” and often require an additional commutation circuit, especially in a DC system.
Spark gaps are constructed from two carbon block electrodes that are connected to the signal line and ground. The electrodes typically are separated by an air gap of 3 to 4 mils. The surge voltage causes an arc to form that shunts the transient voltage to ground.
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| Spark gap |
GDT devices are similar to spark gaps; however, they are constructed with a glass or
ceramic tube that contains an inert gas which ionizes and conducts during a transient event.
If a voltage across the device reaches the breakdown or sparkover voltage, the gas ionizes and the device “fires”. At this point, the GDT provides low impedance and remains in the “ON” state until the voltage falls below the holdover voltage.
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| GDT |
Thyristors are constructed with four layers of P− and N−type semiconductor material. A thyristor surge protection device can be created by combing a SCR and a Zener diode, where the Zener is used to control the gate terminal’s turn−on
voltage.
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| Thyristor |
TSPD devices are another popular thyristor option for surge protection. TSPDs are a two terminal bidirectional device that has a junction capable of handling very high surge currents.
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| Comparison Table |
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