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    Electrical surges are high voltage impulses which last for a very short duration of time, typically in microseconds. These surges are also known as spikes, transients, or glitches. Surges can be introduced in an electrical system either internally or externally. Shut down of motors, electrical faults, and lightning bolts can produce surges in a system. Surges can damage the sensitive electrical/electronic equipment and therefore, adequate protection is necessary for such equipment. [1]

    The devices which are being used in electrical/electronic circuits for the purpose of surge protection are known as surge suppressors, surge protectors, or surge arrestors. The primary function of these devices is to protect the sensitive circuits from the high-magnitude short-duration voltage spikes.

    Commonly used surge suppression devices include MOV (Metal Oxide Varistor), TVS (Transient Suppression Diode), GDT (Gas Discharge Tube) and TSS (Semiconductor Discharge Tube).

    Metal Oxide Varistor

    The varistor is a non-linear, voltage-dependant device used for surge suppression. Varistors are also known as voltage dependant resistors (VDRs). At low voltage level, the resistance of a varistor is very high and virtually no current passes through it. As the voltage level rises, the resistance of the varistor decreases allowing increased current flow. The varistor is placed in parallel to the sensitive circuit and therefore, it allows a low resistance path to the surge current saving the circuit. The destructive energy is then dissipated in form of heat. Varistors are constructed from ceramic materials such as Zinc Oxide and metal plates. [2]

    The major advantages of MOVs include fast response, wide voltage range, larger flow capacity, and long life span. The disadvantages of varistors include stability issues, aging, and higher capacitance. Due to these issues, they are not suitable for RF and high-frequency applications.

    Figure (1)275VAC 6500A METAL OXIDE VARISTOR (MOV)


    Gas Discharge Tube

    The assembly of a GDT consists of a set of electrodes enclosed inside an insulated and temperature-resistant glass or ceramic casing. The casing is usually filled with an inert gas. Under normal voltage condition the gas remains non-conductive and discharge path exists. However, when the applied voltage level exceeds the dielectric strength of the gas, the molecules get ionized and a conductive path is formed. Hence, GDTs can be effectively used in electronic circuits for surge suppression. The major issue with GDTs is their slow response time which proves insufficient for surges with a steep slope. [3]

    Figure(2) SL1021A260R


    Transient Voltage Suppression Diode

    Apart from rectification and voltage regulation, diodes can also be used for surge voltage protection. These special types of diodes are known as transient suppression diodes or TVS diodes. These diodes are placed in parallel to the sensitive circuit and have very high power-dissipation characteristics (several kilo-Watts). The greatest advantage of these diodes is their very fast response time. These diodes are available in unidirectional and bidirectional configurations. The main disadvantage of TVS diodes is their high capacitance. This limits their application in high-frequency circuits. [4]

    Figure (3) P6KE TVS Diode


    Semiconductor Discharge Tube

    The solid state semiconductor devices can be used for the construction of surge suppression devices. These devices are usually based on Thyristors and are known as TSPD (Thyristor Surge Protection Device). Thyristor based crowbar circuits are used in the electronic circuits for over-voltage protection. These devices use advance ion implantation technology which ensures fast response time, high reliability, and precise conduction. [5]

    Figure (4)Basic Crowbar Clamping Circuit


    Multi-stage Protection

    Standalone surge suppression devices fail to provide adequate protection for power supplies and electronic circuits. Hence, it is important to use a combination of multiple SPDs in order to ensure effective surge protection in electronic circuits. Multiple SPDs are used at various stages of the circuit in order to provide multi-stage protection. The GDTs are often used at the first stage, the MOVs are used at 1st or 2nd stage, and the TVS diodes are used at the 3rd stage of the circuit. This scheme ensures efficient and effective surge and over-voltage protection for sensitive circuits.



    [1] bgelectrical. (2019, August) www.bgelectrical.uk. [Online]. https://www.bgelectrical.uk/circuit-protection/surge-protection-devices
    [2] P. Marian. (2011, June) www.electroschematics.com. [Online]. https://www.electroschematics.com/5224/metal-oxide-varistor/
    [3] citel. (2019, August) www.citel.us. [Online]. http://www.citel.us/gas_discharge_tubes_overview.html
    [4] Nick Davis. (2017, May) www.allaboutcircuits.com. [Online]. https://www.allaboutcircuits.com/technical-articles/transient-voltage-suppressors-tvs-an-introduction/
    [5] Vishay. (2019, June) www.vishay.com. [Online]. http://www.vishay.com/docs/88467/thyzorb.pdf


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