Familiarize Electronic Components Part XV – MOV and GDT

Metal Oxide Varistor (MOV) and Gas Discharge Tube ( GDT ) are components used as Spike protectors or Surge arrestors in power supply circuits and Lightning protectors. These devices can protect the electronic devices from high inrush current during a voltage surge. The voltage surge usually occurs when high current inductive loads switches On or when the power resumes after a power failure. Very high momentary voltage surge develops in the power lines when power resumes after a power failure due to very high magnetic field in the distribution transformer. Lighting also results in very high voltage surge since the voltage in the lighting strike may have 5 kV or more.

TV, Computer etc may damage if voltage spikes appear in the power lines. SMPS power supply of Computer and Mobile chargers is highly vulnerable to such voltage spikes( A spark in the socket while plugging can damage a mobile charger)  Very high voltage spikes and transients develop momentarily in the supply lines when lighting occurs. Such short duration high voltage spikes also super impose on the mains when high current loads are switched on or when power resumes after a power failure due to high magnetic field in the distribution transformer. Even though the spikes are short living, they can cause permanent damage in appliances .

How MOV Protects?

Metal Oxide Varistor (MOV) contains a ceramic mass of Zinc oxide grains, in a matrix of other metal oxides such as small amounts of bismuth, cobalt, manganese etc. sandwiched between two metal plates which forms the electrodes. The boundary between each grain and its neighbor forms a diode junction, which allows current to flow in only one direction. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows. Varistor can absorb part of a surge. The effect depends on the equipment and details of the selected Varistor.

The Varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its “clamping voltage”. If a transient pulse is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This will blow up the Fuse and protect the device.

MOV can protect Modem or Set top box from inrush current. Connect like this. Earth should be Ground earth and can be connected to the Earth of Mains if it is perfect.

Gas Discharge Tube

It is a very good device to protect Modem, Set top box etc from inrush current through the Cable during lightning. It can also protect Telephone from lighting strikes.

GDT is the Gas Discharge Tube, is similar to MOV (Metal Oxide Varistor) in function. It diverts the extra current from the hot line to the ground using an inert gas as the conductor between the two lines. It can be incorporated between the Core(Central conductor) and the Shield (Copper clad aluminium braid shield conductor) of the coaxial cable.

GDT functions as an on/off switch with high impedance in the normal voltage conditions. Under surge condition, its internal gas ionizes to turn the device to a low impedance state called Arc mode or Short circuit. In this state the GDT conducts the high voltage transients from hot line to the ground. Once the transient passes to the ground, the GDT returns to its high impedance state.

When the voltage through the cable is at a certain level, GDT will not conduct. When the voltage increases above this normal level due to a surge, the electrical power is strong enough to ionize the gas inside the GDT which act as a conductor to pass the high current to the ground until the voltage returns to the normal level. Again the GDT becomes a poor conductor.