Here is a Power Supply Guard circuit that can be used as a protective device in power supply boxes. This type of Spike protection circuit is present in the multi socket switch boards used to connect computer peripherals. This circuit is useful to guard the electronic or electrical devices from mains transients and spikes. If heavy transients develop in the mains, the MOV in the circuit short circuit the lines and the fuse will blow out.
The Power Supply Guard circuit uses a Metal Oxide Varistor 431KD14, a 0.1 uF X2 rated AC capacitor, a Toroidal transformer and two 2KV disc capacitors.
Metal Oxide Varistor
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 neighbour 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 large current flows. Varistors can absorb part of a surge.The effect depends on the equipment and details of the selected varistor.
A 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.
Toroidal transformers are built around a ring-shaped core. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core’s magnetic field from generating electromagnetic interference. In the circuit, the toroidal transformer is made using primary and secondary winding each having 20 turns of 22 SWG enameled copper wire. The core can be procured from the damaged power supply boards of UPS or SMPS.
Power Supply Guard Circuit
2 KV Capacitor
0.1 uF X rated AC Cap 250 volt
Power supply Guard – Assembled Board