Telephone Guard Circuit

Here is a simple circuit to protect your Telephone, EPBAX, Telephone Modem etc from lightning discharge or line spikes. It uses a safety capacitor and Gas Tube Arrester to prevent damage of the devices.

The circuit is too simple but its use is great. The main elements of the circuit are a Gas Discharge Tube (GDT) and a safety capacitor.

Gas Discharge Tube

The GDT used in the circuit is NS2R – 230 A1 – N make 2 pole Arrester. It is a 2 electrode Ceramic Gas Tube Arrester rated 5KA / 5A. GDT is a surge protection device do the same job as an MOV. GDT divert the extra current from the hot line to the ground line using an inert gas as the conductor between the two lines.

When the voltage is at a certain level, the gas inside the GDT remains non conducting. When the voltage surges above that level occurs, the electrical power is strong enough to ionize the gas, making the gas a very effective conductor. It then passes current to the ground line until the voltage reaches normal level. Then again GDT becomes a poor conductor.

2 Electrode GDT

Safety Capacitor

Safety Capacitors are designed to withstand high impulse voltages in applications where human beings might be exposed to voltage surges. These capacitors will shunt the energy from the impulse to ground, protecting the circuit from the surge. Safety capacitors are available in ceramic disc and metalized film or paper. The film type are made of self-healing metalized polyester, polypropylene or paper and usually come in a “box” style casing as the capacitor is encased is a flame retardant or flame-proof case. The safety capacitor used in the circuit is 220 K 250Volt DC

220K 250 V DC Capacitor

The circuit is self explanatory and easy to make on a piece of Perf board.You can also do lead-to-lead soldering. Enclose the circuit in a small box and connect to the telephones lines and phone / modem as shown in the diagram.

Telephone Guard Circuit Diagram


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