Electrical Neon Tester Diagram, Working Principle, Circuit

Hey, in this article we are going to see what is inside an electrical neon tester. The electrical Neon tester is also known as Phase Tester or Line Tester. The working principle of a neon tester is very simple and its internal construction also simpler and cheaper. A neon tester is a most essential tool for an electrician. Every electrician must have a neon tester with them. The main function of a neon tester is to check the presence of electricity in a conductor, busbar, socket, or any electrical circuit. The metallic rod end is designed like the head of a flat screwdriver, so the neon tester also can be used to tighten or lose screws.

Electrical Neon Tester Internal Parts and Construction

Here you can see the different parts of an electrical neon tester.

electrical neon tester parts, neon tester diagram, neon tester internal circuit, neon tester working

You can see a neon tester has the following parts,
  1. Metallic Rod
  2. Resistor
  3. Neon Lamp
  4. Spring
  5. Metallic Touch screw
  6. Insulated Transpaarent Body

Let's discuss each part of the neon tester,

Metallic Rod

As you see in the above diagram it is a cylindrical metallic rod that can conduct electrical current. Its one end is connected to the resistor and another end or mouth is free to touch the current-carrying part of an electrical circuit. So its main function is to conduct the electrical current from the phase or live conductor to the resistor. You can also see its end is designed like a flat screwdriver head. This cylindrical rod is also covered with an insulating plastic material for human safety.


The main function of the resistor in a neon tester is to block the current or limit the current. You can see in the above diagram, that the resistor is connected between the metallic rod and the neon lamp. So it opposes the high current to flow through the neon lamp or human body to protect the human and the neon lamp. Generally, more than 2 megaohm resistor is used in neon testers.

Neon Lamp

It is connected between the resistor and the metallic spring. The neon lamp consists of a small glass capsule filled with a mixture of neon and other gases. Generally, 80 percent neon gas is used. It also has two electrodes - an anode and a cathode. The main function of the neon lamp is to glow when we touch the tester with a live or phase conductor.

Why Neon Lamp used in tester instead of other lamps?

The main reason for using neon lamps in the line testers is, that the neon lamp can glow with voltage it does not require an electric current to glow. Yes, when we touch the neon tester to any live conductor a voltage is applied across the neon lamp electrodes, and the neon gas inside the lamp produces an orange glow discharge. Once the gas inside the bulb is ionized, it conducts an electric current which is limited by the series resistor.


It helps to connect the neon lamp with the metallic touch screw. Generally, the spring is used instead of any conductor or rod because when the neon tester is used for losing or tightening any screws, mechanical pressure is applied to it. So if any conductor or rod is used, the neon lamp will damage during losing or tightening of screws.

Metallic Touch Screw

It helps to make contact between the human body and the tester circuit. It also helps to hold the side clamp used to hold the tester when we keep it in our pockets.

Insulated Transparent Body

It is made of transparent hard plastic or glass. Its insulating property gives protection against touching the live part and transparent property helps to see if the lamp glows or not.

Electrical Neon Tester Working Principle

The working principle of a neon tester is very simple. When we touch its metallic rod head to a live or phase conductor and the metallic touch screw with our body, a voltage will be applied across the neon lamp and it will glow to indicate the presence of the electricity on that conductor.

Neon Tester can check voltage or current?

As I told you before, a neon lamp requires only voltage to glow a little bit, so a neon tester can voltage not current.

At how much voltage a neon tester can work?

A new neon tester can work or indicate the presence of electricity at the minimum voltage of 80V, after a long time of use neon tester can work properly above 100 volts or even above 150V. Generally, neon testers are designed for 230V circuits. So don't go to check high-voltage circuits with a neon tester.

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Electrical Neon Tester Diagram, Working Principle, Circuit Electrical Neon Tester Diagram, Working Principle, Circuit Reviewed by Author on February 21, 2021 Rating: 5
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