The working principle and usage method of the contact test pen
The test lead is the most commonly used tool for testing whether there is electricity in the wire. According to the level of the measured voltage, it can be divided into: high voltage test pencil (10kv and above), low voltage test pencil (500v and below) and weak current test pencil (6-24v); according to the contact method can be divided into: contact test pencil and induction test pen.
According to the description, here we only analyze and discuss the contact type low-voltage tester. Contact low-voltage tester, used for the detection of charged objects with a line voltage of 500v and below.
The internal structure of the test pen is a "light bulb" with two electrodes, which is filled with neon gas, commonly known as a neon bulb; one pole of it is connected to the tip of the pen, and the other pole is connected in series with a megohm-level resistor, which is connected by a spring to the other end of the pen.
① In the loop, the resistance of the safety resistance R1 in the electric test pen is several megohms, the resistance of the human body resistance R2 is several hundred to several thousand ohms, and the resistance of the insulator on the sole of the foot or the insulator on the ground is R3. The resistance is quite large. C is the capacitance formed between the human body and the earth, because 220v alternating current is added between the live line and the neutral line, the alternating current on the capacitor branch is very little hindered, and the current "passes through" the capacitor C and returns directly to the neutral line, and passes through the human body The current drawn by R2 is tiny (typically less than 1mA).
② The current passes through the pen tip, resistor, neon bulb, spring, and then reaches the human body, that is, the human body becomes a charged body; the human body and the earth form a capacitor, and the capacitor allows alternating current to "pass"; there is a potential difference between the two poles of the neon bulb, and there is an electric field if there is a potential difference; The neon gas in the neon bulb is under the action of the electric field, and the glow discharge emits red light, that is, the neon bulb is bright; the intensity of the red light is proportional to the voltage between the two poles.
③ If the nib is in contact with the zero line, there is no potential difference between the zero line and the ground, which is an equipotential body; there is no potential difference at both ends of the neon bulb, and there is no electric field at the two poles, and the neon bulb will not emit light.
Therefore, when the test pen touches the live wire of AC220v, the neon bulb will light up, but it will not light up when it touches the neutral line.
It is absolutely safe to use a (contact low-voltage) test lead: ① The safety resistance inside the test lead is a megohm-level resistance, and the resistance is usually around a few megohm; ② The resistance of the human body is very small, usually only a few hundred to a few thousand ohms. Therefore, in the test of the low-voltage charged body, the current passing through the human body is very small, usually less than 1 milliampere, and there is no harm to the human body; but as long as the potential difference between the two ends of the neon bulb is sufficient, the electric field can make the neon bulb of the test pen glow.
Special emphasis: the use method and precautions of the test pen,
① Before use, first visually check whether there is any damage, check whether there is a safety resistor in the tester, whether it is damp, and whether it has entered water.
② Confirm whether the neon bulb can emit light: find a known power source to test and verify.
③ Do not touch the metal probe of the test lead with bare hands.
④ Be sure to touch the metal part at the end of the test pen with your hands.
⑤ When testing the charged body under bright light, special attention should be paid to whether the neon bulb is really bright or not. If necessary, the other hand can be used to block the light, carefully judge, and must not be misjudged.
