Questions and answers about test pens
1. Why do neon tubes glow?
2. Since there is current flowing through the human body when using the test pen, why don't people get an electric shock?
3. In fact, wearing insulated shoes can also make the test pen light up, but isn't it disconnecting the path between people and the earth? Why is it still bright?
First question: A neon tube is a small glass tube with two electrodes filled with low pressure neon gas. The characteristic of neon gas is that the gas will be ionized when it bears a certain high enough voltage, and free charges will be generated in the gas, and the free charges will move in a directional manner to form a current, and the accompanying glow will be generated when ionized.
Therefore, when the voltage between the poles of the neon tube increases to a certain value (called the starting voltage, generally 60-90 volts), the gas in the tube conducts electricity, and a beautiful glow discharge phenomenon occurs. The characteristic of glow discharge is that the current is small and the temperature is not high. During normal glow discharge, its voltage does not change with the current. In a word, neon tube luminescence is a gas ionization discharge phenomenon of neon gas under high voltage.
The second problem: the reason for no electric shock is that the current is too small. Someone will definitely ask, how small is it? The human body can feel even a few milliamperes of current. So how small is it? Let's do the math. When a person uses the electric pen, the live wire of the power supply is in contact with the pen tip. If it is in good contact with the ground, the live wire and the internal resistance of the electric test pen, the neon tube, the human body and the ground form a series circuit. The resistance in the test pen is generally about 1 megohm, the resistance of the human body is generally about 1 thousand ohms, the starting voltage U0 of the neon tube is 80 volts, and the power supply voltage is 220 volts, so it can be calculated (220-80)/(1000000 +1000) = 0.14mA. However, the current that people can feel is above 1 mA, and the current that causes electric shock is above 10 mA, both of which are far greater than 0.14 mA. Therefore, we can conclude that the current passes through the human body when using the electric measuring pen, but because of The current is too weak, so people can't feel it at all, let alone cause an electric shock.
The third question: You can see the neon tube glow even if you wear insulated shoes. Although the insulated shoes isolate the human body and the earth, the passage still exists. Because between the human body and the ground, there is equivalent to a distributed capacitance (about hundreds of picofarads when it is close to the ground). In an alternating current circuit, a capacitor, like a resistor, can pass current. Although this tiny distributed capacitance between the human body and the earth has a great hindering effect on the 50 Hz alternating current, it can still generate a current of tens of microamps in the circuit (the calculation method is the same as the method in the second question, the capacitance Capacitive reactance 1/wc can be calculated by yourself), so that the neon tube glows. Of course, if people test in the air far away from the ground and walls, because the distributed capacitance between people and the earth is very small, the capacitive reactance is very large, and the neon tube in the test pen It might not shine.
