The principle and use of the test pencil - Solutions - Huaqiang Electronic Network

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The test pencil is one of the tools that most electricians often use to determine whether an object is charged. Its internal structure is a bulb with only two electrodes. The bubble is filled with helium, commonly known as helium, with one pole connected to the tip of the pen and the other pole connected in series with a high resistance to the other end of the pen. When the voltage between the two poles of the bubble reaches a certain value, a glow is generated between the two poles, and the intensity of the glow is proportional to the voltage between the two poles. When the voltage of the charged body to the ground is greater than the glow voltage at the beginning of the bubble, and the tip of the pen of the test pen is in contact with it, the other end is grounded through the human body, so the test pen will emit light. The effect of the resistance in the test pencil is to limit the current flowing through the body to avoid danger. 
In addition to judging whether an object is charged or not, the test pencil has the following uses:
(1) It can be used to carry out the low-voltage nuclear phase and measure whether any wires in the line are in phase or out of phase. The specific method is: standing on an object insulated from the earth, holding a test pencil with both hands, and then testing on the two wires to be tested. If the two test pencils are bright, the two wires are different. Phase; otherwise, it is in phase, which is determined by the principle that the voltage difference between the two electrodes in the test pencil is proportional to the intensity of the light.
(2) Can be used to distinguish between AC and DC. When testing with a test pencil, if both poles in the test pencil bubble are illuminated, it is alternating current; if only one of the two poles emits light, it is direct current.
(3) It is possible to judge the positive and negative poles of direct current. Connect the test pencil to the DC circuit for testing. The pole where the bubble is bright is the negative pole, and the pole that is not bright is the positive pole.
(4) Can be used to determine if the DC is grounded. In the DC system that is insulated from the ground, the tester can be used to contact the positive or negative pole in the DC system. If the test pencil does not illuminate, there is no grounding. If the bubble is bright, it indicates that there is a grounding phenomenon, and if it is bright as at the tip of the pen, it means that the positive electrode is grounded. If it is lit at the finger end, it is grounded at the negative pole. However, it must be pointed out that in a DC system with a grounded supervisory relay, this method cannot be used to determine if the DC system is grounded.