A method for measuring electrical equipment chassis leakage with a multimeter.
If the metal casing of electrical equipment is well insulated and reliably grounded, there will be no leakage. However, sometimes the equipment has poor insulation or unreliable grounding (it is common in factories to connect the ground wire of the equipment directly to metal sprayed with plastic powder). On the casing), the metal casing will be charged at this time, so we can use the AC voltage range of the digital multimeter to measure the voltage between the metal casing and the ground to know whether the equipment is leaking.
When measuring, set the digital multimeter to the AC 200V range, touch the red test lead to the equipment case, and the black test lead to the ground. If the multimeter shows a voltage of more than dozens of volts, it means that the insulation performance of the equipment is not very good, or the grounding is unreliable. . If the measured voltage reaches 100 volts, you will generally feel an electric shock when you touch the metal casing of the device. At this time, we can measure the leakage current again. Set the multimeter to the AC 2mA (or 20mA) range, touch the red test lead to the equipment case, and the black test lead to the ground. The current displayed at this time is the leakage current, and its size is generally at μA level.
How to use a multimeter to measure the quality of a three-wire sensor
Connect the sensor, 24V power supply, and multimeter to form a circuit, and set the multimeter to the DC setting. The blue wire is connected to the negative power supply, the brown wire is connected to the positive power supply, the black wire is connected to the black or red test lead of the multimeter, and the remaining test lead is connected to the positive or negative terminal of the power supply.
At this time, when the NPN or PNP sensor is working, the voltage measured by the multimeter is the power supply voltage, which should be 24VDC. When the remaining test lead of the multimeter is connected to the positive terminal of the power supply, and an object is close to the sensor, the multimeter has a 24V voltage display, indicating that the sensor is an NPN sensor and is a normally open type. When no object is close, the multimeter has a 24V voltage display, indicating that It is an NPN type sensor and is normally closed.
If the remaining test lead of the multimeter is connected to the negative terminal of the power supply, and there is an object approaching the sensor, and the multimeter has a 24V voltage display, it means that the sensor is a PNP type sensor and is a normally open type. There is no object approaching the sensor and the multimeter has a 24V voltage display. It shows that it is a PNP type sensor and it is a normally closed type.
Therefore, for this problem, if the sensor used cannot see the mark, the only reliable way to judge is through measurement. But remember, the NPN sensor is also a proximity switch. It outputs a low-level signal, which is the negative pole of the 24V power supply. The PNP sensor outputs a high-level signal, which is the positive pole of the 24V power supply.
