How to use a multimeter to measure the quality of AC contactor coils?
The quality of the contactor coil can be judged by measuring the resistance value with a multimeter. Generally, the organization of the contactor coil is several hundred ohms. If the organization you measure is very small (such as a few ohms and does not close after being powered on), it is a short circuit in the coil; If the organization is very large (such as infinite), it is a broken coil.
In addition, it is necessary to check the voltage of the coil entering the contactor to prevent the coil from not engaging due to loose screws, or the contactor from not engaging due to a broken wire entering the contactor coil.
Measure the coil using a digital multimeter with an ohmmeter range of 2K or below to see if it is open circuit. If it is open, it indicates that the coil is damaged.
Measure the normally closed and normally open contacts using a multimeter with an ohm range of 200 (hold or power on the auxiliary contacts to measure the normally open contacts). If the contact is not good, the measured resistance will be high. Indicates poor contact of the contacts. If the auxiliary contacts are connected to a load, the voltage may not reach the rated voltage when measuring the voltage.
Measure each normally open and normally closed terminal with a multimeter to see if it is normal. If a multimeter is not suitable for measuring the coil, the best way is to directly apply the rated voltage and test the operation.
What is the situation where the forward resistance value of the high-voltage diode measured with a mechanical multimeter is 0 and the reverse resistance value is present
A mechanical meter generally refers to a pointer meter, which uses a resistor to shunt the current flowing through the pointer meter head to the rated value of the meter head, which is exactly the full range.
High voltage diodes can be connected in series with multiple secondary high voltage diodes, or they can be sealed in one package after being connected in series. GTR (transistor) type, if it is a single diode, it will be made of materials and processes with high voltage resistance, which cannot be measured by a general multimeter.
The pointer meter Rx10k uses an additional 10F20 15V battery to measure resistance above 100k.
Multiple high-voltage diodes in series, if a single tube is 1000V and withstand voltage of 20000V, 20 diodes need to be connected in series. Each tube has a voltage drop of 0.7V, and 20 diodes are 14V. Therefore, most meters can only measure diodes within 10000V, and higher withstand voltage requires the instrument itself to have a higher voltage.
If the forward measurement is zero, it indicates breakdown, and the normal value is 7k for 10000 volts. (Zero should be set first, otherwise the table may explode)
The reverse measurement is 17.2K, accounting for one-third of the dial. Normally, it should be infinite, but if it is damaged, it should also have zero resistance. If there is no physical object, it can only be inferred that the tube has been broken down, and a large amount of carbon powder has appeared. When measuring in the reverse direction, the diode value is partially partially broken but not broken down. Gently tapping the diode with a tool should cause the value to drift. In the 10K range, if a diode is not or partially broken down and the value itself is 700 Ω, the measuring instrument will display itself as zero.
Another possibility is that the tube is normal and has low voltage resistance, so the forward direction is approximately zero. Do not touch the device when measuring in the reverse direction, otherwise it will measure the resistance of the human body. There is no diode, and measuring the hand directly is also this value.
There is another possibility that is relatively small, but it exists. Germanium material diodes will experience this phenomenon. This type of tube has a large reverse leakage, with a forward conduction of 10000 volts less than 3K, and a resistance value in the reverse direction, but it is random and can be large or small, but the tube is good.
