How to use a multimeter to test whether the AC contactor coil is good or bad?
You can judge whether the contactor coil is good or not by measuring the resistance value with a multimeter. Generally, the coil structure of the contactor is several hundred ohms. If the tissue you measure is very small (such as a few ohms, and it does not pull in after power on), it means that the coil is short-circuited. ; if the tissue is very large (eg infinite) then the coil is broken.
In addition, check the coil voltage of the contactor to prevent the coil from not being pulled in due to untightened screws, or the contactor not being pulled in due to an open circuit of the wire entering the contactor coil.
Use a digital multimeter to measure the coil with an ohm range below 2K to see if the coil is open circuited. If the circuit is open, it proves that the coil is broken.
Use a multimeter with ohm range 200 to measure normally closed and normally open (to measure normally open, hold it down with your hand or energize it). It indicates that the contact is not in good contact. If the auxiliary contact is connected to a load, the voltage will not reach the rated voltage when measuring the voltage.
Use a multimeter to measure the normally open and normally closed terminals to see if they are normal. It is not easy to measure the coil with a multimeter. The best way is to directly add the rated voltage to test the operation.
What is the matter with the multimeter displaying U38 at any file?
The digital multimeter displays U38 in all gears. It may be that I dialed the wrong gear during use and burned out the single-chip A/D converter in the digital multimeter.
For ordinary 3.1/2 digital multimeters, the single-chip A/D converter is the heart of the digital multimeter. Due to the low price of this kind of digital multimeter, ICL7106 chip is often used for sampling operation output display.
Once this happens to the digital multimeter, it can basically be said to be scrapped, that is, to be repaired, and the repair cost exceeds that of buying a new digital multimeter. Therefore, when using various multimeters, you must be careful about the gear and range of the multimeter, handle it with care, and pay attention to the moisture-proof of the meter.
Either it was burnt out, or the display lock key was accidentally pressed. After turning on the power, if there are two buttons on the watch, press each button and check again.
