What may be the reason for the broken resistance file of mechanical multimeter?
If the resistance gear of the mechanical multimeter, that is, the pointer multimeter, is damaged and other gears can be tested normally, it means that the meter head should not be affected. According to the test principle of multimeter resistance, it is most likely that the shunt and voltage dividing precision resistor of resistance is burnt out or the resistance value changes. The most likely reason for the damage of the resistance file of the mechanical multimeter is to take the resistance file as a voltage file to test the voltage. Therefore, before each test, you should get used to seeing whether the gear is selected correctly and develop good test habits.
Pointer multimeter's electric block is broken, and other gears can be used, which shows that the meter head of multimeter is good. This is caused by incorrect use. There are two reasons for this. One is to measure several wire-wound resistances of AC220V voltage burning current block when it is pulled out of DC current block (no matter what model of pointer multimeter, they are all wire-wound resistances, all of which are wound with constantan resistance wires, and the resistance values are very small, for example, there are four resistances in MF-47 with the values of 0.54Ω 5.4Ω 54Ω 540Ω respectively). However, the owner said that the multimeter only has a problem with the electric block, so there is no problem in this gear.
The second reason is; For beginners, electronics and electricians, after using the electrical blocking measuring element of the multimeter or checking the circuit, they did not set the gear of the multimeter to the gear of 500V AC, and then tested the AC in the 220V AC line or jack. There are many models and manufacturers in pointer multimeter. The most common and widely used are the old 500 and the MF-47 multimeter made in Nanjing. Let's analyze and solve the problem with MF-47.
(1) MF-47 multimeter has DC current gear (DCA), with five common gears and a 5A large current expansion jack, 0~0.05mA~0.5mA~5mA-50mA~500mA.
② It has eight commonly used gears of DC voltage (DCV) and an expansion jack capable of measuring DC2500V V.. 0~0.25V~1V~2.5V~10V~50V~250V~500V~1000V~2500V。
③ There are six gears of alternating current voltage (ACV), 0 ~ 10V ~ 50V ~ 250V ~ 2500.
(4) Its direct current blocks (Ω) five gears. R×1Ω R×10Ω R×100Ω Rx1KΩ R×10KΩ and the road buzzer when measuring (when the line resistance value is 3~10Ω, the buzzer will give a prompt sound). Due to the limited space, the functions of transistor DC amplification factor hFE, infrared remote control signal detection, audio level DB and so on are omitted.
First of all, when measuring the electric blocking, the black stylus is inserted into a hole → the negative electrode of the meter head →20.2Ω resistor, 220.4Ω resistor and 2430Ω resistor, all of which are in parallel with the meter head. At this time, the red stylus is inserted into the ten-jack of the multimeter through a 1A fuse → a →1.5V dry battery is connected in series with a resistor → a 20k resistor → a →1..7k electric blocking zero-setting variable potentiometer → a 500Ω resistor → another meter head calibration R+→ meter head positive+. It's easy to understand the closed loop. According to personal experience, you can trace it.
Use another multimeter to find and measure these resistances. Generally, this kind of fault will only burn the resistance of the electrical blocking position that was pulled at that time. These resistors are non-standard resistors, so they can be connected in series close to the damaged resistor or replaced by a piece of constantan resistance wire wrapped around a resistor, and the constantan wire can be intercepted from the wound variable potentiometer.
