Circuit Principle and Troubleshooting of Digital Multimeter Buzzer
The buzzer of the multimeter shows a small reading as soon as it is turned on, and the reading is normal when the two test leads are short-circuited. This may be due to the high humidity of the working environment of the multimeter, which leads to insufficient insulation of some components of the buzzer. due to. Let's first introduce the circuit principle of the buzzer file of the digital multimeter, and then introduce how to repair such faults.
According to the questioner's description, the multimeter with this kind of fault generally uses the buzzer circuit shown in the figure above. This circuit omits the voltage comparator, and uses the analog-to-digital converter ICL7129 of the digital multimeter to detect the value of the measured resistance Rx. size.
In the above circuit, when Rx is greater than 200Ω, the input voltage of ICL7129 is greater than 200mV, the output of its CONT terminal is low level, the voltage-controlled oscillator composed of CMOS gate circuit CD4011 does not work, and the buzzer is silent. When Rx is less than 200Ω, the CONT terminal of ICL7129 becomes high level, the voltage-controlled oscillator composed of CD4011 starts to vibrate, and the piezoelectric ceramic chip buzzes.
The buzzer file of the digital multimeter.
As can be seen from the above circuit diagram, when Rx is not connected to the circuit, if there is a small invisible resistance between the Ω terminal and the COM terminal of the multimeter due to humid or high humidity in the working environment, then when the multimeter is turned on, the multimeter will display There are certain readings. When the two test leads are short-circuited, because the Ω terminal and the COM terminal are short-circuited, the reading displayed by the multimeter is normal again.
When overhauling, you can use a dust-free cloth dipped in some alcohol to wipe off the dirt between the circuit board and the components near the buzzer gear of the multimeter.
