The digital multimeter is composed of a digital voltmeter (DVM) equipped with various converters, and thus has various measurement functions such as AC and DC voltage, AC and DC current, resistance and capacitance.
The following figure is a block diagram of the digital multimeter. It is divided into input and conversion parts, A/D converter part and display part. Input and conversion sections, mainly through current-to-voltage converter (w), AC-DC converter (AC/DC), resistor-to-voltage converter (R/V); capacitor-to-voltage converter (CN) converts each measurement The amount of DC voltage is measured by a range rotary switch and sent to the A/D converter through an amplification or attenuation circuit.
The A/D converter circuit and the display portion are composed of an ICL 7106 and an LCD.
We can see that the digital multimeter is based on DC 200mV for the basic range, and is connected with the DC voltage and current which are linearly transformed. The AC voltage, current, ohmic and capacitance converters can display the corresponding electrical parameters by numbers.
Functional circuit and working principle1. Resistance measurement circuit and decimal point display circuit (see the figure below)
1 The resistance is measured by the proportional method. The measured resistance Rx and the reference resistor are connected in series between V+ and COM, Uin=V+RX/(R+RX). After the measurement gear position is determined, R is determined, then the larger Rx is, the larger Uin is. When the gear position is changed from 200 Ω to 20 MΩ, the corresponding R is also increased. It can be seen from the calculation that the partial pressure on Rx will not exceed A certain value to balance each range.
2ICL7106 only has the liquid crystal pen end and the back electrode driving end. In order to display the decimal point, the op amp OP1 is used to form an inverting amplifier to form a decimal point display circuit, so that the ICL7106 goes to the back electrode BP point pulse signal of the LCD (50 Hz square wave, duty cycle bit 50) %, to ensure that the effective value of the AC voltage is 0, to extend the use time of the LCD) and to reverse the pulse signal of each decimal point BP2, BP20, BP200, according to the display principle of the liquid crystal, the corresponding decimal point is just illuminated at this time.
2. DC voltage measurement circuit and AC voltage measurement circuit (see the figure below)
1 DC voltage measurement uses the resistor divider method to measure the voltage. The input DC voltage is converted into 0~200mV DC voltage through the voltage divider and transfer switch, and finally sent to the A/D converter circuit for display.
The larger the measured value, the higher the voltage applied to the input of ICL7106. When the gear is changed from 200mV to 1000V, the corresponding gear resistance is reduced. It can be seen from the calculation that the voltage at the input of 7106 will not exceed A certain value of 200mV, so that each range can be balanced (as shown in the table below). The core part of the whole circuit is a voltmeter with a range of 200mV composed of ICL7106. For the sake of electrical safety, the second half of the 1000V range (1001~1999V) is not recommended.
2 AC voltage measurement input measurement principle is the same as DC voltage measurement circuit, except that its corresponding switch combination becomes 8-1~8-4, the output is sent to the AC/DC conversion circuit via H point (see the figure below), through the diode After rectification, it becomes DC voltage and then enters the input terminal IN+ of 7106 through G point. Go to the LCD display. It can be seen that the G output is the average of the AC voltage.
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