Principle and maintenance of AC/DC automatic conversion circuit of multimeter

Ordinary digital multimeters rely on manual operation to complete the conversion between AC and DC measurements. The new DT860D digital multimeter uses the NJU9207F automatic range conversion chip and is equipped with peripheral auxiliary circuits to achieve the automatic conversion measurement function of AC/DC. Its principles and common faults are as follows.

The AC/DC automatic conversion circuit is shown in the attached figure. IC1 is NJU9207F made in Japan. The four setting terminals FC1 to FC4 are set externally, and the selection method is shown in the attached table.

As can be seen from the table, when measuring DC and AC voltages, the states of FC2 and FC4 are exactly the same, both at high level. Since the chip is connected with pull-up resistors , FC1~FC4 are at high level when open circuit. If different levels are input on the control terminal FC1, the automatic conversion of AC/DC measurement function can be realized. When measuring current, only a low level needs to be sent on the FC2 terminal.

The AC/DC automatic conversion circuit in the attached figure can be divided into: sampling circuit; voltage amplifier; DC isolation circuit; negative voltage rectifier circuit. The sampling circuit is composed of R1 and R2. After the input voltage is attenuated, it is sent to the voltage amplifier IC2a ​​for amplification through R3, and then coupled to the negative voltage rectifier circuit IC2b through C1. Only when the output of pin 7 of IC2b is negative voltage, D is turned on. The negative pulsating DC voltage output by the rectifier tube D is filtered out by c3 to make VK=0 (low level). At this time, the digital multimeter enters the AC measurement mode. R5 and R7 are the feedback amplifier resistors of the operational amplifier. c2 is the frequency compensation capacitor .

If the multimeter input is a DC voltage, it will be isolated by cl, and the input and output of IC2b are both open circuit. Relying on the internal pull-up resistor of Icl, FC1=1 (high level), and the multimeter automatically enters the DC voltage measurement mode.

Since it is purely controlled by electrical signals, the conversion process of AC/DC measurement function is fast and manual operation is omitted, which is extremely convenient.

Common faults of AC/DC automatic conversion circuits are:

1. It can only measure DC voltage and current and cannot automatically switch to AC measurement.

The reasons are roughly as follows:

1. Function conversion switch Poor contact.

2. The FCl terminal (pin ⑩) of ICl (NJU9207F) is open. The circuit is blocked.

3. Voltage amplifier IC2 (TI062) is damaged.

4. C3 has serious leakage or is short-circuited to the ground.

5. D has an open circuit due to poor soldering, so it cannot generate a DC control voltage when measuring AC.

2. Neither AC nor DC voltage nor current can be measured.

1. Capacitor C1 fails; C3 is short-circuited to ground.

2. IC1 (NJU9207F) is damaged