AC voltage measurement circuit in pointer multimeter
What is the circuit for measuring AC voltage with a multimeter? In order to make it easier for everyone to understand, this article takes the circuit of measuring AC voltage with MF9 multimeter as an example.
Firstly, place the conversion switch in the "~" position of the AC voltage to form a circuit for measuring AC voltage as shown in the figure. The half wave rectification circuit is composed of VD2, and the rectification components used are 2CP6 or 2CP11 silicon diodes. VIA plays a protective role by providing a discharge circuit for reverse voltage to prevent reverse breakdown of VD2. Due to the unidirectional conductivity of diodes, current can only pass in one direction, which is called the positive direction of the rectifier device. The resistance in the positive direction is called forward resistance, and the smaller the forward resistance of a diode, the better.
The circuit still retains the shunt resistor for DC current mode, and it has four AC voltage ranges. By switching the "a" and "b" blades of the switch, four different measurement ranges for AC voltage can be obtained.
It is worth noting that the additional resistance for measuring AC voltage levels is mostly shared with the additional resistance for DC voltage levels. From the overall circuit shown in the figure, it can be seen that the additional resistance for AC 250V level is the additional resistance for DC voltage 50V level. It can be seen that the resistance per volt of AC voltage is 5 times lower than that of DC, due to the lower efficiency caused by half wave rectification after using a rectifier circuit. The 3 μ F electrolytic capacitor connected in parallel with the meter head in the circuit is used to smooth the pulsating voltage after rectification, which can prevent the pointer of the multimeter from shaking when measuring low-frequency voltages below 10Hz. The circuit analysis is as follows:
When the active connecting pieces "a" and "b" of the transfer switch C are connected to the 10V AC voltage range, the AC current flows through the additional resistor - "a" - "b" - of 35.5k Ω, and is rectified by VD2 into DC and sent to the magneto electric meter head.
When the active connection pieces "a" and "b" of switch C are connected to the 50V AC voltage range, the AC signal is rectified by VD2 through 35.5K Ω+10 k Ω+150K Ω+800 k Ω additional resistors - "a" - "b" - and then sent to the magneto electric meter.
From the above analysis, it can be concluded that when measuring high and low voltages in AC voltage mode, different additional resistors are also connected in series. The higher the voltage, the larger the additional resistors connected in series. This circuit adopts a closed circuit design, where the high range gear shares the additional resistance of the low range gear. In addition, in order to simplify the circuit as much as possible, the additional resistance for AC voltage also shares the additional resistance for DC voltage.
When the active connection pieces "a" and "b" of switch C are connected to the 250V AC voltage range, the AC signal passes through 35 5 k Ω+10 k Ω+150k Ω+800k Ω additional resistors "a" - "b" - rectified by VD2 into DC and sent to the magneto electric meter head.
When the active connection pieces V 'and' b 'of the conversion switch C are connected to the 500 V AC voltage range, the AC signal is rectified by VD2 through 35.5k Ω+10k Ω+150k Ω+500k Ω additional resistors' a' - 'b' - to be DC and sent to the magneto electric meter head.
