Voltmeter with improved linearity. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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The linearity of the scale of a simple AC voltmeter can be improved by using a half-wave rectifier on a transistor (Fig. 1). The proposed options are made as modifications of the voltmeter described in "Radio" 1985 No. 11, p. 43.
The device uses a microammeter with a total deflection current of 50 μA and a loop resistance of 2 kOhm. The upper limit of measurements of the voltmeter is 1 V. Moreover, in the range from 0,2 to 1 V, the measurement error due to the non-linearity of the scale is less than 1% of the final value of the scale.
By selecting a resistor R3, the voltmeter is calibrated at the middle mark of the scale, and R1 at its final value. Linearization on the right side of the scale is achieved due to the base current, which creates a voltage drop across the resistor R2. Due to this, the gain of the rectifier to the end of the scale decreases to the extent necessary.
If the measurement limit is 3 - 5 V and higher, the rectifier is best done on a silicon transistor. Such a device based on a KT3102D transistor for a limit of 10 V has an error from scale non-linearity at around 1 V of about 2%, and in the range of 2 ... 10 V - no more than 0,1% of the final scale value. The parameters of the circuit elements in this case have the following data: R1 - 20 kOhm. R2 -180 kOhm and R3-62 kOhm.
A voltmeter with a full-wave rectifier can be made on transistors of the same structure, as shown in Fig. 2. Resistance of resistors R4. R6 is tentatively determined from the relationship: R < Un2ln, where Un is the measurement limit, and 1n is the total deflection current of the microammeter.
The input resistance of this voltmeter is greater than that of a four-transistor bridge circuit, since two base resistors load the measured circuit here, and not four. Yes, and two transistors of the same structure are easier to pick up.
Expansion of measurement limits is possible with the help of an additional voltage divider, provided that the divider current is much greater than the base current. The use of additional resistors violates the balance achieved during graduation, and the linearity of the scale deteriorates.
Author: V. Khvalynsky, Stavropol
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