Device for monitoring small voltage deviations. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Device for monitoring small voltage deviations. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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In amateur radio practice, sometimes it becomes necessary to control small voltage deviations relative to a given threshold level. To solve this problem, the easiest way is to use a conventional digital multimeter, but it is much more convenient to use a specialized millivoltmeter with a "stretched scale". In such a device, almost the entire scale is occupied by a relatively small area - usually a few percent - of the upper part of the operating voltage range.

To provide measurement accuracy sufficient for most practical applications, a voltmeter must respond to voltage changes in tenths of a volt, have a high input resistance and a small dependence of readings on temperature. These requirements are met by the device described below.

It is assembled according to the measuring bridge scheme (Fig. 1). The left shoulder of the bridge contains a reference voltage source on a VD3 zener diode with a stable current generator assembled on a VT1 transistor. The right shoulder is formed by the diode VD4 and resistors R3, R4. A microammeter RA5 is included in the measuring diagonal of the bridge through resistors R6, R1.

The small dependence of the readings on the ambient temperature is achieved by the fact that a precision Zener diode KC405A for a voltage of 6,2 V with a temperature coefficient of stabilization voltage of 0,002% / ° C is used to form the reference voltage, and due to the low stabilization current (about 2 mA), a rather high input millivoltmeter resistance.

In the device, you can use fixed resistors - MLT, tuning - multi-turn SP5-11. Microammeter RA1 - any (M263, M24, etc.) with a full deflection current of the arrow 50 ... 100 μA. When replacing a zener diode, you should choose one that has the lowest temperature coefficient of stabilization voltage.

Device for monitoring small voltage deviations

To set up the device, you will need an adjustable DC voltage source and a digital multimeter. First, a threshold (minimum measurable) voltage is applied to the input of the device, and with the resistor R3 "Balance", the arrow of the microammeter PA1 is set to zero. Then the voltage is increased to the maximum controllable level and the resistor R6 "Sensitivity" set the arrow to the final division of the scale. By changing the input voltage within the specified limits, the scale is calibrated using a standard voltmeter in units of voltage deviation.

I used the described millivoltmeter to assess the state of charge of a twelve-volt car battery by its EMF. For this, the following voltage levels are set: threshold - 11 V, maximum - 13 V ("length" of the scale is 2 V). A variant of the scale of such a specialized millivoltmeter, if it works with an M24 microammeter with a total deflection current of 50 μA, is shown in Fig. 2; the scale is shown in a "straightened" form. On the scale, the letter N indicates the divisions of the scale already applied on it, E - EMF (unloaded battery voltage) in volts, Q - the degree of charge of the battery in percent.

The device is mounted on a printed circuit board made of foil fiberglass 1 mm thick. The board drawing is shown in fig. 3.

If you set the levels of 10 and 15 V during adjustment, you will get an accurate meter for the on-board voltage of the car.

The temperature dependence of the voltmeter readings will be even less if the terminals of the VD4 diode are closed.

Author: B.Tatarko, Tver

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