Converter for powering a digital multimeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Among radio amateurs and professionals, digital multimeters have gained great popularity due to their versatility and currently relatively low cost. To power such devices, as a rule, a nine-volt Krona battery (6F22) is used, which is not very convenient, since these batteries have a small capacity, noticeable self-discharge, and, moreover, a higher price compared to other elements. The device proposed in the description will allow you to avoid the above "disadvantages" and operate the multimeter using only two AA (R6) type elements.

Feature of digital multimeters - low current consumption from the power supply. Below are the results of measuring the current consumption of the multimeter (in mA) of the M838 series from a 6F22 battery with a voltage of 9,0 V when operating in the modes:

• Measurement of direct and alternating currents and voltages at all limits, resistance of resistors within 20 kOhm ... 2 MΩ and temperature ...... 2,2
• Resistance measurement at the limit of 2 kOhm......2,3
• Measurement of resistance at the limit of 200 Ohm......3,5
• Continuity, probes shorted......4,5
• Measuring hFF transistors......up to 9,5

Thus, the converter must have not only high efficiency, but also a minimum quiescent current in the absence of load. In addition, the converter must be placed in the multimeter case so as not to impair its performance. Taking into account the above requirements, a converter was developed with the following technical characteristics:

• Supply voltage, V ...... 1,8 ... 4
• Quiescent current, mA......3,4
• Maximum load current, mA......20
• Conversion frequency, kHz ...... 45. .85
• Ripple amplitude at a current of 10 mA, mV......30
• Relative instability of the output voltage when the supply voltage changes within 1,8 ... 4 V,%...... 7

The dependence of the current consumed by the converter on the load current is shown in fig. 1

The influence of the load current on the efficiency at fixed values ​​of the supply voltage is shown in fig. 2.

The converter circuit is shown in fig. 3. Elements C1, T1, R1, R2, VD1 and VT2 form a single-cycle oscillator with inductive feedback. The conversion frequency depends on the inductance of section 1-2 of the transformer T1 and the capacitance of the capacitor C1. The presence of an additional section 4-5 doubles the amplitude of the pulses at the anode of the VD2 diode. This solution slightly improved the conversion efficiency, and most importantly, it made it possible to achieve the required level of constant voltage at the output of the converter at a lower supply voltage.

The output voltage is stabilized by the elements VT1, R3, R4 and VD3. As soon as the voltage at the output of the converter reaches the sum of the breakdown voltages of the zener diode VD3 and the saturation of the base-emitter transition of the transistor VT1, the latter, opening, limits the base current of the transistor VT2. As a result, the collector current of the transistor VT2 decreases, the increase in the magnetic flux in the transformer is limited, which allows you to maintain the output voltage at a given level.

A drawing of a printed circuit board and the arrangement of elements on it are shown in fig. 4 (scale 1:1). Capacitors C1, C3, C5 - ceramic type K10-176 or similar imported ones. Capacitor C1 must have a TKE group no worse than M750 (preferably M47). Resistors can be of any type with a power of 0,125 or 0,25 watts. Diode VD2 - any Schottky diode suitable in size, for example, 1N5817, 1N5818, 1N5819. It does not make sense to use a conventional silicon diode due to a noticeable decrease in efficiency. Zener diode VD2 can be of any type, with a stabilization voltage of 8 ... 8,5 V.

Transistor VT2 - 2T665A9, designed for surface mounting, so its conclusions must be extended with pieces of copper wire 7 ... 10 mm long. Instead of the 2T665A9 transistor, you can use the KT961 or KT961B transistor, however, in this case, the efficiency will deteriorate, since the collector-emitter saturation voltage of the KT961A is 0,2 V higher than that of the 2T665A9. Transistor VT1 can be replaced with any other n-p-n structure with a static current gain of at least 100.

The T1 transformer is made on the B14 armored magnetic circuit made of M2000NM grade ferrite. Winding 1-2 has seven turns of wire LESHO 10x0,07, windings 3-4 and 4-5 - 16 turns of the same wire. The cups of the magnetic circuit are glued with BF-2 or BF-19 glue without a dielectric gasket.

Adjusting the converter comes down to checking the output voltage without load, which should be 8,9 ... 9,4 V. Otherwise, it is necessary to select a VD2 zener diode with a suitable stabilization voltage. After that, it is necessary to check the load capacity of the converter. Connect it to a 1,9V power supply and terminate the converter output with a 910 ohm resistor. Measure the output voltage, if it differs from the no-load voltage by more than 7...10%, reduce the value of the resistor R1 and repeat the measurement. If the described converter is installed in the M838 multimeter, no effect of the converter on the measurement accuracy was found.

The placement of the converter and batteries inside the multimeter is shown in fig. 5.

Literature

1. Pakhomov A. Converter for powering radio receivers. - Radio, 2000, No. 2, p. 19.
2. Biryukov S. Low-voltage voltage converter. - Radio, 2002, No. 2, p. 41,42.
3. Bukreev S. S., Golovatsky V. A., Gulyakovich G. N. Sources of secondary power supply. - M.: Radio and communication, 1983.

Author: S. Belyaev, Tambov

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