Power supply of multimeters of the M-83x series from one accumulator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The article proposes options for powering popular multimeters of the M-83x (DT-83x) series from one nickel-cadmium or nickel-metal hydride battery of size AAA or 2/3 AAA using a boost converter. Such a power supply also performs the function of a timer, automatically turning off the operating device a few minutes after turning it on. In addition, it is possible to turn the multimeter on and off manually without using the switch for operating modes and measurement limits. In the case of using a 2/3 AAA battery, it was possible to assemble the power supply unit in the dimensions of the Krona battery (6F22).

A lot has been written about the shortcomings of the M-83x series multimeters and similar ones. One of the essential ones is the absence of a separate power switch, which makes using the multimeter not very convenient and leads to increased wear of the mode switch and measurement limits. Forgetfulness when turning off the power leads to useless expenditure of energy source - battery size 6F22 ("Krona" or similar). Cheap batteries are not of high quality and high capacity, and high quality ones cost about the same as the multimeter itself.

Fig. 1

The proposed device largely eliminates these disadvantages, it contains a step-up voltage converter, which allows you to use an AAA or 2/3 AAA battery to power the multimeter, and, in addition, automatically turns off the power some time after turning on, performing timer function. It is designed to be built into M-83x (DT-83x) series multimeters. The scheme of the device is shown in fig. 1. Its basis is a specialized chip NCP1400ASN50T1 (DA1). The combination of functions turned out to be possible due to the fact that this microcircuit has a control input, by applying a voltage of the appropriate level to it, you can turn the converter on and off.

In the initial state, the capacitor C3 is discharged and the converter is turned off. The voltage on the capacitor C4 is not enough to open the field effect transistor VT1, so it is closed and the supply voltage is not supplied to the multimeter. In this state, the current consumed from the battery does not exceed several tens of microamperes. If you briefly press the SB2 "On" button, the capacitor C3 will quickly charge to the battery voltage and the converter will turn on. Diode VD2 rectifies the pulsed output voltage, and capacitor C4 smoothes it. The rectified voltage will open the transistor VT1 and through the LC filter C5L2L3C6 will go to the power lines of the multimeter. After a few minutes, when the capacitor C3 is discharged to about 0,5 V, the converter turns off and the transistor closes - the multimeter will be de-energized. You can also turn off the converter earlier. To do this, briefly press the button SB1 "Off", and the capacitor C3 is quickly discharged through the resistor R1, which limits its discharge current.

In a typical variant of turning on the microcircuit, the output voltage of the converter is stabilized by applying a negative feedback voltage from the output to the input (pin 2) of the DA1 microcircuit. In this case, the output voltage is 5 V. But this is not enough to power the multimeters of the M-83x series. Connecting between the output of the converter and the input OUT of the microcircuit of the elements VD1, R3 and C2 increases the output voltage to 8,5 ... 9 V. The required output voltage is set with the resistor R3, the zener diode limits its value, and the capacitor makes the start of the converter more stable.

The converter will not turn on if the battery voltage drops to about 0,5 V. But this corresponds to a very deep discharge of it. Therefore, to check the condition of the battery, briefly and simultaneously press the buttons SB1 and SB2. In this case, the voltage of the battery loaded with resistor R1 is supplied to the input of the CE microcircuit. If the battery is close to a state of full discharge, at a current of about 100 mA, its voltage will not be enough to turn on the converter - the multimeter will not turn on. This means that the battery needs to be charged.

The device uses elements for surface mounting: resistors - RN1-12 size 1206, capacitor C3 - tantalum size D, the rest - ceramic. Inductors L2, L3 - SDR0703 series with an inductance of 220 ... 1000 μH, L1 is wound with PEV-2 0,3 wire on a ferrite ring with an outer diameter of 6 and a height of 3 mm from the compact fluorescent lamp converter transformer and contains 6 turns with a tap from the 2nd . Buttons - any small-sized with self-return (clock) and a pusher length of 2 ... 3 mm.

Fig. 2

Fig. 3

All elements, except for the battery, are mounted on a printed circuit board made of 1 mm thick fiberglass laminated on one side. Its drawing is shown in Fig. 2. Buttons are glued on the side free from printed conductors. The board, together with the AAA battery holder, is placed in the battery compartment of the multimeter (Fig. 3). It is fixed on the side wall, in which holes are made for the pushers of the buttons. If you plan to charge the battery without removing it from the multimeter case, additionally introduce the elements shown in Fig. 1 with dashed lines. Charging can be carried out from a cell phone charger with an output voltage of + 5 V. In this case, the required charging current is set with resistor R4.

Fig. 4

A variant of the placement of elements in the dimensions of the Krona battery case was developed. The design of such a device and the option of placing it in a multimeter are shown in Fig. 4. The base of the structure is a 2/3 AAA battery holder, which is cut out of the body of a failed LED lawn lamp. The length of the board (see Fig. 2) must be reduced. To do this, it is shortened along the dashed line, and the elements L2, L3 (EC24 inductors) and C6 are installed on the connector (terminal block) from the Krona battery. The board with the buttons installed on it, the battery holder and the connector are fastened with hot glue. After checking the operability of the device, two side walls and a "bottom" are glued to complete the structure.

Fig. 5

Fig. 6

Fig. 7

Fig. 8

As a result, on one narrow side there will be a groove for installing the battery (Fig. 5), on the other, there are buttons (Fig. 6), which should not protrude beyond the dimensions of the structure. The device is installed in the battery compartment of the multimeter (Fig. 7), and in the wall of the multimeter, opposite the buttons, holes are made in which rubber pushers are placed (see Fig. 4). The length of the pushers is made such that, on the one hand, it is convenient to press them, on the other hand, the probability of accidentally pressing the buttons is minimal.

The appearance of a multimeter with a built-in device is shown in fig. eight.

Author: I. Nechaev

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