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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Power supply of the M-832 multimeter from two batteries. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology The author proposes a way to power the popular multimeters of the M-83x (DT-83x) series from two high-capacity AA nickel-metal hydride batteries, which can significantly extend the operating time of the devices without turning off the power. In my amateur radio practice, I use an M-832 digital multimeter for measurements. The main disadvantage of such devices is the lack of a separate power switch. Therefore, in order not to get into a sad situation due to the short duration of operation from a nine-volt galvanic battery, when I forgot to turn off the power several times and the "Krona" has already "sat down", you have to constantly manipulate the switch of the operating mode and the measurement limit, turning the device on and off. In this case, the switch contacts are subjected to significant wear. I would like not to turn off the multimeter at all, leaving the measurement type switch in the working position, the most frequently used, which will extend the life of the switch, and the multimeter will always be in a state of readiness for the next measurement. Replacing the galvanic battery with a rechargeable battery does not solve the problem. Firstly, it requires frequent charging due to the low capacity of the "rechargeable" Crowns. "Secondly, the battery will quickly fail if measures are not taken to disconnect it from the operating device when completely discharged. This led to the decision to use it for The multimeter is powered by two AA nickel-metal hydride batteries with a capacity of 2,7 Ah, placed in a standard battery compartment, and a voltage of 9 V is obtained from a voltage converter. we charge the phone much more often.Of course, the converter is equipped with a node that turns off the power when the batteries are discharged.
The scheme of the device is shown in fig. 1. On field-effect transistors VT1 and VT2 with low threshold voltages, an electronic switch is assembled that turns off the battery when it is discharged to 2 V [1]. The transistors are connected according to the well-known trigger "latch" scheme. When you press the SB1 button, the transistor VT2 opens first, and then VT1. From the output voltage of the switch (on the drain VT1), equal to the voltage at its input (source VT1), the transistor VT2 is kept open until the voltage across the resistive divider R3R4 drops to its threshold value. If the voltage is less than the threshold, both transistors will close due to positive feedback, which will lead to the disconnection of the battery. The current consumed by the converter in this case is practically zero. The SB1 button is used to turn on the converter after installing the battery or after charging it, if it was turned off when it was completely discharged, and also if, after disconnecting the discharged batteries, several measurements are urgently required before putting the battery on charge. For this purpose, a capacitor C1 is connected between the gate and source terminals of the transistor VT1 and in parallel with the resistor R1. When VT2 closes when the battery voltage is less than 2 V, the capacitor, discharging through resistor R1, keeps transistor VT1 open for several tens of seconds, which allows you to make several measurements with discharged batteries by periodically pressing the button. The turn-off delay time is directly proportional to the capacitance of the capacitor C1 and can be changed up or down. A voltage doubler is assembled on the DA1 chip according to a typical scheme. At the output (pin 5) of DA1, the voltage is -8 V relative to pin 5. The efficiency of this converter at a low load current (a few milliamps), as you know, is close to 100% [2], and at an input voltage of 2,5 V, its own consumed by such converter, the current does not exceed 25 μA. The output voltage of the doubler on DA1 is again raised to -9 V, which is necessary for the operation of the ADC chip (ICL7106), by the converter assembled on the DD1 chip, and is fed to the ADC pin 26 (-9 V). After power is supplied from the battery, a voltage of -5 V through the diodes VD1, VD2 is supplied to pin 26 of the ADC. Its built-in clock generator is launched, rectangular pulses from pin 38 are fed to the input of DD1 - the Schmitt trigger. This chip belongs to the high-speed CMOS series with increased load capacity [3]. Its output is loaded on a rectifier with voltage doubling, assembled on diodes VD1, VD2 and capacitors C5, C6, at the output of which a voltage of -5 V is formed from -9 V. The efficiency of this converter depends only on the voltage drop across the Schottky diodes VD1, VD2 at the above load current. The current consumed by the Schmitt trigger is approximately 10 ... 20 μA and depends only on the duration of the pulse changes of the ADC clock generator. Another circuit solution, according to the author, will be less economical.
The power supply is assembled on a printed circuit board made of fiberglass foiled on one side (Fig. 2), placed in the multimeter compartment intended for the power battery. All elements are for surface mounting, with the exception of the DA1 chip, which can be in the package not only SOIC, but also PDIP (DIP-8), for which the corresponding contact pads are provided on the board. The board is designed to install resistors of size 1206, capacitors C1, C2, C4 - size B, C3 - 1206, C5, C6 - 0805. Schottky diodes BAT54WS (VD1, VD2) are replaceable by any similar ones with a reverse current of not more than 2 μA and a capacitance of less than 5 pF at a reverse voltage of 5 V. Transistor IRLML2244TR (VT1) - with a channel resistance of not more than 0,5 Ohm at a gate-source voltage of 2 V, it is replaced, for example, Si2301BDS, IRLML6402TR, VT2 - any low-power transistor with a threshold voltage of not more than 2 V , except for that indicated on the diagram, for example, IRLML6346TR is suitable. The NC7SZ14 chip (Dd 1) can be replaced with an imported 4093V or 40106V microcircuit, as well as domestic KR1561TL1, KR1561TL2. Their inclusion is shown in Fig. 3, while pin 14 of the microcircuit must be connected to the 0 V line, and pin 7 to the -5 V line. The printed circuit board will, of course, have to be finalized.
The DA1 chip, produced by several companies, is more accessible with the initial abbreviation ICL. The copies purchased by the author (both in SOIC and PDIP packages) with the letter Z at the end of the chip designation (for example, ICL7660ACBAZ) had twice the output impedance (at an input voltage of 2,5 V - about 200 versus 90.100 Ohm without the letter Z). Instances with this output impedance can be installed in the power supply if the current consumed by the ADC does not exceed 0,6 mA (usually about 1 mA) or a more economical ADC is installed, for example ICL7126.
The block fits easily into the body of the device (Fig. 4). To charge the GB1 battery, the DS-313 (XS1) socket and the TC-0403 (SB1) tact switch are fixed on the device board with glue. Opposite them, two holes are drilled in the side wall of the case. The adjustment comes down to setting the power off resistor R3 after the appearance of a stable and brightly displayed message on the multimeter indicator about the low battery in the form of BAT characters (in other models there is LO BAT, LOW BAT). With a nominal battery voltage of 2,5 V, the ADC supply voltage should also be measured. If it exceeds 1 V between pins 26 and 9,3, which is possible if the device has an ADC with a current consumption of less than 0,3 mA, one of the VD1 or VD2 diodes should be replaced with any low-power silicon, for example 1N4148W, to obtain the required voltage. In the event that the ADC clock generator does not start, which is quite possible, it is necessary to connect the output 37 "TEST" of the ADC (see Fig. 1) to the -5 V line. Literature
Author: S. Glibin
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