Attachment to the multimeter for measuring battery parameters. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The prefix is ​​intended for connection to the MY-63 multimeter, which measures, along with other parameters, direct and alternating voltage, as well as the capacitance of capacitors and the current transfer coefficient of bipolar transistors. The presence of these modes is necessary for the operation of the set-top box. Its scheme is shown in Fig. 1. On the OS DA1. 1 and field-effect transistor VT1, a voltage-controlled current stabilizer is assembled.

Fig. 1

An exemplary voltage Uobr = 5 V is supplied to its input from the engine of the tuning resistor R0,1. Current Ia flows through the transistor VT1, which is discharge for the battery. It depends on the reference voltage and on the resistance of the current sensor (Rt) - resistors R8, R9 or R10: Ia = Uobr / Rt. The discharge current is selected by the switch SA1. In position 3, the discharge current is 1 A, in position 2 - 0,1 A, and in position 1 - about 10 μA (it can be considered equal to zero). On the op amp DA1.2, an amplifier with unity gain for direct current (K == 1) and about 100 (K = 100) for alternating current is assembled. Switch SA2 disconnect the battery from the measuring circuit. All elements of the set-top box are powered by the multimeter's internal voltage regulator (+3 V), the current consumed by the set-top box does not exceed 35...40 μA. This efficiency is achieved through the use of a micro-power dual op amp OPA293.

To measure the battery voltage, the multimeter is switched on to the DC voltage measurement mode at the limit of 2 V. The battery installed in the holder is connected to the measuring circuit with the SA2 switch. By changing the discharge current with the switch SA1, take readings of the voltmeter Ua0 (at Ia = 0), Ua0,1 (at Ia = 0,1 A) and Ua1 (at Ia = 1 A). Based on these data, the internal resistance of the battery is calculated, which can be called static. For example, for Ia \u1d 0 A, Rct \u1d (Ua1 - Ua0,9) / XNUMX. In this mode, you can also determine the capacity of the battery. To do this, measure the duration of discharging t times a fully charged battery with a stable current Ia to a voltage of XNUMX V and calculate its capacity: C \uXNUMXd Ia * t times (Ah). During discharging, the multimeter cannot be turned off, since the current stabilizer will also turn off.

In order not to calculate the internal resistance, the set-top box provides for its measurement mode, in which the method given in the article by B. Stepanov "Measuring battery parameters" ("Radio", 2001, No. 9, p. 42) is used. It is based on the fact that an alternating component is added to the exemplary voltage of the current stabilizer. By measuring the variable component of the voltage on the battery, you can determine its internal resistance. The source of alternating voltage in the attachment is a signal with a frequency of about 400 Hz and an amplitude of 50 mV, which is present in the MY-b10 multimeter on the left contact of the "Cx" connector, designed to connect the measured capacitor. An alternating voltage is supplied to the input of a voltage-controlled current stabilizer, and leads to the appearance of an alternating component of both the discharge current (Ia„) and the battery voltage Ua, = IaRd, where Rd is its internal differential resistance. The prefix is ​​set to Ia, = 1 mA. In order for the variable component of the current to be the same for different discharge currents, the alternating voltage coming from the "Cx" socket, at Ia \u1d XNUMX A, further reduces the tuning resistor RXNUMX.

The voltage Ua amplifies the op-amp DA1.2, and then it is fed to the input of the multimeter, which is switched on to measure the alternating voltage at the limit of 2 V. The alternating voltage at the output of this op-amp is: UOU, = UaK = IaRdK. The variable component of the discharge current (Ia,) and the gain of the OA DA1.2 (K,) are chosen so that the measured alternating voltage at the output of the set-top box (UОУ,) is numerically equal to the internal differential resistance (Rd) of the battery. For example, for Rd \u0,1d 0,01 Ohm, we get UOU \u0,1d 100 * 0,1 * XNUMX \uXNUMXd XNUMX V. It is this voltage that the voltmeter will show. It should be noted that the calculated and measured values ​​of the internal resistance will differ slightly, since in the first case it is determined by the difference in the values ​​​​of the no-load voltage and under load, and in the second - by the slope of the battery load characteristic at a particular point.

Fig. 2

Most of the attachment elements are placed on a printed circuit board made of fiberglass laminated on one side with a thickness of 1,5 ... 2 mm, the drawing of which is shown in fig. 2. Fixed resistors for surface mounting RN1-12 of size 1206 (resistor R10 of size 2512) are used, tuning resistors - SPZ-19. The oxide capacitor is tantalum for surface mounting, size B or C, the rest are ceramic sizes 1206 (C2, C4) and 0805 (C3). The field effect transistor must be in the D2Pak package, it is soldered to a metallized pad to improve heat dissipation. In addition, it must be with the so-called "logic level control", i.e. at a gate-source voltage of 2,5 V, the drain current must be at least 2.3 A. In the name of some of these transistors, the letter L is present in the prefix. In addition to the indicated on the IRL2505S diagram, for example, IRLR2905 is suitable.

Switch SA2 must have a low contact resistance in the closed state and is rated for a current of at least 1.2 A, suitable, for example, B3009. If a switch with such parameters is not available, it is better to exclude it by installing a jumper wire instead. Switch SA1 for three positions and two directions for a current of at least 1 A - SS23F07. Some other 23A SS1,5 series will work, such as SS23E24, SS23E28, SS23E29, but the PCB will need to be changed because these switches have a different pinout.

The op amp can be replaced with a similar micro power Rail-to-Rail such as the LMV358DR2G. Plug XP1 - a piece of tinned copper wire with a diameter of 1 and a length of 15 mm, connected by an insulated wire to the board. This plug is inserted into the "C NPN" jack for connecting bipolar transistors. Plugs XP2 and XP3 - pins with a diameter of 4 and a length of 35 mm, which are fixed in the holes of the board. XP4 plug - a strip of tinned brass or copper 0,5 thick, 4 wide and 20 mm long, it is soldered from the side of the printed conductors to the pad on the board. When installing the attachment, the XP2 and XP3 plugs must go into the corresponding sockets of the multimeter, and XP4 - into the left socket of the "C" connector. After checking and adjusting the attachment, the XP1-XP3 plugs are fixed on the board with epoxy glue. its contacts should not be in the form of coil springs, but petal ones.

The device can be simplified by eliminating the differential resistance measurement mode and its corresponding elements. In this case, the SA1 switch can be one direction and three positions, the op-amp is single (DA1.2 is not needed), the LMV321SQ3T2G will do. The drain of the field-effect transistor and contact X1 are connected to the XP2 plug, the resistors R1, R2, R4, R11, R12 are excluded (a wire jumper is installed instead of R4), capacitors C2, C4. In this version, the prefix, having changed its design, can be used in conjunction with simpler and more affordable multimeters of the M-83x fT-83x series), in which there are modes for measuring DC voltage and current transfer coefficient of bipolar transistors.

Adjust the prefix in the following sequence. Connect it to a multimeter, install a fully charged battery in the holder, and switch SA1 to position 1 ("0 A"). The slider of the resistor R2 is moved to the lower position according to the scheme, the multimeter is switched to the DC voltage measurement mode at the limit of 2 V and the power is turned on. The readings of the multimeter should correspond to the battery voltage, which is controlled by an exemplary voltmeter. After turning off the multimeter, between one of the terminals of the battery and the contact of the holder, a plate-insert made of double-sided foil fiberglass with a thickness of 0,5, a width of 10 and a length of about 15 mm is installed. Beforehand, a thick insulated wire is soldered to each side of the plate, to which a DC ammeter is connected. Switch SA1 is set to position 3 ("1 A") and the multimeter is turned on. The trimming resistor R5 sets the corresponding stabilizer current (1 A). In switch position 2 ("0,1 A"), the current should decrease to this value, and in position 1 ("0 A") it should not exceed 20 μA.

The multimeter is turned off and, instead of wires, a resistor Rdop with a resistance of 0,1 Ohm is soldered to the plate. The engines of the resistors R2 and R11 are set to the middle position, the multimeter is switched to the AC voltage measurement mode at the limit of 2 V, and then the power is turned on. Set the discharge current to 0,1 A. In this case, the voltmeter will show a voltage (U2) proportional to the sum of the internal differential resistance of the battery and the additional resistor Rd + Rdop. If, for example, the resistor Rdop is closed with tweezers, it will be excluded from the discharge current flow circuit and the voltmeter readings will decrease (U1). The slider of the resistor R2 is set to a position where U2-U1 = 0,1 V.

Fig. 3

In this case, it may be necessary to change the position of the resistor R11 slider. Similarly, adjustment is carried out at a discharge current of 1 A, but only resistor R1 is used. Adjustment is desirable to carry out two or three times. The appearance of the console is shown in fig. 3.

Author: I. Nechaev

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