New functions of the DT-830B multimeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The popular digital multimeter DT-830B (M-830B) will become even more necessary if you add it with a capacitor capacitance meter and a buzzer for "continuity" circuits. The article describes a simple addition to the device that implements these functions.

A schematic diagram of additional nodes built into the multimeter is shown in fig. 1 (the binding is made to the device scheme published in "Radio", 2001, No. 9, p. 26, Fig. 2). The unit for measuring the capacitance of capacitors is made on a DD1' chip. In fact, these are single vibrators made on D-flip-flops. The supply voltage is stabilized by the DD1 chip of the multimeter and is equal to 3,1 V.

Consider the operation of a single vibrator on a DD1M trigger. The pulses of the dynamic "sweep" of the indicator are used as triggers. In the absence of a measured capacitor Cx, the duration of the output pulses of a single vibrator is extremely small and is determined mainly by parasitic capacitances and the speed of the microcircuit. When the measured capacitor is connected to terminals X1, X2 ("Cx-nF"), the single vibrator generates pulses, the amplitude of which is constant (about 3 V), and the duration is proportional to the capacitance. The integration of these pulses and the selection of the constant component of the voltage are carried out by the R29C2 circuit of the multimeter when its probe is connected to the output of a single vibrator (X5 "Cx, nf") in the constant voltage measurement mode. The upper limit of capacitance measurement when the device switch is set to the "200 mV" position is 200 nF, to the "2000 mV" position - 2 μF (the resolution in the first case is 100 pF, in the second - 1 nF).

The second node (on DDV.2) works similarly. The pulses of the clock generator of the DD1 microcircuit of the multimeter are used as triggers. Their repetition rate is 800 times higher than the "sweep" frequency and is approximately 30 kHz. The upper limits of capacitance measurement in this case are 200 pF and 2 nF with a resolution of 0,1 and 1 pF, respectively.

When measuring small capacitances, the influence of the parasitic capacitance of the mounting and the speed of the microcircuit becomes noticeable. Because of this, the lower limit of measurement rises to several tens of picofarads. To set zero readings in the absence of the measured capacitor, resistors R7, R8 are used, through which a small negative bias is applied to the output of the measurement unit from the second stabilized source DD1. This voltage is used to stabilize the voltage on the indicator and, as a result, the contrast of the information displayed on the display. It should be noted that the variation in mounting capacity and speed of the microcircuit can be quite large, therefore the values ​​​​of the resistors R7 and R8 are indicated in the diagram approximately.

The stability of the operation of the described capacitance measurement nodes is relatively low, due to the low stability of the clock generator of the DD1 microcircuit. This parameter of the generator can be somewhat improved by replacing the resistor R26 and capacitor C6 with elements with high temperature stability (for example, resistor C2-29 and a capacitor with TKE of the MP0 or M47 group).

On the transistor VT1, an audible signaling unit for "dialing" circuits is assembled. Its base is connected to the lower (according to the multimeter circuit) terminal of the resistor R9, and the emitter to the upper one. The transistor load is a piezoelectric emitter with a built-in generator HA1.

In the attachment, you can use any low-power diodes, for example, the KD521, KD522 series. Transistor VT1 - any of the KT3107 series. K561TM2 is replaceable by the K1561TM2 chip. Trimmer resistors R2, R5 it is desirable to use multi-turn wire.

The parts are mounted on a printed circuit board (Fig. 2) made of foil fiberglass with a thickness of 0,5 mm. It is designed for the installation of fixed resistors MLT-0,125, tuning resistors SP5-3 (R2, R5) and SPZ-38d (R8), diodes KD522 and piezoelectric sound emitter HRM14AX from JL World. The conclusions of the latter before installation are shortened in such a way that they protrude above the printed conductors by no more than 1 mm. The same is done with the conclusions of the remaining details. Trimmer resistors R2 and R5 are fixed with brackets of tinned wire with a diameter of 0,4 ... 0,5 mm, the ends of which are passed through the holes in the board and soldered to the corresponding pads with an interference fit. Transistor VT1 is mounted parallel to the board. The height of all solder joints (above the plane of printed conductors) must not exceed 1 mm.

The mounted board is placed above the middle part of the multimeter board (upper - according to Fig. 2 - side to the LCD indicator) and connected with short segments of a thin mounting wire (for example, MGTF) to the corresponding points of the device. In order to avoid touching the printed conductors of the instrument board with metal cases of trimmer resistors, as well as wire brackets securing them, a gasket made of varnished cloth or other thin dielectric is placed between the boards.

Clamps (or sockets) X1 - X4 and contacts X5, X6 are installed on the side wall of the device.

To calibrate the capacitance meter on the DD1M trigger, a 1 ... 2 μF capacitor is used with a permissible deviation from the nominal value of not more than 1%. In the extreme case, the capacitor K73-17 or similar can serve as an exemplary one, the capacitance of which is measured by another device with a sufficiently high accuracy. Calibrate the meter with a trimming resistor R2. Resistor R3 protects the output of the one-shot in the event of an accidental short circuit.

The capacitance meter on the trigger DD1'.2 is calibrated with a trimmer resistor R5 using a reference capacitor with a capacity of 1...2 nF.

For normal operation of the sound signaling unit, it is necessary to select the resistor R13 of the multimeter. For the time of adjustment, it is replaced with a trimming resistor with a resistance of 2,2 kOhm. Turning on the multimeter in the mode of measuring resistances up to 200 ohms, connect a 100 ohm resistor to the probes and, slowly turning the trimming resistor slider, achieve sound in the HA1 emitter. Then measure the resistance of the entered part of the tuning resistor and replace it with a constant with the closest value. After such a refinement, the readings of the device will change somewhat when checking the diodes, but they are more of a qualitative nature than a quantitative one.

On the basis of a single vibrator on a D-trigger, it is not difficult to implement the function of measuring the frequency of signals. (However, in this case, the frequency meter will be analog or, more precisely, pseudo-digital). If pulses of an unknown frequency are fed through the simplest shaper-limiter to the input C of the trigger, and the elements that form the duration of the one-shot pulse are selected accordingly, then the result will be a frequency / duty cycle converter. Otherwise, the mechanism for extracting the constant component and its measurement are similar to those described above. The frequency meter is calibrated by selecting elements that form the duration of the single vibrator pulses.

Author: S. Kostitsyn, Izhevsk

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