RMS voltmeter - prefix to the multimeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The prefix expands the capabilities of the 83x series digital multimeters, it allows you to measure the rms values ​​​​of alternating voltage of various shapes, and with an additional shunt - and current. The attachment is powered by the internal ADC stabilizer of the multimeter, does not require adjustment, is easy to repeat and provides high measurement accuracy.

About the measurement of root-mean-square (other names - effective, effective, and in the English abbreviation - RMS) voltage and current values ​​\u736b\u1bin radio engineering literature, there was a conversation more than once. Therefore, for those radio amateurs who do not have such a measuring device and cannot purchase it, it may be of interest to make it on their own for a home laboratory. The task is simplified if a similar device is made on the basis of a specialized ADXNUMX microcircuit as an attachment to a digital multimeter powered by its internal stabilizer. A diagram of such an attachment for measuring the rms value of an alternating voltage of various shapes is shown in fig. XNUMX.

Rice. 1. Attachment circuit for measuring the rms value of alternating voltage of various forms (click to enlarge)

Main Specifications

• Limits of measurement, V ....... 0,2; 2; twenty
• Frequency range when measuring voltage in the range 0,1....1
• maximum value for each measurement limit, Hz, not less than....... 50...10000
• Voltage measurement error in the above interval and frequency range, %, not more ....... 2
• Input resistance, MΩ ...... 1
• Current consumption, mA, not more ....... 0,8

More detailed capabilities and features that determine the characteristics of voltmeters assembled on the basis of the AD736 microcircuit, which is used in the set-top box, can be found in [2].

The measured alternating voltage is supplied to the sockets "U_vh"and through the separating capacitor C1 - to the resistive divider R1 -R3, with which you set the measurement limit by selecting the appropriate position of the SA1 switch. From its movable contact, it enters the input of the DA1 AD736JN microcircuit. Resistor R4 and diodes VD1, VD2 are protective. They protect chip DA1 from failure from high voltage, which may appear at its input if the measurement limit is chosen incorrectly.The capacitances of capacitors C2 and C3 are selected according to the recommendations for the use of the microcircuit and can be changed depending on the specific application of the attachment: audio voltmeter, voltmeter of average values ​​and etc. The signal from the output of the DA1 chip in the form of a constant voltage of positive polarity is fed to the "VΩmA" input of the multimeter for subsequent measurement.

The AD736 requires a bipolar voltage source for power. The minimum requirements for the supply voltage are +2,8 V and -3,2 V. Therefore, to power the set-top box from the built-in ADC stabilizer of the multimeter +3 V, a negative voltage converter with high efficiency, assembled on the DA2 chip, is used, the description of which is given in [3] . The converter provides power to the microcircuit with a voltage of -5,5 V.

The prefix is ​​assembled on a board made of fiberglass laminated on one side. A drawing of a printed circuit board and the arrangement of elements on it are shown in fig. 2. A photograph of the assembled attachment is shown in fig. 3. There are three tinned wire jumpers on the board. They are soldered before the installation of the remaining elements. Chips DA1 and DA2 - in the DIP8 package. Capacitors C1, C4 - output ceramic, imported analogues of K10-17B. Oxide capacitors - imported output (mini version). Resistors - output power 0,125 or 0,25 watts. Pin XP1 "NPNc" - suitable from the connector or a piece of tinned wire of suitable diameter. A hole for it in the board is drilled "in place" after installing the pins XP2, XP3. Pins XP2 "VΩmA" and XP3 "COM" - from the probes for the multimeter. Input sockets XS1, XS2 "U"_vh"- screw terminal block 350-02-021-12 of the 350 series from DINKLE, DEGSON. SA1 switch - sliding series MSS, MS, IS, for example, MSS-23D19 (MS-23D18).

Rice. 2. Drawing of a printed circuit board and the location of elements on it

Rice. 3. Assembled prefix

The prefix does not require adjustment. When working with it, the switch of the kind of work of the multimeter is set to the position of measuring the DC voltage at the limit of 200 mV. For the above accuracy of measuring the resistance of resistors R1, R2 in the divider, you can choose from the nominal series E12 (10%) or E24 (5%) as follows. First, by sampling 900 and 90 kΩ from resistors with nominal resistances of 910 and 91 kΩ, respectively. Secondly, the resistor R1 can be set with a resistance of 1 MΩ, R2 - 100 kΩ, R3 - 11,1 kΩ, selecting 11 kΩ from the resistors. However, there are many variations here, taking into account "everything is a little more" or "everything is a little less". Before connecting the set-top box to the multimeter, you should check the current it consumes from another 3 V power source with current protection so as not to disable the built-in low-power ADC supply voltage regulator in the event of a malfunction of any element or an accidental short circuit of the current-carrying tracks of the board.

The input impedance of the set-top box can be increased up to 10 MΩ. To do this, it is enough to increase all the resistances of the resistors of the input divider R1-R3 ten times. Preservation of the frequency range, if necessary, is achieved by making the divider frequency independent. In this case, as is known, in parallel with each resistor of the divider, it is necessary to connect a capacitor of the corresponding capacitance. On the board from the side of the printed conductors for these capacitors, the corresponding contact pads are provided. These capacitors are surface-mounted, sizes 0805, 1206. The approximate time constant of each such RC circuit is 100 µs and is selected according to the well-known method for setting up input dividers of measuring instruments.

The presence of reverse current protective diodes VD1, VD2 (1N4148) can introduce an additional error in the measurements. To eliminate it, it is necessary to use diodes with a reverse current of not more than 1 ... 2 nA at a reverse voltage of 5 V, for example, FDLL300A, or select from the available ones. You can measure the reverse current with any multimeter at the limit of 200 mV with an input resistance of 1 MΩ. The cathode of the diode is connected to the plus of the power source, and the anode is connected in series with the multimeter probes to the minus. Readings in millivolts will be numerically equal to the reverse current in nanoamps. With an input resistance of the device equal to 10 MΩ, the readings should be divided by ten.

To measure the rms values ​​of alternating current, for example, the no-load current of network transformers, power supplies based on them, switching power supplies and other similar devices, it is useful to supplement the attachment with a measuring shunt with clamps. To do this, connect to the terminals "U_vh"1 ohm resistor with two insulated wires, to the opposite ends of which are soldered clips, for example, "crocodile". If you install a 2 W resistor, you can measure alternating current within 200 mA and 2 A, which will correspond to the positions of switch SA1 "200 mV" and "2 V". The author's version of the set-top box with such a shunt connected to the multimeter is shown in Fig. 4.

Rice. 4. Author's version of a set-top box with a shunt connected to a multimeter

When checking the author's prefix for the accuracy of measuring voltages of triangular and rectangular shapes (meander) with a frequency of 10, 50, 500 and 5000 Hz, the multimeter readings differed from the readings of the reference voltmeter by no more than 1% in the range of 0,1 ... 1 maximum value at each measurement limit .

Literature

1. Long A. What does the AC voltmeter show? - Radio, 2006, No. 6, p. 23-27.
2. AD736 Low Cost, Low Power, True RMS-to-DC Converter. - URL: analog. com/static/imported-files/data_sheets/AD736.pdf.
3. Glibin S. Obtaining doubled bipolar voltages in a converter based on ICL7660 series microcircuits. - Radio, 2015, No. 3, p. 35.

Author: S. Glibin

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