LC-meter - prefix to the multimeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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This article continues the theme of expanding the capabilities of the popular 83x series multimeters. The small current consumed by the set-top box allows you to power it from the internal stabilizer of the ADC of the multimeter. Using this attachment, you can measure the inductance of coils and chokes, the capacitance of capacitors without soldering them from the board.

The designs of measuring attachments to multimeters, in addition to the difference in circuit solutions and methods for measuring a particular parameter, are also different in their ability to work from their own power source and without it, using the ADC voltage regulator of the multimeter. Set-top boxes powered by the multimeter's ADC stabilizer, according to the author, are more convenient to use, especially "outside the house". If necessary, they can also be powered from an external 3 V source, for example, from two galvanic cells. Of course, the question arises about the current consumed by such a prefix, which should not exceed a few milliamps, but the use of a modern element base in combination with optimal circuitry solves this problem. However, the issue of current consumption has always been and will be relevant, especially for self-powered measuring instruments, when the duration of operation from an autonomous source often determines the choice of device.

When developing the LC-meter, the main attention was paid not only to minimizing the current consumed, but also to the possibility of measuring the inductance of coils and chokes, the capacitance of capacitors without soldering them from the board. This possibility should always be taken into account when designing such measuring instruments. Many examples can be given when radio amateurs in their designs, unfortunately, do not pay attention to this. If, for example, we measure the capacitance of a capacitor by charging with a stable current, then even with a voltage on the capacitor of more than 0,3 ... 0,4 V, without desoldering it from the board, it is often impossible to reliably determine the capacitance.

The principle of operation of the LC meter is not new [1, 2], it is based on the calculation of the square of the measured period of natural oscillations in the resonant LC circuit, which is related to the parameters of its elements by the relations

T = 2π √LC or LC = (T/2π)².

From this formula it follows that the measured inductance is linearly related to the square of the oscillation period with a constant capacitance in the circuit. It is obvious that the measured capacitance is also connected with the same linear dependence at a constant inductance, and for measuring inductance or capacitance it is enough to convert the oscillation period to a convenient value. From the above formula it can be seen that with a constant capacitance of 25330 pF or an inductance of 25,33 mH for multimeters of the 83x series, the minimum measurement resolution is 0,1 μH and 0,1 pF in the intervals of 0 ... 200 μH and 0 ... 200 pF respectively, and the oscillation frequency with a measured inductance of 1 μH is 1 MHz.

The prefix contains a measuring generator, the frequency of which is determined by the LC circuit and, depending on the type of measurement - the inductance connected to the input sockets of the coil, or the capacitance of the capacitor, the generator output voltage stabilization unit, a pulse shaper, frequency dividers to expand the measurement intervals and a pulse repetition period converter into a voltage proportional to its square, which is measured by a multimeter.

Main Specifications

• Inductance measurement limits ......200 µH; 2 mH; 20 mH; 200 mH; 2 H; 20 Gn
• Capacitance measurement limits ....... 200 pF; 2 nF; 20 nF; 0,2uF; 2 uF; 20 uF
• Measurement error on the first four limits from 0,1 limit value and above, no more, % .......3
• Measurement error within 2 μF and 2 H, no more than, % .......10
• Measurement error within 20 μF and 20 H, no more than, % .......20
• Maximum current consumption, no more than, mA ....... 3

The inductance measurement error within 2 and 20 H depends on the self-capacitance of the coil, its active resistance, the residual magnetization of the magnetic circuit, and the capacitance within 2 and 20 μF depends on the active resistance of the coil in the LC circuit and ESR (ESR) of the measured capacitor.

The attachment diagram is shown in fig. 1. In the "Lx" position of the SA1 switch, measure the inductance of the coil connected to the sockets XS1, XS2, in parallel with which the capacitor C1 is connected, and in the "Cx" position, the capacitance of the capacitor, in parallel with which the inductor L1 is connected. On transistors VT1, VT2, a measuring generator of a sinusoidal voltage is assembled, the frequency of which, as mentioned above, is determined by the elements of the LC circuit. This is an amplifier covered by positive feedback (POS).

The first stage of the amplifier is assembled according to a common collector circuit (emitter follower), it has a large input resistance and a small output, and the second - according to a common base (CB) circuit - has a low input and high output resistance. Thus, a good agreement is achieved when the output of the second is closed with the input of the first. Both stages are non-inverting, so this connection covers a XNUMX% PIC amplifier, which, in combination with the high input impedance of the emitter follower and the output stage with OB, ensures that the oscillator operates at the resonant frequency of the LC circuit over a wide frequency range.

Rice. 1 (click to enlarge)

Consider the operation of an LC meter with an inductor or capacitor connected to the sockets XS1, XS2 "Lx, Cx". The voltage from the generator output is fed to an amplifier with a high input impedance, assembled on a VT3 transistor, which amplifies it five times, which is necessary for the normal operation of the generator output voltage stabilization unit. The stabilization unit is assembled on diodes VD1, VD2, capacitors C3, C5 and transistor VT4. It maintains the generator output voltage at a constant level of about 100 mV rms, at which measurements can be taken without desoldering elements from the board, and also increases the stability of the generator oscillations at this level.

The output voltage of the amplifier, rectified by the diodes VD1, VD2 and smoothed by the capacitor C5, is fed to the base of the transistor VT4. When the voltage amplitude at the generator output is less than 150 mV, this transistor is opened by the base current flowing through the resistor R7, and the full supply voltage of +3 V is supplied to the generator (this voltage must be applied to the generator for its reliable start, as well as when measuring the inductance 1.. .3 µH). If, during the measurement, the amplitude of the generator voltage becomes more than 150 mV, a voltage of polarity closing the transistor VT4 will appear at the output of the rectifier. Its collector current will decrease, which will lead to a decrease in the generator supply voltage and restoration of the amplitude of its output voltage to a predetermined level. Otherwise, the reverse process occurs.

The output voltage of the amplifier on the transistor VT3 through the circuit C4, C6, R8 is supplied to the pulse shaper, assembled on transistors VT5 and VT6 according to the Schmitt trigger circuit with emitter coupling. At its output, rectangular pulses are formed with a generator frequency, a short decay time (about 50 ns) and a swing equal to the supply voltage. Such a fall time is necessary for the normal operation of the decimal counters DD1-DD3. Resistor R8 ensures stable operation of the Schmitt trigger at low frequencies. Each of the counters DD1 - DD3 divides the signal frequency by 10. The output signals of the counters are fed to the SA2 measurement limit switch.

From the movable contact of the switch, depending on the selected measurement limit "x1", "x10²", "x10⁴" square wave pulse signals U_и (Fig. 2, a) are fed to the "period-voltage" converter, assembled on the op-amp DA1.1, field-effect transistors VT7-VT9 and capacitor C8. With the arrival of the next signal pulse with a duration of 0,5 T, the transistor VT7 closes for this time. The voltage from the resistive divider R13R14 (about 2,5 V) is fed to the non-inverting input of the op-amp DA 1.1. A stable current source (IT) is assembled on this op-amp and transistor VT9. The IT current of 140 μA is set by parallel connection of resistors R16 and R17 with the contacts of the SA3 switch closed ("x1" position) and ten times less - 14 μA - by resistor R16 when open ("x10" position).

Fig. 2

At the moment of arrival of a pulse with a duration of 0,5T, the transistor VT8 through the differentiating circuit C7R15 opens for 5 ... 7 μs, discharging the capacitor C8 during this time, after which it closes and the capacitor C8 starts charging with a stable current from IT (Fig. 2, b). At the end of the pulse, the transistor VT7 opens, closing the resistor R13, and the IT current becomes zero. During the next interval of 0,5T, the voltage U1 on the capacitor C8 remains unchanged until the next pulse arrives and is equal to

U₁ = U_С8 = I_IT1xT / (2xC8) \uXNUMXd K₁xT,

where K₁ = I_IT1/(2хС8) - constant coefficient.

It follows from this expression that the voltage across the charged capacitor C8 is proportional to the period T of the incoming pulses. In this case, a voltage of 2 V corresponds to the maximum value of the measured parameter at each measurement limit. The capacitor is connected to the input of the buffer amplifier on the op-amp DA1.2 with a unity gain, the input current of which is negligible (a few picoamperes) and does not affect the discharge (and charging) of the capacitor C8.

From the output of the buffer amplifier, it goes to the next converter - "voltage-current" to the op-amp DA2.1. On this op-amp and resistors R18-R21, another IT (IT2) is assembled. The current of this IT is determined by the input voltage supplied to the left output of the resistor R18 according to the circuit, and its resistance, and the sign depends on which of the resistors (in our case it is R18 or R20) is included in the input. IT is loaded on capacitor C9. During the action of the input pulse with a duration of 0,5 T, the transistor VT10 is open and the voltage U₂ on the capacitor C9 is zero (Fig. 2, c). At the end of the pulse, the transistor closes and the capacitor starts charging with direct current from the voltage supplied to the resistor R18 from the buffer amplifier to the op-amp DA1.2. As can be seen from the diagram (Fig. 2, c), the voltage on the capacitor increases linearly in the form of a saw until the next pulse appears after a time of 0,5 T. By the time it appears, the voltage across the capacitor will reach the value

U_2max = U_С9max = I_IT2xT / (2xC9) \uXNUMXd U_C8xT/(2xR18xC9) = K₂xU_C8xT = K₁xK₂xT²,

where K₁, K₂ - constant coefficients; To₂ = 1/(2xR18xC9).

It follows from this expression that the amplitude of the voltage across the capacitor C9 is proportional to the square of the period of the incoming pulses, i.e., it depends linearly on the measured inductance or capacitance. Such a transformation "to the square of the period" is logically understandable even without the above expression, since the voltage across the capacitor C9 depends linearly simultaneously on both the period and the voltage at the IT input, which also depends linearly on the period. In this case, the voltage U2max, equal to 2 V, corresponds to the maximum value of the measured parameter at each measurement limit.

To the capacitor C9 is connected the input of the buffer amplifier to the op-amp DA2.2. From its output, the sawtooth voltage, reduced to the required level by the R22R23 divider, is fed to the "VΩmA" input of the multimeter (XP2 connector). The built-in integrating RC-circuit of the multimeter, connected to the ADC input (time constant 0,1 s), and the external one - R22C12 smooth out the sawtooth pulses to an average value for the period, which is equal to a quarter of the amplitude. So, with a "saw" amplitude at the XP2 "VΩmA" connector of 0,8 V, the voltage at the input of the multimeter's ADC is 200 mV, which corresponds to the upper limit of the DC voltage measurement at the limit of 200 mV.

The prefix is ​​assembled on a board made of fiberglass laminated on both sides. The PCB drawing is shown in fig. 3, and the location of the elements on it - in Fig. 4.

Fig. 3

Fig. 4

Photos of the printed circuit board are shown in fig. 5, 6. Pin XP1 "NPNC" - suitable from the connector. Pins XP2 "VΩmA" and XP3 "COM" - from failed test probes for the multimeter. Input sockets XS1, XS2 - screw terminal block 350-02-021-12 of DINKLE series 350. Sliding switches: SA1 - SS12D07; SA2, SA3 - MSS, MS, IS series, such as MSS-23D19 (MS-23D18) and MSS-22D18 (MS-22D16) respectively. Coil L1 is self-made, contains approximately (to be specified when setting up) 160 turns of PEV-2 0,2 wire, wound in four sections of 40 turns each on an annular magnetic core of size 10x6x4,5 made of ferrite 2000NM1, 2000NM3 or N48 (EPCOS). Ferrites of these grades have a low temperature coefficient of magnetic permeability. The use of ferrites of other brands, for example N87, will lead to an increase in the measurement error of capacitance when the temperature changes already by 5...10 ^оC.

Fig. 5

Fig. 6

Capacitors C1, C8 and C9 - film imported output for a voltage of 63 V (for example, WIMA, EPCOS). The deviation of the capacitance of capacitors C8, C9 should be no more than 5%. The rest - for surface mounting: C2, C10, C11 - size 0805; C4, C6, C7 - 1206; oxide C3, C5, C12 - tantalum B. All resistors of size 1206. Resistors R13, R14, R16-R21 should be used with a tolerance of no more than 1%, and resistors R18, R20 and R19, R21 should be selected with a multimeter with as close resistances as possible in each couple. Often, a tape pack of 10 ... 20 resistors of the E24 series of a five percent accuracy class is enough to select.

Transistors VT1 -VT5 must have a current transfer ratio of at least 500, VT6 - from 50 to 200. BSS84 transistors are replaceable with IRLML6302, and IRLML2402 with FDV303N. For other replacement, it should be taken into account that the threshold voltage of the transistors should be no more than 2 V, the open channel resistance should be no more than 0,5 Ohm, and the input capacitance should be no more than 200 pF at a drain-source voltage of 1 V. AD8542ARZ micropower op-amps are interchangeable, for example , MSR602 or domestic QF1446UD4A. It is advisable to select the latter by a zero bias voltage of no more than 2 mV to reduce the measurement error when its result does not exceed 10% of the set limit. Decimal counters 74HC4017D of high-speed logic can be replaced with similar ones from the 4000B series from NXP (PHILIPS) - HEF4017B. Similar meters from other companies, especially domestic K561IE8, should not be used. With a supply voltage of 3 V, the input frequency of 1 MHz from the measuring generator for such counters is too high, and the duration of the decay of the pulse at their input (50 ns) is short. They may not "feel" such a signal.

The conclusions of the capacitors C8, C9, going to the common wire, are soldered on both sides of the printed circuit board. Similarly, the conclusions of the SA3 switch and the conclusion coming from the movable contact SA2, as well as the XP1-XP3 plugs, are soldered. Moreover, XP2 and XP3 are fixed by soldering in the first place, and then a hole is drilled “in place” and the XP1 plug is soldered. Pieces of tinned wire are inserted into the holes of the pads near the source of the transistor VT10 and resistor R14 and soldered from both sides. Before mounting on DD2, DD3 microcircuits, pin 4 should be bent or removed.

When working with an LC meter, the multimeter's type of operation switch is set to the position for measuring direct voltage at the limit of "200mV". The measurement limits of the LC-meter, corresponding to the positions of the switches SA2, SA3, are given in the table.

Calibration of the LC-meter is carried out depending on the availability of the necessary instruments and qualifications. In the simplest case, you will need a coil with a precisely known inductance, the value of which is close to the corresponding measurement limit, and the same capacitor with a measured capacitance. To eliminate the error from the input capacitance of the LC meter, the capacitance of the capacitor must be at least 1800 pF (for example, 1800 pF, 0,018 μF, 0,18 μF). The set-top box is first connected to an autonomous power supply with a voltage of 3 V and the consumed current is measured, which should not exceed 3 mA, and then connected to a multimeter.

Next, set the switch SA1 to the "Lx" position and connect a coil with a known inductance to the sockets XS1, XS2 "Lx, Cx". Switches SA2 and SA3 are set to the appropriate limit and achieve readings on the indicator that are numerically equal to the inductance (the comma of the indicator is not taken into account), connecting, if necessary, an additional capacitor C1 with an additional capacity of up to 3300 pF in parallel. Capacitors C1, C8, C9 have pads on the printed circuit board for desoldering additional sizes 0805 for surface mounting.

A more accurate correction of the readings is possible by changing the resistance of the resistor R22 or R23 within small limits. Similarly, the LC meter is calibrated when measuring capacitance, but the corresponding readings on the indicator are set by changing the number of turns of the coil L1.

When measuring capacitance with a prefix, it is necessary to take into account its input capacitance, which in the author's sample is 41,1 pF. This value is displayed by the multimeter indicator if you set the SA1 switch to the "Cx" position, and SA2 and SA3 to the "x1" position. When changing the topology of the printed circuit board, the connections of the terminals of the capacitors C8 and C9 with the terminals of the transistors VT9 and VT10 must be made by separate conductors.

The prefix can be used as a generator of fixed frequencies of sinusoidal and rectangular shape. A sinusoidal signal with a voltage of 0,1 V is removed from the emitter of the transistor VT3, a rectangular amplitude of 3V - from the movable contact of the switch SA2. The desired frequencies are obtained by connecting capacitors of the appropriate capacitance to the input of the set-top box in the "Cx" position of switch SA1.

PCB drawing in Sprint Layout 5.0 format can be downloaded from ftp://ftp.radio.ru/pub/2014/08/Lc-metr.zip.

Literature

1. Universal LC generator. - Radio, 1979, No. 5, p. 58.
2. L-meter with linear scale. - Radio, 1984, No. 5, p. 58, 61.

Author: S. Glibin

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