Capacitance meter and EPS of capacitors - attachment to the multimeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Nowadays, almost every radio amateur has a digital multimeter, but not every model has a function for measuring the capacitance of capacitors. Both in radio repair and in evaluating the suitability of reused capacitors, measuring the capacitance and equivalent series resistance (ESR) of "suspicious" capacitors is very useful.

The main criteria in the development of the meter were the simplicity of the circuit, the cheapness and availability of elements, ease of adjustment and small dimensions. We can say that this is a "weekend construction" that can be assembled in a few hours

The operation of this device when measuring capacitance is based on the principle of charging a capacitor of unknown capacity to a certain voltage through a resistor of known resistance. The duration of this process is directly proportional to the capacitance of the capacitor.

The principle of measuring EPS is as follows, a discharged capacitor is connected to a voltage source through a resistor of known resistance. then, at short intervals, the microcontroller measures the voltage on the charged capacitor twice and calculates its ESR.

With a decrease in capacitance, the measurement error of the ESR increases. Therefore, this measurement is disabled by software when the capacitance of the capacitor is less than 2 uF.

Main Specifications

• Capacitance measurement interval, uF......0,02...10000
• Capacitance measurement error, no more than, % ......5
• EPS measurement interval, Ohm.......0.. 50
• EPS measurement resolution, Ohm ...... 0,2
• EPS measurement error, Ohm .......±0,45
• Maximum voltage on the tested capacitor, V ....... 5
• Current consumption, mA in rest mode ....... 5,5
• in measuring mode.....11

The meter circuit is shown in fig. 1 The basis of the device is a microcontroller PIC 12F683 (DD1) It operates at a clock frequency of 4 MHz from an internal RC oscillator. After turning on, the microcontroller enters the capacitance measurement mode, and then the configuration of the input / output ports is as follows: GP0 and GP4 work as outputs and control the charging of the capacitor through resistors R1 and R3, respectively; GP1 - inverting input of the comparator built into the microcontroller, while its non-inverting input is connected to an internal reference voltage source, which determines the voltage threshold, until which the capacitor charging time is calculated; GP3 - signal input from the SB1 button for switching to the EPS measurement mode; GP5 - capacitance subrange indication control output and, finally, SSR1 - output of the SHI signal, the average voltage of which is proportional to the measured parameter. The calculated value of the period of the SHI signal is 4096 μs.

The probes of a digital multimeter switched on in the DC voltage measurement mode at the limit of 2 mV are connected to the output jacks Х2000 and ХЗ.

Rice. 1 (click to enlarge)

The subranges of the measured capacitance are indicated by the green LEDs HL1, HL2 and red HL3, HL4. When measuring capacitance less than 1 μF, as well as when measuring ESR, the LEDs are off. If the capacitance is greater than 1 μF, but less than 10 μF, only the red LEDs are lit. If the capacitance is greater than YumkF, but less than 100 microfarads, they all burn. If the capacitance is more than 100 uF, but less than 1000 uF, only green LEDs are lit Finally, if the capacitance is more than 1000 uF, but not more than 10000 uF, the red and green LEDs flash In this subrange, the maximum value on the multimeter display is "1000" in the rest - "999 "

If the measured capacitance is greater than 10000 uF, the LEDs remain in a state of alternate flashing, and the multimeter display shows the threshold value, which is described below.

The measured capacitor is discharged through resistors R1 and R2, while the GP1 port also switches to output mode. The total time between charge/discharge cycles in the last measurement sub-range reaches 10 s, in other sub-ranges it is less.

When the SB1 button is pressed, the device switches to the ESR measurement mode for 5 s, then returns to the capacitance measurement mode. In the ESR measurement mode, the configuration of the I / O ports of the microcontroller is as follows - GP0 and GP1 synchronously control the charging of the capacitor through resistors R1 and R2; GP4 - input of the built-in analog-to-digital converter;

GP5 and CCP1 perform the same functions as in capacitance measurement mode. During the EPS measurement, the LEDs do not light up, the indication is displayed in tenths of an ohm with a resolution of 0,2 Ohm. This is due to the fact that the resolution of the built-in ADC of the microcontroller is about 5 mV, and the capacitor charging current in this mode is 25 mA. If the measured ESR of the capacitor exceeds 50 ohms, then the multimeter will display a threshold value.

The meter is powered by a 9 V battery, size 6F22, which is connected to connector X1. The battery voltage is supplied to the stabilizer chip 78L05 (DA1) with an output voltage of 5 V. Capacitors C1 and C2 ensure the stability of its operation. If possible, instead of the 78L05 microcircuit, it is better to use the LP2950CZ-5.0 - this will reduce the current consumption to 1,5 mA in rest mode and to 7,5 mA in measurement mode. Diodes VD1 and VD2 and a zener diode VD3 are used to protect the input / output lines of the microcontroller from failure when a charged capacitor is connected. When choosing a VD3 zener diode, it must be taken into account that at a voltage of 5 V, a current of more than 0,5 mA should not flow through it. For example, you can apply BZX55C5V6. Diodes VD1 and VD2 - any silicon impulse, for example, from the KD521, KD522 series. But the 1N4148 diodes were chosen because of the larger maximum permissible pulsed forward current. The VD4 diode can be replaced by a jumper if the wrong polarity of the battery connection to the X1 connector is excluded.

Due to the simplicity of the device, a printed circuit board was not developed for it; it was assembled on a breadboard with dimensions of 26x40 mm. The microcontroller is installed in the panel. When programming, the reset permission of the microcontroller must be disabled - there should not be a checkmark in the "MCLR Enable" box, since this pin is used as a signal input. LEDs HL1-HL4 - any different color of glow with a noticeable brightness at a current of 5 ... 6 mA, the author's copy used DFL-3014RC and DFL-3014LGC with a diameter of 3 mm. A necessary condition is that a chain of four LEDs connected in series should not glow when connected to a 5 V source, so four LEDs are used, although only two are needed for indication. If the brightness of the glow of LEDs of different colors differs markedly, it is equalized by the selection of resistors R8 and R9.

Fig. 2

Connector X1 - terminal block from a 6F22 battery. Sockets X2 and X2 for connecting the multimeter are taken from the power connector of the computer motherboard (Fig. 2). The positive socket X1 has no special features. The negative socket HZ, combined with the SA3 power switch, is a self-made design shown in fig. 3. One of the two springy contact strips is removed, an insulating pad made of fiberglass with a square side of 4 ... 0,5 mm is installed nearby. A bent spring wire with a diameter of 0,6 ... 1 mm is fixed on it, which acts as a SA3 power switch. When the negative probe of the multimeter is inserted into socket X1, it touches the spring wire, as a result of which the circuit of the negative power wire of the meter is closed. Of course, when repeating the design, you can use any industrial miniature power switch SA2 and a negative socket, such as XXNUMX.

Ris.3

Trimmer resistor R7 - SPZ-19a or similar miniature. Resistor R3 determines the charging current for the range of measured capacitances up to 15 μF, it is better to take it with a tolerance of 1% or take it with a digital ohmmeter. Resistor R1, which determines the charging current for capacities more than 15 μF, can be selected from a nominal value of 1 kOhm 5%, its calculated resistance is 980 Ohm, but it is quite acceptable to put 1 kOhm 1% without selection, since such a capacity is typical for oxide capacitors, and for them accuracy measuring their capacitance of 5% is sufficient.

Instrument calibration can be done in two ways.

The first way is to connect one or more capacitors with a total capacitance of more than 10000 uF to the meter and set the threshold value "7" on the multimeter's display using the R1023 trimming resistor. You can also connect a circuit of a 62 ... 100 Ohm resistor and a 50 ... 1000 uF capacitor to the meter input, press the SB1 button and similarly set the same threshold value on the display. Since the meter stays in this mode for only 5 seconds, this operation may need to be repeated several times.

The calibration error can be about 3% in the worst case, since it consists of the errors of the internal generator and the differences in the resistances of the resistors R1-R3 from the calculated values. % in the range of 1...1 °С.

The second way is to connect a film or ceramic capacitor with a known capacitance within 4,7 ... 9 μF to the meter and set the value of its capacitance on the multimeter display using the R7 trimming resistor. It is first necessary to measure the capacitance of this capacitor with an exemplary device with an accuracy of no worse than 1%. When calibrating by this method, the threshold value may slightly differ from "1023" The choice of the calibration method is not fundamental - the spread of readings of several instances of the device calibrated in different ways did not exceed 3%.

Of course, only a pre-discharged capacitor should be connected to the meter. When measuring the capacitance of oxide capacitors, the polarity of the connection must be observed. Touching the measuring clamps with your hands will distort the readings.

Microcontroller programs can be downloaded from ftp://ftp.radio.ru/pub/2013/02/van.zip.

Author: Yu. Vanyushin

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