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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING LC meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology I want to offer a direct reading LC meter. This probe, despite its simplicity, has great potential. It allows you to measure:
On the elements DD1 and DD2, a generator is assembled, the timing element of which is the measured capacitance or inductance. On elements DD3 and DD4, a frequency divider with a maximum division ratio of 16777211 is assembled. The entire probe scale includes 25 values that differ from each other by 2 times. When the probe is in operation, it is visually determined which LED blinking frequency is closest to 1 Hz. The readings opposite it are the result of the measurement. Diode VD2 protects the device from power reversal. Capacitance measurement. The capacitor must be discharged before measurement. Set switch S1 to open position (capacitance measurement). Depending on the required accuracy, the measurement can be carried out in three ways. Specifications:
Method 1. The measured capacitor is connected to the probe probes (you can not solder it from the circuit) and it is determined which LED blinks at a frequency of about 1 Hz. On the scale against it, the capacitance value is read. Method 2. For a more accurate capacitance measurement, you need to do everything as in method 1, only look at the LED that blinks at a frequency greater than 1 Hz, count the number of blinks in 10 seconds, and calculate the blinking frequency by dividing the counted number by 10. Reading opposite this LED divided by the received frequency. The result will be the capacitance value of the capacitor. Method 3. For an even more accurate determination of capacitance, you can use an oscilloscope or frequency meter. Moreover, when using an oscilloscope, you can also evaluate the quality of the tested capacitor (determine the loss tangent). Having connected an oscilloscope or frequency meter to the probe probes, you need to touch the tested capacitor with the same probes. If the capacitor has low losses, then the waveform will be as shown in Fig. 2a. For large losses, the oscillogram will look like in Fig. 2b. Determine the value of the period T and, using the formula (1), calculate the capacitance of the capacitor: C=T/40-5*10-9 (F). (one) When repairing radio equipment, it is enough to measure the capacitance of the capacitor according to method 1. If the obtained capacitance value is less than the nominal value indicated on the capacitor by 2 or more times, such a capacitor must be replaced.
Measurement of inductance. Inductance, like capacitance, can be measured in three ways. Method 1. It is similar to method 1 for capacitance measurements. Only switch S1 needs to be closed. Method 2. Similar to method 2 for measuring capacitor capacitances. Switch S1 put in position for measuring the inductance (close). Method 3. Similar to method 3 for capacitance measurements. Inductance is calculated by the formula L \u40d 2 * T (H), (XNUMX) and the view of the oscillograms for coils with low and high losses is shown in Fig. For and 3b, respectively. The values of capacitances of capacitors and inductances of coils with losses, determined using a probe, will contain an error - the greater, the greater these losses.
Signal frequency measurement. The probe allows you to measure the frequency of a TTL-level signal, provided that the power supply of the probe is galvanically isolated from the power supply of the circuit under test. Switch S1 must be set to the position to measure the inductance. Touch the common wire with one probe, and the signal source with the other. Opposite the LED flashing at a frequency of about 1 Hz, read the indication of the signal frequency. For a more accurate determination of the frequency, you can use method 2. Determination of the loss tangent of capacitors. The loss tangent (tg d) can be accurately determined using an oscilloscope. Method 1. To do this, you need to connect an oscilloscope and the capacitor under test to the probe probes. If the waveform looks like in Fig. 2b, the capacitor has losses, the value of which can be calculated. A lossy capacitor can be replaced by an equivalent circuit - a capacitor and a loss resistance connected in series. Then the loss tangent is: tg d = Rp/Xc = Rp/(2*pi*f*C), (3) where Rp - loss resistance (Ohm); Xc - capacitor reactance (Ohm); f is the frequency at which the capacitor operates (Hz); C is the capacitance of the capacitor (F). For this probe: Rp \u0,03d Up / 4 (Ohm). (four) Up - measured on an oscilloscope, according to fig. 2b. When a capacitor is connected to the probe, the period T, taking into account the loss resistance Rp, is equal to: T \u3,33d 12 * (5-Rp) * (C + 10 * 9-5) (s) (XNUMX) If Rp=0 is substituted into this formula, then formula (1) is obtained. Method 2. Measure the capacitance of the capacitor using a probe. If the probe showed a capacitance 2 or more times less than the value of the capacitor (indicated on it), this capacitor has a large loss resistance Rp, and, accordingly, a large tg d. Then, according to formula (5), the loss resistance can be found. The calculation results are summarized in the table:
In the top line of the table - the multiplicity of the probe readings (how many times the capacitance of the capacitor is less than the capacitance indicated on the capacitor case. In the bottom line - the corresponding loss resistance. Determination of the quality factor of inductors. Determine the inductance of the coil L1. Using an ohmmeter (preferably digital), measure the active resistance of the coil R. Calculate the reactance at a given frequency. XL= 2*pi*f*L (ohm), (6) where XL is the reactance of the coil (Ohm); f - operating frequency (Hz); L - coil inductance (H). The quality factor of the inductor is calculated by the formula; Q=XL/R. (7) On this probe, readings are noticeable at Q> 11.
Determination of the magnetic permeability of a ferrite core. Consider three types of cores (Fig. 4). Let us calculate the values necessary to determine the magnetic permeability of the cores. lM \u2d (D + d) * pi / 9 (XNUMX) SM \u2d (D - d) * h / 10 (XNUMX) lM=2*(A+B-2*C) (11) SM=h*c (12) lM=2*(h+а+с)+3/2*а (13) SM \u14d a * b (XNUMX) Formulas (9) and (10) are used for the ring, (11) and (12) for the U-shaped core, and (13) and (14) for the W-shaped core. All dimensions in formulas (9)...(14) are taken in centimeters. Wind at least 15 turns of wire (in bulk) on the core and measure the resulting inductance with a probe (for an E-shaped core, the turns must be wound in size a). The effective magnetic permeability of the core is calculated by the formula ue=(L*lM)/(u0*n2*SM) (15) where L is the inductance of the coil wound on this core (H); lm is the length of the average magnetic field line (cm); SM - cross-sectional area of the magnetic circuit (cm2); u0 - vacuum magnetic permeability (u0=4*pi*10-9 H/cm); n is the number of turns. Identification of short-circuited turns. To determine the presence of short-circuited turns in coils wound on ring-shaped, U-shaped and W-shaped cores, it is necessary to compare the inductance measured by the probe and the calculated one: L=u0*ue*n2*Sm/lm, (16) where ue is the effective magnetic permeability for ferrite materials (indicated on them). If it is unknown, it can be determined as described above. If the inductance determined by the probe is 2 or more times less than the calculated one, then there are short-circuited turns in the coil. Details. Formulas (1, 2, 4, 5) are valid only for a probe assembled on 74HC00 microcircuits. If the probe generator is assembled on microcircuits of other series, including domestic ones, correction factors will appear in the formulas. When choosing chips, you need to remember that:
The author tested microcircuits of the K155, K555, K531, K131, KR1533, 7400, 74LS00, 74NS00 series. The KR1533LAZ chip most of all met all the requirements. She had a voltage swing on the probes of about 0,02 V. But because of this, she turned out to be too sensitive to interference and interference from hands. It was necessary to apply special measures that sharply reduced the measurement range. IC K155LAZ had a large voltage swing, which opened p-n junctions even of silicon transistors and diodes. K555LAZ opened p-p junctions only of germanium transistors and diodes. So from these series it is best to use the 74HCOO chip. It is insensitive to interference and interference from hands, does not open p-n junctions even of germanium transistors and diodes. In addition, it has low energy consumption. For counters, it is also better to use CD74HCT4040 series chips, because. they are sufficiently high-frequency, have an output current sufficient for a good glow of the LEDs, and consume little energy. The supply voltage must be stable. It is selected 4,4 V. When choosing the supply voltage, it must be remembered that its change leads to a change in the coefficients in the formulas (1, 2, 4, 5), and therefore affects the readings of the probe. By changing Un, you can change the range of measured values in one direction or another. Changing the supply voltage also affects the sensitivity of the probe to lossy capacitors. If you decrease it, the sensitivity drops, increase it - it increases. LEDs in the probe - any, red glow. You can not install all of them, but install, for example, through one. True, the scale step will increase in this case. Setting. The probe is placed on a 105x30 mm board. The probe scale is calculated according to formulas 1 and 2 and is true only when using the 74NSOO chip and a supply voltage of 4,3 V. It is advisable to install the DD2 chip in the socket, because. if you accidentally touch the probe to an undischarged capacitor under high voltage, the microcircuit may burn out. Therefore, it is imperative to discharge the capacitors before measuring. Probe probes should be as short as possible. even a very small inductance of the probes affects its performance. In the author's version, the length of one probe (together with the cable) is 22 cm, and the other is 10 cm. Author: S.Volodko, Gomel.
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