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This easy-to-build test instrument can measure induc­tances over the range from 10µH to 19.99mH with an accuracy of about 5%. It uses readily available parts and has a 4-digit LCD readout. By RICK WALTERS Build this: 10uH to 19.99mH Inductance Meter 64 Silicon Chip’s Electronics TestBench A N INDUCTANCE METER can be a handy test instrument in many situations. It can be used for servicing (eg, in TV sets), se­lecting coils for RF circuits, checking coils for switchmode power supplies and for measuring coils in many other applica­tions. The instrument to be described here measures from 10µH to 19.99mH over two ranges and has the twin virtues of being easy to build and easy to use. As shown in the photos, there are just three front panel controls: a range switch (µH or mH), a pushbutton switch and a potentiometer. An AC plugpack is used to supply power, so there is no on/off switch to worry about. To make a measurement, you first connect the inductor to the test terminals and switch to the µH range. You then press the “Null” button and rotate the knob until the LCD panel meter reads zero, or as close to zero as you can get (ie, a null). This done, you release the button and read the inductance directly off the display. If the meter over-ranges (ie, it only displays a 1 at the lefthand digit), you simply switch to the mH range before reading the inductance value from the meter. The value indicated on the scale by the potentiometer is the DC resistance of the inductor (although, in practice, this reading may not be all that accu­rate). Block diagram Fig.1 shows the block diagram of the Digital Inductance Meter. It uses a 3.2768MHz crystal oscillator (IC1a) to generate a precise clock frequency and this is divided by 20 and filtered by IC5 to give a 163.84kHz sinewave signal. In addition, the signal from the divide-by-20 stage is divided by 100 and filtered by IC6 to give a second frequency of 1638.4Hz. Main Features • Two ranges: 10-1999µH & 1-19.99mH • Indicates inductor DC resistance • Operates from a 9V AC plugpack supply • Accuracy typically 5% from 10µH to 19.99mH Range switch S2a selects between these two frequencies and feeds the selected signal to a nulling circuit. This circuit is used to null out the DC resistance of the inductor being measured. The output from the nulling circuit is then fed to positive and negative peak detectors and these in turn drive a digital panel meter (DPM). Circuit details Let’s now take a look at the circuit diagram of the Induc­tance Meter – see Fig.2. NAND gate IC1a and its associated components function as a square wave oscillator. It oscillates at a frequency of 3.2768MHz, as set by crystal X1. The 33pF, 270pF and 100pF ca­pacitors provide the correct loading for the crystal and ensure that it starts reliably when power is applied. Pushbutton switch S1 is used to disable the oscillator. Normally, the output of IC1a (pin 3) clocks the pin 15 (CA-bar) input of IC2b. However, when S1 is pressed, pin 1 is pulled low and IC1a’s pin 3 output remains high. We’ll explain why this is done later on. IC2b, part of a 74HC390 dual 4-bit decade counter, divides the clock signal from IC1a by 10. The divided 327.68kHz output appears at pin 9 and in turn clocks pin 1 of IC3a. IC3a is one half of a 74HC112 dual J-K flipflop. In opera­tion, it toggles its Q and Q-bar outputs on each falling edge of the clock pulse and thus divides the frequency on its pin 1 input by 2. The resulting 163.84kHz square wave signal on the Q output (pin 5) is then applied to op amp IC5 which is configured as a Multiple Feedback Bandpass Filter (MFBF). Because a square wave is made up of a fundamental sinewave frequency plus multiple harmonics, we can configure IC5 to recov­er virtually any harmonic. In this case, we are using IC5 to recover the 163.84kHz fundamental frequency, as determined by the three resistors and two capacitors between the output of IC3a and its inverting input. The recovered 163.84kHz sinewave output appears on pin 6 of IC5 and due to the bandwidth limitations of the IC, it is a little “notchy”. For this reason, it is further filtered using a 1.5kΩ resistor and a 470pF capacitor to remove these high fre­quency artefacts. This filter circuit also reduces the amplitude of the sinewave to around 5V peak-to-peak. The filtered sinewave is then fed to VR1 which is the calibration control for the µH (microhenry) range. Similarly, for the mH range, IC3a’s Q-bar output is fed to pin 4 of IC2a which in conjunction with IC1c and IC1d is wired as a divide-by-5 counter. Its output appears at pin 3 and clocks decade counter IC4. IC4 divides the frequency on its pin 15 input by 10 and in turn clocks JK flipflop IC3b which divides by two. The signal is then fed to MFBF filter stage IC6, in this case centred on 1638.4Hz. The output from pin 6 of IC6 is a 1638.4Hz sinewave (also at 5V p-p) and this is fed to calibration control VR2. Range switch S2a selects between the two output frequencies Fig.1: the block diagram for the Digital Inductance Meter. Two precise sinewave frequencies are derived and these are fed to a null circuit which contains the inductor under test. The following circuitry then measures the impedance of the inductor and displays its inductance in µH or mH. Silicon Chip’s Electronics TestBench  65 Parts List 1 PC board, code 04107991, 124mm x 101mm 1 plastic case, Jaycar HB6094 1 front panel label 1 Digital Panel Meter, Jaycar QP5550 (or equivalent) 1 9V AC plugpack 1 chassis mount power socket, to suit plugpack 1 DPDT toggle switch (S1) 1 pushbutton switch, (PB1), Jaycar SP0710 (or equivalent) 1 speaker connector panel, Jaycar PT3000 (or equivalent) 1 knob to suit front panel 1 ferrite core set, Altronics L5300 (or equivalent) 1 bobbin, Altronics L5305 (or equivalent) 20m 0.25mm enamelled copper wire 2 5kΩ multi-turn trimpots (VR1-2) 1 10Ω wirewound potentiometer (VR3) (see text for alternative) 3 20kΩ vertical mounting trimpots (VR4-VR6) 1 3mm x 20mm bolt 1 3mm nut 1 3mm flat washer 1 3mm fibre washer 13 PC stakes Semiconductors 1 74HC00 quad 2 input NAND gate (IC1) 1 74HC390 decade counter (IC2) 1 74HC112 dual JK flipflop (IC3) 1 4029 binary decade counter (IC4) and applies the selected signal to the bases of transistors Q1 and Q2 via a 10µF capacitor. Nulling circuit OK, we now have two precise frequencies, either of which can be selected and fed to the bases of PNP transistors Q1 and Q2. These are wired in a nulling circuit. Let’s take a closer look at their operation. The thing to remember here is that the emitter of a PNP transistor is always 0.6V more positive than its base (0.6V more negative for an NPN transistor). Thus, if the base of Q1 is at 5.7V, its emitter sits at 6.3V. Because the supply voltage is 9V, this means that 2.7V must appear across 66 4 LM318 op amps (IC5, IC7-IC9) 1 TL071 op amp (IC6) 1 TL072 dual op amp (IC10) 1 7809 TO-220 9V regulator (REG1) 1 78L05 TO-92 5V regulator (REG2) 1 79L05 TO-92 -5V regulator (REG3) 2 BC559 PNP transistors (Q1,Q2) 4 1N914 silicon diodes (D1-D4) 2 1N4004 1A power diodes (D5,D6) 1 3.2768MHz crystal (X1), Jaycar RQ5271 (or equivalent) Capacitors 4 470µF 16VW PC electrolytic 7 100µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 7 0.1µF monolithic ceramic 5 0.1µF MKT polyester 3 .01µF MKT polyester 1 .0047µF MKT polyester 1 470pF ceramic or MKT polyester 2 270pF NPO 5% ceramic 1 220pF NPO 5% ceramic 3 100pF NPO 5% ceramic 1 33pF NPO 5% ceramic 2 22pF NPO 5% ceramic Resistors (0.25W, 1%) 1 8.2MΩ (select on test) 1 1MΩ 2 5.6kΩ 2 820kΩ 3 4.7kΩ 2 200kΩ 1 1.5kΩ 5 100kΩ 2 1kΩ 1 68kΩ 2 470Ω 1 47kΩ 2 270Ω 1 33kΩ 1 180Ω (calibration) 2 20kΩ 4 100Ω 14 10kΩ 1 3.3Ω (calibration) 1 7.5kΩ the associated 270Ω emitter resistor and this translates into a current of 10mA through the resistor. This (constant) current will also flow in the collector circuit of Q1, regardless of the load resistance (provided this resistance is not too large). If the base of Q1 is now modulated by a sinewave, its collector current will vary sinusoidally, the average still being 10mA. Q2 has the same value of emitter resistor as Q1 so its col­lector current will be the same as Q1’s; ie, 10mA. This collector current flows through potentiometer VR3 to ground. Note that high beta (gain) transistors are used for Q1 and Q2 to reduce the base current, which is a small fraction Silicon Chip’s Electronics TestBench of the emitter current. Because the current through Q2 is 10mA, VR3 (10Ω) will have the same voltage across it as an inductor with a 10Ω resistance connected between Q1’s collector and ground. This position is labelled on the circuit as “DUT”, which means “Device Under Test”. The scale for VR3, on the front panel, is calibrated from 0-10. We will come back to it shortly. Q1’s collector is connected to the positive (red) input terminal of the inductance meter, while the other input terminal is connected to ground. When an inductor is connected across these terminals, a voltage appears across it. This voltage con­sists of two components: (1) a voltage due to the DC resistance of the inductor (as just described); and (2) a voltage due to the inductive reactance. In operation, Q1 drives pin 3 of differential amplifier stage IC7 via a resistive divider (10kΩ & 20kΩ), while Q2 drives the pin 2 input via VR3. IC7 and the following parts, including the LCD readout, function as a digital voltmeter. Before taking a measurement, the resistive voltage compon­ent must be cancelled out. This is done by pressing switch S1 which shuts down oscillator stage IC1a and effectively “kills” the sinewave signals selected by S2a. Potentiometer VR3 is then adjusted so that the signal on pin 2 of differential amplifier stage IC7 is the same as the signal on pin 3, as indicated by a 0.00 reading on the LCD readout. Note that when the meter reads zero, the control knob on VR3 indicates the inductor’s DC resistance on the calibrated scale. Making the measurement If S1 is now released, the selected sinewave modulates the 10mA collector current of Q1. This in turn generates a sinusoidal voltage across the inductor (DUT), the amplitude of which is proportional to the inductance. The resulting sinewave signal from IC7 is subsequently rectified by peak detectors IC8 & IC9, summed Fig.2: the complete circuit diagram of the Digital Inductance Meter. IC1 is the oscillator, while ICs2-5 divide the oscillator signal to produce the two precise sinewave frequencies. Constant current sources Q1 & Q2 form the null circuit. ~--------------------------------------------------------+5V ICla 74HC00 1 0.1+ 15 CA IC2b 12 - 7 4HC390 +10 9 13 CB QA QD 3 o 1.I. 1 . NULL1• Sl I -• 7 Xl 270pF . H [}-----I + 33pF.L • D6 1N4004 15 IN 7809 470'1 T 1001i 0.1 + + REG3 470pF w uHVRl CAL 5k I _• + -5V 16 15 lcK +9V 11 J 13 COUT 7 PO 4 ICld 163 ' 84kH z RANGE S2a uHo._ mH S Q~m n n, n•m n ~ I IC3b +2 7 -5V SINE WA VE SHAPER 7 51- mH CAL VR2 5k .0047+ 7 7 ~--------------1......,.__ +9V 470-:; i + + E' +6.3V +9V 10k 0.1+ 0.1! 470~ 0.1! 7 lk Silicon Chip’s Electronics TestBench  67 7 + ~~~•~VR4 20k IN- +12V jh•~ 10k V+ DIGIT AL PANEL METER POSITIVE PEAK DETECTOR 0.1! 4.7k 7 10k C IC4 Pl 12 4029 p2 13 1.QJu/D +10 ~ P3 CIN B/D 9 PE 1 18 +5 CONST ANT CURRENT SOURCES i ,-----41--+------<----+5V 10 O.lI I- t O.lI G 7 -5V SINE WA VE SHAPER !JcA CB t 470;: -. 7IQD +12V QGI ('.;i') ~ ~-~ 79L05 VIEWED FROM BELOW 4 1N4004 t ~2 +2 -Q 6 I R 2 K 7 D5 FROM PLUG· PACKI . IC3a 74HC112 l00pFI OSCILLATOR 12VAC J 1 C- 78L05 4 116 3 R "--~--,~ 8 14 lk 3.27 68MHz Effie 1Wo oWG ,l~ia 16 14 - BC559 OFF +5V 7 22pF . ,;>-'-f-1 IN+ V- ON 3x100k 10k 7 7 +5V OR -5V .... *HYW 6 6 0.1 I 10k 22pF I 7 O.lI _ _ _ _.......,...... 5v 1001 20k -5V 7 2 DIFFE:ENTIAL -5V AMPLIFIER +5V 7 NEGATIVE PEAK DETECTOR DVM ZERO +5V 7 DIGIT AL INDUCTANCE METER RANGE S2b Pl IP2 P3 Fig.3: install the parts on the PC board as shown here, taking care to ensure that all polarised parts are correctly oriented. Note that two 8.2MΩ resistors are shown connected to pin 2 of IC7 but only one is used in practice and is selected on test (see text). Note also that the metal case of the pot is connected to earth via one of its terminals. 68 Silicon Chip’s Electronics TestBench in IC10b and applied to the digital panel meter. IC8 is used to detect and rectify the positive sinewave peaks. It works like this: when the output of IC7 swings posi­tive, pin 6 of IC8 swings negative and charges a 100µF capacitor via D4 and a series 100Ω resistor to the peak level of the wave­form. As a result, the voltage across the 100µF capacitor is equal to but opposite in polarity to the peak positive input voltage. D4 prevents the 100µF capacitor from discharging as the input level falls and the voltage on pin 6 starts to rise. In addition, D3 is reverse biased during this time and so has no effect. Conversely, when IC7’s output swings negative, IC8’s output swings positive and is clamped by D3 so that it is 0.6V above the virtual earth input at pin 2. As a result, the voltage across the 100µF capacitor is “topped up” only during positive signal excur­sions at the output of IC7. IC9, the negative peak detector, works in exactly the same way but with opposite polarity. It charges its 100µF capacitor to the positive peak of the applied waveform. Thus, the positive peak voltage is represented by a negative DC voltage, while the negative peak voltage is represented by a positive DC voltage across the lower 100µF capacitor. Due to the bandwidth limitations of the ICs, this rectifi­cation is not perfect at the higher frequency. This limits the accuracy below 10µH and readings below this value should only be used for comparison measurements. The output signals from the positive and negative peak detectors are summed in amplifier stage IC10b. This stage oper­ates with a gain of .056, as set by the 5.6kΩ and 100kΩ feedback resistors, to match the signal to the sensitivity of the DPM (200mV FSD). IC10b drives op amp IC10a which operates with a gain of two and this then drives the IN+ input of the panel meter. Note that the IN- input of the panel meter takes its refer­ence from the 9V supply rail and normally sits at about 6.3V. As a result, IC10a must also operate as a level shifter. This is achieved by biasing pin 3 of IC10 to half the IN- reference voltage (using two 10kΩ resistors). Thus, under no signal condi­tions, pin 1 also sits at 6.3V and the meter reading is zero. Trimpot VR6 is used to compensate Table 1: Capacitor Codes           Value IEC Code EIA Code 0.1µF 100n 104 .01µF   10n 103 .0047µF   4n7 472 470pF 470p 471 270pF 270p 271 220pF 220p 221 100pF 100p 101 33pF   33p   33 22pF   22p   22 for any offset voltage at the output of IC10a and allows us to set a zero reading on the DPM when the output of IC7 is at ground. Similarly, VR4 and VR5 compensate for any offset voltages at the outputs of the peak detectors. Range switch S2b switches the decimal point on the panel meter, so that it displays the correct value when we switch from µH to mH. In effect, this switch divides by 10 while S2a divides by 100, so that we get an overall range division of 1000 when switching from the µH to the mH range. Power supply Power for the Digital Inductance Meter is derived from a 12VAC AC plugpack supply. Its output is halfwave rectified by diodes D5 and D6 to derive +12V and -12V rails and these are filtered and fed to 3-terminal regulators REG1 & REG3 respective­ly. Quite a few changes were made to the PC board of the Digital Inductance Meter after this photograph was taken. Table 2: Resistor Colour Codes  No.    1    1    2    2    5    1    1    1    2  14    1    2    3    1    2    2    2    1    4    1 Value 4-Band Code (1%) 5-Band Code (1%) 8.2MΩ grey red green brown grey red black yellow brown 1MΩ brown black green brown brown black black yellow brown 820kΩ grey red yellow brown grey red black orange brown 200kΩ red black yellow brown red black black orange brown 100kΩ brown black yellow brown brown black black orange brown 68kΩ blue grey orange brown blue grey black red brown 47kΩ yellow violet orange brown yellow violet black red brown 33kΩ orange orange orange brown orange orange black red brown 20kΩ red black orange brown red black black red brown 10kΩ brown black orange brown brown black black red brown 7.5kΩ violet green red brown violet green black brown brown 5.6kΩ green blue red brown green blue black brown brown 4.7kΩ yellow violet red brown yellow violet black brown brown 1.5kΩ brown green red brown brown green black brown brown 1kΩ brown black red brown brown black black brown brown 470Ω yellow violet brown brown yellow violet black black brown 270Ω red violet brown brown red violet black black brown 180Ω brown grey brown brown brown grey black black brown 100Ω brown black brown brown brown black black black brown 3.3Ω orange orange gold brown orange orange black silver brown Silicon Chip’s Electronics TestBench  69 This photograph shows the completed Digital Inductance Meter with the calibration inductor connected to its test terminals – see text. REG1 provides a +9V rail, while REG3 provides a -5V rail. In addition, REG1 feeds REG2 which provides a regulated +5V rail. The ±5V rails supply most of the op amp stages, while the +9V rail supplies the digital panel meter and the constant current sources in the null circuit. The +12V rail is used for the positive supply to IC10, as its output needs to swing up to near the 9V supply of the DPM. Putting it together Building the circuit is a lot easier than understanding how it works. 70 Most of the parts are mounted on a single PC board and this is coded 04107991. This, together with the digital panel meter, fits inside a standard plastic case with a sloping front panel. As usual, check the PC board for etching defects by compar­ing it with the published pattern (Fig.4). Any defects should be repaired before proceeding. In addition, part of the PC board will have to be filed away along the bottom lefthand and bottom righthand edges, so that the board will fit between the mounting pillars of the case. Check also that the body of switch Silicon Chip’s Electronics TestBench S1 fits through its matching clearance hole in the board. Enlarge this hole with a tapered reamer if necessary, so that it clears the switch. The same goes for the threaded bush of pot VR3. Fig.3 shows the assembly details. Begin by fitting 13 PC stakes for the external wiring points, then fit the 11 wire links on the top of the board (including the one under VR3). This done, fit the resistors, diodes and transistors. Table 2 shows the resistor colour codes but check them with a DMM as well, just to make sure. Take care to ensure that all the transistors and diodes are installed the correct way around and make sure the correct part is used at each location. Once these parts are in, install the capacitors (watch the polarity of the electros), the regulators and the ICs. We used IC sockets in the prototype but suggest that you solder your ICs directly to the PC board. Again, be sure to use the correct device in each location and note that the ICs don’t all face in the same direction. The trimpots can now all be installed, followed by poten­ tiometer (VR3). As shown in the photo, VR3 is installed from the component side of the PC board and is secured using a nut on the copper side. Its terminals are connected to their pads on the PC board using short lengths of tinned copper wire. Once the pot is in, you have to run two insulated wire links between its terminals and points CT & CW on the PC board – see Fig.4. These points are located near Q2, towards the bottom righthand corner. Note also that the metal case of the pot is connected to earth via one of its terminals. That completes the board assembly. Before placing it to one side though, go over your work carefully and check for errors. In particular, check for missed solder joints and incorrectly placed parts. Final assembly Next, attach the artwork to the front panel and use it as a drilling template for the switches, the potentiometer, the test terminals and the panel meter. The square cutout for the meter is made by first drilling a series of small holes around the inside of the marked area, then knocking out the centre piece and filing the edges to shape. This done, use a sharp chisel to remove the short mounting pillar inside the case, to prevent it from fouling the panel meter. You will also have to drill a hole in the top rear panel for the 3.5mm power socket – see photo. Be sure to position this hole so that the socket clears the panel meter when it is mounted. The various components can now all be installed in the case, starting with the switches and the input connector block which carries the test terminals. Bend the lugs on the input connector block so that they are parallel to the front panel, to prevent them shorting to the PC board. The board can then be fitted inside the case and secured using two self-tapping screws into the short mounting pillars. Before fitting the digital panel meter, it should have a link fitted from N to OFF (to disable the polarity indication). In addition, you have to fit three 100kΩ resistors from P1, P2 and P3 to OFF. These modifications are all shown on Fig.3 (do not forget the link). The panel meter we used has an external dress bezel with two captive mounting screws. This bezel is mount­ed from the front and the panel meter then fitted over the screws and secured using nuts and fibre washers. The assembly can now be completed by running the point-to-point wiring. Note the connections between S2 and the panel meter. In particular, the middle lefthand terminal of S2 goes to the ON pad on the meter board (not to resistor P3). By contrast, the top and bottom lefthand terminals are connected to the resis­tors on P2 and P1 respectively. Fig.4: two insulated flying leads must be run on the copper side of the PC board, between the pot terminals and points CT & CW, as shown in this diagram. Test & calibration Before you begin testing, you need to wind an inductor which is used later during the calibration procedure. To do this, wind around 300 turns of 30 B&S wire on the L5305 bobbin, then fit the cores and clamp them together using a 20mm bolt, flat washer, fibre washer and nut. Once the coil has been wound, clean and tin the ends, then connect a 180Ω 1% resistor in parallel with it. Now put the coil to one side – you’ll need it shortly, for Step 7 of the following procedure. To test the unit, apply power and check that D5’s cathode is at about 12V. This voltage will depend on the particular plugpack you use and is not too critical. Next check the +9V, Fig.5: check your PC board by comparing it with this full-size etching pattern before installing any of the parts. Silicon Chip’s Electronics TestBench  71 H SILICON CHIP INDUCTANCE METER 5 4 6 7 3 2 8 9 1 0 PRESS AND ADJUST FOR METER NULL +5V and -5V rails – these should all be within 5%. The panel meter should show a reading of around 16.00 or 160.0, depending on the range. Now check the supply rails at the IC pins. If these are OK, you are ready to calibrate the instrument using the following step-by-step procedure: Step1: connect a multimeter across the test terminals and set it to a range suitable for measuring 10mA DC. Step 2: press S1 and check the current on the multimeter. It should be close to 10mA. Step 3: release S1, rotate VR3 fully anticlockwise (0Ω), remove the multimeter and connect a 3.3Ω resistor across the test terminals. Step 4: switch your multimeter 72 10 Fig.6: this full-size artwork can be used as a drilling template for the front panel. mH to a low voltage range and connect it between pin 6 of IC7 and ground. Short switch S1’s terminals using an alligator clip, then adjust VR3 (on the front panel) for a 0V (or as close as you can get) reading on the multimeter. Step 5: connect the multimeter across the 100µF capacitor at the output of IC8 and (with S1 still shorted) adjust VR4 for a read­ing of 0V. Now adjust VR5 for 0V across the 100µF capacitor at the output of IC9. Step 6: adjust VR6 for a zero reading on the panel meter and remove the shorting clip from S1. Step 7: remove the 3.3Ω resistor from the test terminals and fit the inductor that you wound earlier (with its parallel 180Ω 1% resistor). Silicon Chip’s Electronics TestBench Step 8: rotate VR3 to the zero ohms position and measure the voltage on pin 6 of IC7. It must be adjusted to zero by fitting a resistor between pin 2 and either the +5V or -5V rail. Two sets of pads have been placed on the PC board for the resistor, from pin 2 to each supply. Our unit needed an 8.2MΩ resistor to the negative rail. Step 9: set S2 to µH and adjust VR1 until the panel meter reads 174.9. Step 10: switch to the mH range and adjust VR2 for a reading of 17.49. That completes the calibration procedure. You can now close the case and begin using your new inductance meter. By the way, if you find that you cannot zero (or null) the panel meter when measuring an inductor, even with VR3 rotated fully clockwise, it means that the resistance of the inductor is greater than 10Ω. Despite this, the inductance reading displayed when S1 is released should be close to the correct value. What if it won’t work? If you have problems, the first step is to check your sol­ dering. In particular, look for missed solder joints and shorts between adjacent tracks and IC pins. A few voltage checks can also help pinpoint problems. First, check for + 2.5V on pins 5, 6 and 9 of IC3. Pin 6 of IC5 and pin 6 of IC6 should be around 0V DC and 4-5V AC. Most meters will give quite a low reading on the AC output of IC5. As long as you get an indication, the signal is probably OK. The bases of Q1 and Q2 should be at 5.7V and their emitters at 6.3V. The collec­tor of Q2 should read 100mV. Note that when the unit is working properly and there is no inductor across the terminals, the meter will read around 16.00 or 160.0, depending on the range. This is due to the positive peak detector swinging to full output and is normal. Variations VR3 can be changed if you wish to measure inductors with DC resistances greater than 10Ω. For example, a 25Ω pot will allow inductors with resist­ances up to 25Ω to be measured. Naturally you will have to recalibrate the potentiometer scale or you can simply multiply the front panel readSC ing by 2.5.