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By RICK WALTERS Build A Digital Capacitance Meter Got a junk box with a stack of capacitors with the values rubbed off? Maybe you are building a filter & need to match some capacitors closely. Or maybe you just can’t read the capacitor labels. This neat little Capacitance Meter will soon let you check their values. It measures capacitors from a few picofarads up to 2µF. Every multimeter will read resistance values but few will read capacitance or if they do, they don’t read a wide enough range. This unit can be built in several forms. It can be a self-contained unit with its own digital display or it can be built as a capacitance adaptor to plug into your digital 66  Silicon Chip multimeter. And you can run it from batteries or an AC or DC plugpack. Our preferred option is to build it as a self-contained instrument running from a DC plugpack. Batteries are OK but we prefer to do without them wherever possible. If you only use the item on infrequent occasions, the batteries always seem to be flat. Our new Digital Capacitance Meter is a simple instrument with no-frills operation. It is housed in a small plastic utility box with an LCD panel meter and a 3-position switch labelled pF, nF and µF. There are two terminal posts for connection of the capacitor to be checked and no On/Off switch. To turn it on, you plug in your 12V plugpack. The unit will measure capacitance values from just a few picofarads up to 2µF. Its accuracy depends on calibration but it should be within ±2%. Theory of operation The theory of operation of the capacitance meter is simple and is illustrated in Fig.1. We apply a square wave to Parts List 1 main PC board, code 04101991, 89 x 48mm 1 switch PC board, code 04101992, 44 x 30mm 1 plastic case, 130 x 68 x 41mm, Jaycar HB-6013 or equivalent 1 front panel label, 120 x 55mm 1 3-pole 4-position rotary switch 1 knob to suit switch, Jaycar HK7020 or equivalent 1 power input socket, 2.1mm x 5.5mm, Jaycar PS-0522 or equivalent 1 red binding post 1 black binding post 2 3mm x 10mm countersunk head screws 4 3mm nut 2 3mm star washer 1 20kΩ multi-turn top adjust trimpot (VR1) 1 2kΩ multi-turn top adjust trimpot (VR2) 1 100kΩ vertical trimpot (VR3) Semiconductors 1 74HC132 quad NAND Schmitt trigger (IC1) one input of an exclusive-OR gate and feed the same square wave through a resistor to charge the capacitor we are measuring. The voltage on the capacitor is fed to the other input of the XOR gate. While the capacitor’s voltage is below the input switching threshold the output of the gate will be high (+5V). An XOR gate’s output is low when both inputs are the same (low or high) and high when they differ. The larger the value of the capacitor the longer it will take to reach the threshold and consequently the higher the duty cycle of the output pulse waveform (ie, wide pulses). Putting it another way, if the capacitor is small, it won’t take long for it to charge and so the resulting pulses will be very narrow. This pulse waveform is integrated (filtered) and fed to a voltmeter. The circuit time constants are arranged to make the voltage reading directly proportional to capacitance. How it works Of course, like all theory, the practical realisation is a lot more complicat- 1 74HC86 quad exclusive-OR gate (IC2) 1 TL071, FET-input op amp (IC3) 1 2N2222, 2N2222A NPN transistor (Q1) 1 78L05 5V 100mA regulator (REG1) 2 1N914 signal diodes (D1,D2) Capacitors 4 100µF 25VW PC electrolytic 1 1µF 25VW PC electrolytic 1 0.1µF MKT polyester 2 .01µF MKT polyester 1 12pF NPO ceramic Resistors (0.25W, 1%) 1 8.2MΩ 1 15kΩ 1 820kΩ 1 10kΩ 2 220kΩ 1 8.2kΩ 1 20kΩ 1 1.5kΩ Panel Meter Option Resistors (0.25W, 1%) 1 1.5MΩ 2 20kΩ 3 100kΩ 4 10kΩ 1 39kΩ 1 1kΩ 1 100kΩ vertical trimpot (VR4) Battery Option 1 SPST toggle switch (S2) 1 9V battery (216) 1 battery clip to suit Plugpack Option 1 12VDC or 9VAC plugpack 1 panel mounting socket to suit plugpack 1 78L05 5V 100mA regulator (REG2) 1 3.9V 400mW/500mW zener diode (ZD1) 1 1N4004 1A power diode (D3) 1 470µF 25VW PC electrolytic capacitor 1 2.2kΩ resistor (0.25W, 1%) 1 panel meter, Jaycar QP5550 or equivalent 1 TL071 FET-input op amp (IC4) 1 0.1µF MKT polyester capacitor Miscellaneous Hookup wire, machine screws & nuts, solder. ed. The circuit of the Capacitance Meter is shown in Fig.2 and you may find difficulty in seeing any resem­blance between it and the simple circuit of Fig.1. Never fear; we will explain it all. First, IC1a is a Schmitt trigger oscillator and it oscil­lates at a rate determined by the switched resistors and the .01µF capacitor. IC1a has an output frequency of 16kHz on the pF range, 160Hz on the nF range and 16Hz on the µF range. The (approximate) square wave output is buffered and inverted by gates IC2b, IC2c and IC2d which have their outputs wired in parallel. These outputs are fed directly to pins 9 and 12 of IC1 and through trimpot VR2 and the 15kΩ resistor to the capacitor we are measuring (CUT). The XOR gate IC2a corresponds to the single XOR gate shown in Fig.1. Note that Q1, the transistor that discharges the ca­ pacitor at the end of each charge cycle, is a 2N2222. This has been specified instead of the more common varieties such as BC547 or BC337, in order to get sufficiently fast switching times. Fig.1: this is the principle of the Digital Capacitance Meter. A square wave is fed to an XOR gate and the time delay in charging the capacitor produces a pulse waveform with its duty cycle proportional to the capacitance. February 1999  67 Fig.2: this circuit can be built as a capacitance adaptor for a digital multimeter or as a self-contained instrument with its own LCD panel meter. It can be powered from a 9V battery or a DC plugpack, in which case the circuit involving REG2 is required. We use two of the Schmitt NAND gates of IC1 (74HC132) as the inputs to IC2a and this has been done to ensure that these inputs make very fast transitions between low and high and vice versa. Without the Schmitt trigger inputs, the XOR gate circuit of Fig.1 tends to have an indeterminate performance and the pulse output can be irregular. The “capacitor under test” (CUT) charges via VR2 and the 15kΩ resistor and eventually the voltage at the input of IC1c (pin 10) will reach its switching threshold and pin 8 will go low. The capacitor is then discharged by transistor Q1 which is driven from the output of oscillator IC1a. The cycle then repeats, with the capacitor being charged again. The waveforms of Fig.3 illus­trate the circuit operation. This output pulse from IC2a is integrated by a 220kΩ resistor and a 1µF capacitor to provide a DC potential to the pin 3 input of op amp IC3, which is connected as a voltage fol­ lower. Trimpot VR3 is used to set the output at pin 6 to zero when the input is zero. This “offset adjust” is most important as an offset as low as 1mV is equivalent to a reading of 1pF on the most sensitive range. Since the output of IC3 must be able to swing to zero, IC3 needs a negative supply rail and this is provided by IC1b which is connected as a 10kHz oscillator. Its square wave output is rectified by diodes D1 & D2 in a diode pump circuit. The result­ing DC supply is about -3V. Stray capacitance Even with no external capacitor connected, the stray ca­pacitance on the PC boards and the interconnecting-wiring will have to charge and discharge. This stray capacitance will thus be seen by the rest of the circuit as a capacitor connected across the terminals. In effect, the stray capacitance will slightly slow the charging and discharging of the real capacitor under test. 68  Silicon Chip To compensate for the stray capacitance, we’ve added a delay circuit to the pin 13 input of IC1d. The idea is to provide the same delay to IC1d as the stray capacitance causes to pin 10 of IC1c. Then both delays will cancel out. The delay circuit con­sists of a variable resistor (VR1) and a 12pF capacitor. VR1 can be adjusted so that with no external capacitor connected, the output of IC2a (pin 11) always stays low. So far then we have described all the circuit you need if you plan to use your multimeter as the readout. The output of IC3 is can be fed directly to a digital multimeter and the reading in mV corresponds to the capacitance in pF, nF or µF. So if the reading is 0.471V and you are switched to the pF range, the capacitance is 471pF. Digital panel meter Unfortunately, we can’t simply feed the output of IC3 to a digital panel meter to make the instrument self-contained. This is because currently available digital panel meters appear to take their reference from their 9V supply rail and so their input voltage needs to be offset with respect to the 0V line. That means that the panel meter usually needs a separate isolated 9V power supply which could be a big drawback. Fortunately, John Clarke has figured out an elegant way to solve the problem. As the negative input of the panel meter sits around 2.6-2.8V below the positive rail (say 6.3V for a 9V supply), we need an op amp to shift the output of IC3 from a 0-1.999V range to a 6.38.2999V range. IC4 does this for us. The output of IC3 is attenuated by a factor of 4 by the two 20kΩ resistors and the 10kΩ resistor connected to pin 3 of IC4, while the gain of 2 is determined by the 10kΩ feedback resis­tors connected to pin 2. The 1.5MΩ resistor has a negligible effect. Thus, the 0-1.999V variation at the output of IC3 is trans­lated to a 1V swing at the input of the digital panel meter. Resis­tors RA and RB are chosen to be 10kΩ and 39kΩ respectively for the meter’s attenuator, which gives it a full scale sensitivity of 1V for a display of 1999. Trimpot VR4 sets the panel meter’s readout to zero when the output of IC3 is zero. The decimal points on the display are all tied to the OFF connection through 100kΩ resistors. Fig.3: these waveforms show the operation of XOR gate IC2a. The bottom trace is the oscillator square wave while the top trace is the output with a small capacitor under test. The middle trace shows the output waveform for a larger capacitor. The output waveform is then integrated (filtered) to produce a DC voltage which is proportional to capacitance. To illuminate a decimal point it is connected to the ON terminal by S1b, the second pole of the range switch. Power supply As already noted, the Capacitance Meter can be run from a 9V battery or from a DC or AC plugpack. If you plan to use a 9V battery, then you will have to fit an on/off switch instead of the plug­pack socket. The 9V battery then feeds the panel meter, IC3 and IC4 directly and the 3-terminal 5V regulator REG1. REG1 supplies CMOS gates IC1 and IC2. This is necessary to ensure that the meter’s cali­bration does not vary with changing supply voltage. If you plan to use a plugpack, more circuitry is required and this involves diode D3 and the additional 3-terminal regula­tor REG2. Diode D3 ensures that a DC plug­ pack cannot cause any damage if it is connected with the wrong lead polarity. It then feeds REG2 which is jacked up by 3.9V zener diode ZD1 so that it deliv­ers 8.9V to IC3, IC4 and the digital panel meter. REG2 also supplies REG1. PC board assembly The Digital Capacitance Meter uses two PC boards as well as the digital panel meter. The main PC board houses most of the circuitry while there is a smaller board for the range switch. Before starting assembly, check each PC board for defects such as shorted or broken copper tracks or undrilled holes. The diagram of Fig.4 shows the details of the two PC boards and all the interconnecting wiring. You can begin by assembling the switch board which mounts just the 3-position switch and three resistors. Note that the specified switch is a 3-pole 4-position rotary type and it will have to be changed to give just three positions. This is done by removing the switch nut and washer, then prising up the flat washer which has a tongue on it. Move the tongue to the next anticlockwise hole and refit the washer and nut. It may sound complicated but once you are actually doing it, it will be straightforward. Make sure the switch provides three posi­tions before you solder it to the board. Next, fit and solder the links, resistors and diodes into the main board, then mount the trimpots, capacitors, 3-terminal regulators and transistor. By the way, the 78L05 regulators February 1999  69 Fig.4: this is the complete wiring of the Digital Capacitance Meter. The LCD panel meter is shown as well as the optional regulator (REG2) required for plugpack operation. Fig.5: this diagram shows the connections and formulas to be used when calculating a capacitor’s value for the calibration method. The digital multimeter used is assumed to have a typical accuracy of 2%. Once everything fits OK, wire the boards together following Fig.4 carefully. Make the leads long enough to be able to test the unit on the bench but not too long or they will be a nuisance when assembling the boards into the case. When all the wiring is complete, check your work carefully and then apply power to the unit. The display should light and you should be able to make some measurements on capacitors although the readings probably won’t be too close to the mark at this stage. It will be need to be calibrated. Calibration procedure look like ordinary plastic TO-92 transistors because they have the same encapsulation. They don’t work like transistors though, so don’t confuse them with the TO-18 metal-encapsulated 2N2222 transistor. Finally, mount the op amps and lastly, the two CMOS ICs. Once the two PC boards are assembled, it is time to work on the plastic case which needs the cutout for the 70  Silicon Chip LCD panel meter and the other holes drilled. The specified panel meter comes with a bezel surround so you don’t need to be ultra-neat when making the cutout for it. It is easier to drill all the holes in the plastic case and check that everything fits before wiring the units together. If you don’t intend to use the LCD panel meter you may be able to use a slightly smaller case. Now that you have a working capacitance meter how do you cali­brate it? We have used 1% resistors on the range switch, so range-to-range accuracy should be within 1%. The basic accuracy of the instrument is set by the .01µF capacitor at the input of IC1a, along with VR2 and the associated 15kΩ resistor. The input thresholds of IC1 also affect the accuracy. These input thresholds can have a variation in excess of 1V from device to device, when using a 5V supply. If we could get a precise .01µF capacitor we could specify an exact resistor value to replace the 15kΩ resistor and trimpot VR2. Unfortunately, this would not solve the input threshold variation problem. These two photos show how the PC boards and the LCD module all fit inside the plastic case. Note that the LCD module is optional – see text. As well, virtually all MKT capacitors have 10% tolerance (K), so we accept the supplied value of the capacitor and adjust the trimpot to calibrate the meter. Having said all this, we still need an accurately known value of capacitor to carry out the calibration. One way is to obtain five or more of the same value (preferably .015µF or .018µF) and measure them all using the uncalibrated meter. Having measured them, add up the values and calculate the average and then use the capacitor which is closest to the average as the calibration unit. The problem with this method is that the whole batch could have its tolerance in the same direction. If you have a digital multimeter there is a much better way. Power up an AC plugpack and set your DMM to read AC volts. Connect a 150kΩ resistor and a .015µF or .018µF capacitor in series across the AC output. Measure the AC voltage across each. We then use the formula shown in Fig.5 to calculate the capacitor value. By measuring the voltage across the resistor we can calculate the current through the capacitor and February 1999  71 on the panel meter’s PC board until the correct reading is displayed. Fault finding F F F Digital Capacitance Meter SILICON CHIP Fig.6: this actual size artwork for the front panel can be used as a drilling template for the switch and the display cutout. we then divide the capacitor voltage by the capacitor current to find its im­ped­ance. This method should give you an accuracy better than 2%, depending on your multimeter’s AC performance, although it does assume that the mains frequency is exactly 50Hz. Testing Once you know the capacitor’s value you can use it to do the calibration. Firstly, with power applied and nothing connect­ed to the input terminals, connect your multimeter to pins E & F on the main PC board. Adjust trimpot VR1 until the DC voltage at pin 11 of IC2 is a minimum (5-10mV depending on the setting of Fig.7: the actual size artworks for the two PC boards. Check your boards carefully before installing the parts. VR3). Note that it dips to a minimum then rises again. Then adjust VR3 until the meter reading is 0mV. Connect the known capacitor to the input terminals and, on the appropriate range, adjust trimpot VR2 for the correct read­ing. If you get close but cannot reach the value, add an extra capacitor in parallel with the .01µF capacitor on pin 2 of IC1, as ex­plained in the fault finding section. If you elected to use the Digital Panel Meter, carry­out the calibration described above, then adjust VR4 for a zero reading with no capacitor connected. This done, connect the stan­dard capacitor across the terminals and adjust the trimpot The first check to make, if the circuit is not working, is to measure the DC voltages. Check that the input to REG1 is around 9V with either battery or plugpack supply. Its output should be 5V ±5%. If any of these voltages are missing, you will have to trace from where they are present along the track (or tracks) to where they vanish. Obviously, if the 9V battery supply measures low or 0V, disconnect it quickly as you may have a short and the battery will be rapidly flattened. For this reason, it is wise to use a bench power supply with an ammeter, if you have one, to do the initial testing. Next, check the negative voltage at pin 4 of IC3. This voltage will vary depending on the current drawn by IC4 but it should be somewhere around -3V. If there is no negative voltage, it is likely that IC1b is not oscillating, so check the soldering and tracks around this device and the polarities of D3 and D4. When it is oscillating the DC voltage at pin 6 should be about +2.3V. The AC voltage should be around 2.75V. Similar DC and AC readings should be present at pins 3 and 12 of IC1 and pins 3, 6 & 8 of IC2. If you discover any voltages that are wildly different then you have found one (or all) of your faults. If you cannot adjust trimpot VR2 to get the meter reading high enough then add a 470pF or .001µF capacitor in parallel with the .01µF capacitor at pin 2 of IC1. Provision has been made on the PC board for this additional capacitor. The value will depend on all the component tolerances, as previously explained. Using it Always start from the pF range and turn the switch clock­wise if the readout indicates over-range. The pF range covers from 1-1999pF; the nF range covers 0.1nF to 199.9nF (or if you prefer .0001µF to .1999µF); and the last range covers .001µF to 1.999µF. If you don’t like nanofarads, and would like the middle range to display µF, disconnect the P1 decimal point wire from S1b. Of course, you will have to alter the label lettering to SC agree with this modification. 72  Silicon Chip