Autoranging Capacitance Meter wide-range, PIC-controlled Des

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Autoranging Capacitance Meter wide-range, PIC-controlled Des

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TEST&MEASUREMENT
Autoranging Capacitance Meter
wide-range, PIC-controlled
Design Flemming Jensen
Most commercial digital Capacitance meters measure capacitors from (picofarads) 2,000 (microfarads). able measure Circuits like audio power amplifiers, switchedmode power supplies, printers photocopiers often incorporate really large capacitors hundreds range 1,000 µF). With this cheap circuit, you'll able measure capacitors almost `size', from picos Farads.
Main Features
cost, direct reading, Wide range, (from several Farads) Auto-ranging, range switches) Auto-zeroing, (nulls stray capacitance connected test leads test power Low-Battery indication Simple setup
Although specifications Autoranging Capacitance Meter pretty impressive, circuit diagram uncluttered free from surprises Figure Using connector capacitor tested, connected into circuit based CMOS version good timer Here, TLC555 configured monostable multivibrator (MMV). real workhorse circuit PIC16F84 microcontroller running MHz. Using output port lines, controls 555's
Elektor Electronics
2/2003
TEST&MEASUREMENT
BATT.
100n 100n MCLR
Module Trimods 1535 Farnell 142-554
CONTRAST
BC557B
CNTR
RB0/INT
PIC16F84
1N4148
OSC2
OSC1
TLC555
20MHz
BATT. 1N4148 BATT.
78L05
330n 330n
020144
Figure Circuit diagram Autoranging Capacitance Meter.
(reset) (trigger) inputs same time senses logic level 555's (output) pin. larger capacitance capacitor under test, longer High time pulse 555's output. long this output high, increments software counter (i.e., variable). When 555's output drops again result processed sent readout. handles necessary range switching controls module well. meter three capacitance ranges: used capacitors from 9,999 which covers 9999 µF/mF which covers your convenience, read-out modified meter will show 1.00 9.99 same goes range.
oscillator frequency hence calibration each range determined capacitor resistance between parallelconnected (discharge) (threshold) pins TLC555 chip. resistance range-dependent derived from resistor/preset combinations which switched into circuit using port lines. LCD, 2-line, 16-character type controlled 4-bit mode. backlight switched optionally using jumper JP1. power supply totally standard based 78L05 threepin fixed voltage regulator. extra zener diode, included prevent risk damage circuit when input overloaded with direct voltage excess supply voltage circuit powered 9-volt battery. Current consumption region with backlight left off.
Auto-zeroing auto-ranging
power-up, runs routine that checks stray capacitance input caused test leads. puts result variable. This result later subtracted from result obtained from capacitor measurement. Note, however, that this only true range. therefore important that capacitor connected when instrument switched unless, course, intend cancel certain amount stray capacitance. other ranges than range, capacitor connected power-up. After zeroing routine, meter enters range. this point, measured capacitance will recorded placed variable. auto-ranging functions like this: capacitor large range, count will overflow, will select range, i.e., select lower value charging resistor, then continue performing measurement. capacitance value still large, range selected, result displayed line alphanumeric module.
2/2003
Elektor Electronics
TEST&MEASUREMENT
1-441020 ROTKELE
ELEKTOR
020144-1
020144-1
Figure design project (board available ready-made).
noise cancellation
range, input represents very high impedance. this range capacitor charged Megohm resistor therefore meter sensitive noise picked capacitor leads test leads, used. When measuring capacitors near range essential meter kept well removed from transformers etc., otherwise unsteady readout result. reduce noise pickup even further, measurement range performed twice, with 10-ms interval. mean value results calculated result sent readout. This method sure give steadier reading. ranges, resistor values relatively special precautions called for, allowing every single measurement read out.
COMPONENTS LIST
Resistors: (see text) 10-turn multiturn preset, vertical mounting 10-turn multiturn preset, vertical mounting 10-turn multiturn preset, vertical mounting Capacitors: 47nF C2,C7 100nF C3,C4 4pF7 C5,C6 330nF Semiconductors: zener diode D2,D3 1N4148 TLC555 (Texas Instruments), 3V555 TS555 (Thomson)
PIC16F84A-20/P programmed, order code 020144-41 78L05 BC559B Miscellaneous: (6F22) battery with clip-on connector 2-way pinheader with jumper 7-way socket strips, lead pitch inch 16-way boxheader solder terminals on/off switch 20MHz quartz crystal display, 2x16 characters, e.g., Trimods 1536, Farnell 142-554 Length 16-way flat cable with socket Enclosure: e.g., Pactec HPLS series, type HPS-9VB (28x53x91x146 mm), Farnell 736351, also www.pactecenclosures.com PCB, order code 020144-1 (see Readers Services page) Disk, source code files, order code 020144-11 Free Download
Mode change when measuring large capacitors
Capacitors below measured continuously, i.e., meter faithfully starts range. count overflows, routine run. count still overflows then routine run, Once proper range reached, result sent readout. Next, whole thing starts over again: routine overflow routine, Larger capacitors, excess (10,000 10-3 measured
continuously four times. message `wait' displayed until four measurements have completed. result fourth measurement then read out, message `ready' displayed result valid. This method ensures
that capacitor been fully discharged charged, gives reliable reading guarantees current consumption. prepare measurement on/off switch operated.
Elektor Electronics
2/2003
TEST&MEASUREMENT
Construction Troubleshooting
Populating printed circuit board shown Figure should present problems follow component overlay printed ready-made board else gleaned from Figure information parts list. Connector consists socket strips that allow capacitors with different spacings easily connected without introducing much stray capacitance. Wander (banana) sockets traditional colours black used test leads. Depending exact type intend need establish suitable value resistor accordance with current drawn backlight lamp inside LCD. Here, value watt, given initial guidance. three wire links board should installed first they forgotten later. Carefully inspect board solder blobs, short circuits, joints orientation polarised components. working prototype board shown Figure Before fitting board, check presence +5-volt supply volt-
Figure Close-up view prototype board.
Figure look inside case. copper foil shielding optional, only necessary interference from external sources problem.
2/2003
Elektor Electronics
TEST&MEASUREMENT
Things Remember
Discharge capacitors before testing. range: first power meter, then connect capacitor. Capacitors larger than measurements performed. When meter signals `ready' result valid. Turn meter back measurement. Large capacitors: don't impatient. 370-mF will take about minutes test.
relevant points circuit (use circuit diagram reference). everything checks okay, switch install ICs. connected circuit boxheader short length flatcable. suggested mounting method Pactec case gleaned from photograph Figure board fits exactly three moulded bosses Pactec case, secured using small self-tapping screws. will have secured sloping part enclosure using glue similar substance, only connections circuit known correct. don't anything display, turn fully anticlockwise. this result dark upper line, text, probably have short-circuit between some tracks going display connector, have some display wires mixed Check repair before proceeding. Figure shows pieces copper laminate foil fixed inside case. pieces also present under board. This method screening sensitive circuit helps avoid interference. laminate should connected ground plane. suggested Pactec HPS-9VB enclosure internal battery compartment requires minimum amount mechanical work. However, you'll have carefully file away about each side module board. This shouldn't difficult. There's need make clearance module, because enclosure comes with loose display panel.
tolerance number makes CMOS been examined. conclusion. prefer genuine Texas Instruments TLC55, Thomson TS555 3V555IN. decide other brands necessary alter range resistors slightly. Using close tolerance capacitors reference devices, Thomson proved possible next Capacitance Meter achieve repeatable results using nothing more than ordinary multimeter. only requirement that capable reliably measuring resistance values excess This should possible Elektor Electronics readers avoid special reference capacitors when setting instrument. Remove from their sockets. range: measure resistance between pin6/7 collector adjust reading. range: measure resistance between IC2, then adjust 5.94 reading. Finally, range: unsolder (the nearest connector), then measure resistance between this IC1. Adjust preset reading.
connect 220-nF cap, then adjust 220-nF reading. range will probably possible hold low-tolerance capacitor have access commercial capacitance meter ohmmeter adjust series combination R1-P3 total resistance
trimming capacitors
construct simple test (fixture) hold capacitors, auto-zero function will cancel capacitance make very easy test pF-range caps. same goes trimmer tuning capacitors (ceramic, PTFE types). Make simple, mechanically stable that allows capacitor soldered into). Switch instrument with test connected stray capacitance will cancelled out. Next, solder trimmer measure. Adjust trimmer observe varying capacitance. Make note lowest highest capacitance.
Burn your
Assembly-code lovers, well those with access programmer blank 16F84 chips will pleased know that author's source code files this project available free charge from Free Downloads page www.elektor-electronics.co.uk. file number 020144-11 under month publication.
Setting instrument using reference caps
Turn contrast preset fully anticlockwise then slightly clockwise until usable contrast reached. adjust Capacitance Meter should hold precision capacitor whose value falls range, e.g., that falls range e.g., These values obtained tolerance from Farnell, among others. value like 1,000pF advisable because will cause meter switch back forth between 1.00 reading during adjustment. Keep instrument well away from mains transformers. aware that your soldering iron halogen light source generate strong magnetic field. Switch instrument connect 470-pF precision cap. Adjust 470-pF reading. Remove capaci-
Final notes
sure discharge capacitor before testing Sure, Capacitance Meter equipped with simple diode input protection, would like stress into actual operation, would you? Various other, more advanced, protective circuits have been tested they seemed degrade either wide range accuracy meter. Still, Capacitance Meter rather robust just keep mind that capacitors should fully discharged before testing.
(020144-1)
Setting instrument using
Turn contrast preset fully anticlockwise then slightly clockwise until usable contrast reached. production
Elektor Electronics
2/2003

Autoranging Capacitance Meter wide-range, PIC-controlled Des

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