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TEST&MEASUREMENT

Autoranging Capacitance Meter

TEST&MEASUREMENT
Autoranging Capacitance Meter
wide-range, PIC-controlled
Design by Flemming Jensen
Main Features
- Low cost, direct reading, - Wide range, (from a few pFs up to several Farads) - Auto-ranging, (no range switches) - Auto-zeroing, (nulls stray capacitance of connected test leads or test jig on power up) - Low-Battery indication - Simple setup
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TEST&MEASUREMENT
+5V +U BATT.
C7 R5 39k 100n C2 100n D3 17 4 MCLR 14 6 7 8 9 10 11 12 13 R2 5k6 T1 22k
LCD Module Trimods 1535 16 x 2 Farnell 142-554 +5V
CONTRAST P4
K2 BC557B
RB0 / INT RB1
RA0 RA1 RA2 RA3 RA4
IC2 PIC16F84
RB2 RB3 RB4 RB5 RB6 RB7
1N4148
11 D4 13 D6 15 +BL
OSC2 5 15 X1
OSC1 16
TLC555
JP1 100 1k 120 R1 1M 5M6 470 R3 R7 1W
20MHz C3 10p C4 10p
R6 10k
+U BATT. 1N4148 +U BATT. IC3
78L05
D1 5V6
330n 330n
Figure 1. Circuit diagram of the Autoranging Capacitance Meter.
The MMV oscillator frequency and hence the calibration for each range is determined by capacitor Cx and the resistance between the parallelconnected DIS (discharge) and THR (threshold) pins of the TLC555 chip. The resistance is range-dependent and is derived from resistor / preset combinations which can be switched into the circuit by the PIC using its RB1 and RB2 port lines. The LCD, a 2-line, 16-character type is controlled in 4-bit mode. Its backlight may be switched on optionally using jumper JP1. The power supply is totally standard and based on the 78L05 threepin fixed voltage regulator. An extra zener diode, D1, is included to prevent any risk of damage to the circuit when the input is overloaded with a direct voltage in excess of the supply voltage (5 V). The circuit is powered by a 9-volt PP3 battery. Current consumption is in the region of 7 mA with the LCD backlight left off.
Auto-zeroing and auto-ranging
At power-up, the PIC runs a routine that checks the stray capacitance at the input caused by test leads. and puts the result in a variable. This result is later subtracted from the result obtained from the capacitor measurement. Note, however, that this is only true in the pF range. It is therefore important that no capacitor is connected when the instrument is switched on, unless, of course, you intend to cancel a certain amount of stray capacitance. In all other ranges than the pF range, the capacitor may be connected at power-up. After the zeroing routine, the meter enters the pF range. At this point, any measured capacitance will be recorded and placed in a variable. The auto-ranging functions like this: if the capacitor is too large for the pF range, the count will overflow, and the PIC will select the nF range, i.e., select a lower value charging resistor, and then continue by performing a new measurement. If the capacitance value is still too large, the µF range is selected, and so on. The result is displayed on the two line alphanumeric LCD module.
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TEST&MEASUREMENT
P4 1-441020 IC3 C6 R7 D1 S1 T1 K3 P3 C5 X1 ROTKELE )C( C4 C3
(C) ELEKTOR
JP1 020144-1 K2
P1 R4 R2 C1 020144-1
IC2 C7
Figure 2. PCB design for the project (board available ready-made).
Hum and noise cancellation
In the pF range, the input represents a very high impedance. In this range the capacitor is charged via a 5 to 6 Megohm resistor and therefore the meter is sensitive to hum and noise picked up via the capacitor leads and the test leads, if used. When measuring capacitors near the low end of the pF range it is essential for the meter to be kept well removed from transformers etc., otherwise a unsteady readout may be the result. To reduce hum and noise pickup even further, the measurement in the pF range is performed twice, with a 10-ms interval. The mean value of the two results is calculated and the result is sent to the readout. This method is sure to give a steadier reading. In the nF and µF ranges, the resistor values in the MMV are relatively low and no special precautions are called for, allowing every single measurement to be read out.
COMPONENTS LIST
Mode change when measuring large capacitors
that the capacitor has been fully discharged and charged, gives a reliable reading and guarantees a low current consumption. To prepare for a new measurement the on / off switch has to be operated.
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TEST&MEASUREMENT
Construction and Troubleshooting
Populating the printed circuit board shown in Figure 2 should not present problems if you follow the component overlay printed on the ready-made board (or else gleaned from Figure 2) and the information in the parts list. Connector K1 consists of two socket strips that allow capacitors with different pin spacings to be easily connected without introducing too much stray capacitance. Wander (banana) sockets in the traditional colours red and black are used for the test leads. Depending on the exact LCD type you intend to use you may need to establish a suitable value for resistor R7 in accordance with the current drawn by the backlight lamp inside the LCD. Here, a value of 470 , 1 watt, is given for initial guidance. The three wire links on the board should be installed first so they are not forgotten later. Carefully inspect the board for solder blobs, short circuits, dry joints and the orientation of all polarised components. Our working prototype board is shown in Figure 3. Before fitting IC1 and IC2 on the board, check the presence of the +5-volt supply volt-
Figure 3. Close-up view of our prototype board.
Figure 4. A look inside the case. The copper foil shielding is optional, and only necessary if interference from external sources is a problem.
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TEST&MEASUREMENT
Things to Remember
tolerance on a number of makes of the CMOS 555 IC has been examined. In conclusion. we prefer the genuine Texas Instruments TLC55, Thomson TS555 or 3V555IN. If you decide to use other brands it may be necessary to alter the range resistors slightly. Using close tolerance capacitors as reference devices, and the Thomson 555 ICs it proved possible to set up the next Capacitance Meter and achieve repeatable results by using nothing more than an ordinary multimeter. The only requirement is that the DMM is capable of reliably measuring resistance values in excess of 6 M. This way it should be possible for Elektor Electronics readers to avoid special reference capacitors when setting up the instrument. Remove IC1 and IC2 from their sockets. For the µF range: measure the resistance between pin6 / 7 of IC1 and the collector of T1, and adjust P3 for a 190 Ohm reading. For the nF range: measure the resistance between pin 6 / 7 of IC1 and pin 8 of IC2, then adjust P2 for a 5.94 k reading. Finally, for pF range: unsolder one end of R3 (the end nearest the K1 connector), then measure the resistance between this end and pin 8 of IC1. Adjust preset P1 for a 6.0 M reading.
tor and connect the 220-nF cap, then adjust P2 for a 220-nF reading. For the µF range it will probably not be possible to get hold of a low-tolerance capacitor and if you do not have access to a commercial capacitance meter you may use an ohmmeter to adjust the series combination R1-P3 for a total resistance of 190 .
SMD and trimming capacitors
If you construct a simple test jig (fixture) to hold SMD capacitors, the auto-zero function will cancel out the capacitance of the jig and make it very easy to test pF-range SMD caps. The same goes for trimmer and tuning capacitors (ceramic, PTFE or air types). Make a simple, mechanically stable jig that allows the capacitor to be soldered to (or into). Switch on the instrument with the test jig connected up. The stray capacitance of the jig will be cancelled out. Next, solder the trimmer to the jig and measure. Adjust the trimmer and observe the varying capacitance. Make a note of its lowest and highest capacitance.
Burn your own
Setting up the instrument using reference caps
Final notes
Setting up the instrument using a DMM
Turn the contrast preset P4 fully anticlockwise and then slightly clockwise until a usable contrast is reached. The production
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