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Capacitance meter |
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Last update: January 21, 2006
No, my meter does not look as fancy as this....
Every home constructor should own at
least two pieces of test equipment: a digital (or analog) multimeter and a
simple frequency counter. Most digital multimeters can measure capacitance, but
the capacitance range is rather limited (in my case, from about 10pF to
maximally 10 µF). It is not possible to check the value of large capacitors
with such a meter, e.g. bipolar electrolytics in a cross-over filter, or
reservoir caps in a power supply. In my attic, I have not only a digital
multimeter but also a cheap frequency counter (0...10MHz). So I decided to
build a simple tool which would allow me to check the value of large
capacitors, using the frequency counter.
The tool is based
on a NE555 (or LM555) timer i.c., wired as an astable multivibrator. The
frequency of the multivibrator is determined by the value of the unknown
capacitor Cx. The circuit can be fed by a small battery. I use myself a 9 V
block battery which is connected to pins 4 and 8 of the i.c. via an on/off
switch.
The output of the
multivibrator is connected to the digital frequency counter. The counter should
assess the cycle length of the square wave, not the frequency (on most
frequency counters, one can select either frequency measurement or measurement
of the cycle length). A wave with a frequency of 1 kHz (i.e. 1000 Hz) has a cycle
length of 1 millisecond.
The cycle length
of the square wave which is produced by the NE555 can be determined using the
following formula:
T = 0.7 * Cx *
(R1 + 2 * R2)
[Cx in Farad, R
in Ohms, R1 is the resistor between pins 7 and 8 and R2 the resistor between
pins 7 and 6 of the i.c.]
When R1 = 1000
Ohms and R2 = 215 Ohms, a capacitor of 1 uF will produce a 1 ms cycle. If you
measure large electrolytics (1000 uF or more), you must wait several seconds
before you get any display reading on the frequency counter. With this
combination of R1 and R2, you can measure capacitors between about 0.1 uF and
5000 uF. Smaller capacitors can be tested if R1 and R2 are made larger (e.g.,
using 1M and 215k, an 1 nF capacitor will produce a cycle length of 1 ms). However,
when small capacitors are measured the capacitance of the wiring must be taken
into account (e.g. a capacitor of 1000 pF may give a reading of 1050 pF if the
wiring has a capacitance of 50 pF). When Cx is greater than 1 uF, stray
capacitance can be neglected. For an accurate reading, R1 and R2 should be 1%
metal oxide resistors.
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Questions, Comments: aren.van.waarde@hetnet.nl |
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(Use only my initials if you like an answer to your mail)