The document discusses digital instruments, focusing on the digital tachometer and digital capacitance meter. It explains the measurement techniques for rotary shaft speed using a conventional frequency counter and characterizes capacitance measurement in RC circuits with a 555 timer. Key concepts include the relationship between pulse frequency, gate period, and direct RPM reading, as well as methods for measuring capacitance directly in microfarads or nanofarads.

1. BIE-601
Electronic Instrumentation-II
Unit-5
Miscellaneous Instruments
Lecture-3
Mohd. Umar Rehman
umar.ee.amu@gmail.com
24. 4. 2020
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2. Digital Tachometer
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3. Digital Tachometer
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4. Digital Tachometer...Contd I
The technique employed in measuring the speed of a rotating shaft
is similar to the technique used in a conventional frequency counter.
However, the selection of the gate period is in accordance with the
rpm calibration.
Let us assume, that the rpm of a rotating shaft is R. Let P be the
number of pulses produced by the pick up for one revolution of
the shaft.
Therefore, in one minute the number of pulses from the pick up is
R × P.
Then, the frequency of the signal from the pick up is
R × P
60
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5. Digital Tachometer...Contd II
Now, if the gate period is G s the pulses counted are:
R × P × G
60
In order to get the direct reading in rpm, the number of pulses to be
counted by the counter is R, So we select the gate period as 60/P,
and the counter counts
R × P × 60
60 × P
= R pulses
and we can read the rpm of the rotating shaft directly.
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6. Digital Tachometer...Contd III
The relation between the gate period and the number of pulses pro-
duced by the pickup is G = 60/P.
If we fix the gate period as one second (G = 1 s), then the revolu-
tion pickup must be capable of producing 60 pulses per revolution.
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7. Digital Tachometer
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8. Digital Capacitance Meter
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9. Digital Capacitance Meter
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10. Digital C Meter...Contd
In a RC circuit, capacitance can be considered to be proportional to
the time constant.
When a capacitance is charged through a constant current source
and then discharged through a fixed resistance, 555 timer can be
used to measure the capacitance in a digital manner, along with
other necessary test equipment.
One obvious method to measure the time period of the oscillations.
By choosing the right size of the charging resistance, reading can
be obtained directly in µF or nF.
This measurement scheme can handle electrolytic capacitors of the
order of few thousands of µF
A better way is to measure the only the capacitance discharge time
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11. Digital C Meter
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12. Digital C Meter
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