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Using the general-purpose analog inputs (channels 3 and
higher) on the ZENSOL
CBA-32P, it is possible to measure
AC voltages where the peak-to-peak amplitude is less than
10 volts. It is necessary to assemble a special circuit
to obtain the desired results.
Direct
connection
If
a transformer's sine wave output is directly connected
to an analog input, as shown in Figures 1 and 2, a basic
problem already shows itself:
Fig.
1 - Schematic of the basic circuit
Fig.
2 - Photo of the basic circuit
Results:
Having
deactivated the Relative and Auto-amplification options
of the analog input used in CBA Win©, we observe
the following:
Fig. 3 - Resulting curve for the basic circuit
Observed
problem:
In
effect, the average value of the signal centers itself
on level zero, and we only see the part of the signal
that is above the average value (the positive swing of
the signal).
Solution:
To
see the entire signal, a battery has to be inserted in
series to add an offset to the signal.
This
offsets the curve by the amount of the battery's
voltage (in this example, the battery is at 6 volts)
Figures 4 and 5 show the connection of the battery into
the original circuit.
Fig.
4 - Schematic of the modified circuit
Fig.
5 - Photo of the modified circuit
Results:
Fig.
6 - Curve obtained with the modified circuit
Precautions:
Care
must be taken to choose the correct battery voltage in
order to raise the signal to the right level.
Example:
battery is too weak
If
we use a battery that is too weak (here, a 1.5V battery),
we have:
Fig.
7 - Curve produced with a weak battery
The
signal was not sufficiently raised because the battery
voltage is too low, and so the lower end of the signal
is cut off. The average value is at 1.5 volts. So a battery
with a higher voltage must be used.
Example:
battery voltage too high
Also
beware of the reverse effect : one must not use a battery
with too high a voltage. The example of figure 8, below,
shows what happens with an 8.5 volt battery.
Fig.
8 - Curve produced with a battery where the voltage
is too high.
This
time, the signal is cut off on the positive end. A battery
with less voltage must then be used to see the entire
signal.
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