voltage is detected by the Galvanometer (G).
Light from the galvanometer
mirror falls on a pair of photo transistors, which, depending on the amount
of illumination received, control the current in the loop.
A continuous
correction to the current value is thus applied in the presence of changes
in line voltage and load resistance.
a. Control circuit. Normally, the voltage across the variable resistor
is equal to the standard cell voltage, and no current flows in the
galvanometer.
Any change in the current causes a change in the voltage
across the variable resistor, and allows current to flow in the
galvanometer. The resulting displacement of the galvanometer mirror, away
from its center position, causes a change in the light-dependent transistors
which alters the current so as to return the mirror to its center position.
Thus there tends to be zero current flow in the galvanometer circuit and the
voltage across the variable resistor is kept equal to the standard cell
voltage.
Because of the very high gain in the current control circuit, the
output resistance of the CCS is greater than 109 ohms, therefore a change in
the lead resistance of 100 ohms will result in a current change of only
0.1ppm.
The output terminals can be open or short-circuited indefinitely
without damage to any components. An increase in load resistance to a value
that produces an output voltage of 30V or more will cause the bypass circuit
to operate and the current will cease to be controlled.
b. Overload relay.
The function of this relay is to protect the
standard cell from current overload.
Normally, the relay is continuously
energized by a circuit which detects that the galvanometer light spot is
correctly positioned.
A photo transistor is positioned so that it is
illuminated only when the light spot is correctly positioned over the two
control photo transistors.
When the overload relay is energized, the
standard cell is connected into the galvanometer circuit.
If the
galvanometer lamp should burn out, the photo transistor will not be
illuminated, the overload relay will release, and the standard cell will be
disconnected. Similarly, if an external standard cell should be connected
in the wrong polarity, the galvanometer light beam will be violently
deflected away from the photo transistor, thus disconnecting the standard
cell.
The OVERLOAD lamp is illuminated whenever the overload relay is
released.
c. Bypass circuit.
When the voltage across the load exceeds 30V, a
zener diode bypass circuit operates and takes all or part of the output
current. Therefore the terminals can be open circuited, and no damage will
occur.
d. Standard cell.
An external standard cell is used as a reference
with the CCS. The gain of the control circuitry is such that a 20% change
in load current is produced by a 102 A change in the galvanometer (and
hence standard cell) current, and typical changes in load resistance during
operation produce standard cell currents of only a few picoamperes.
The
characteristics of a standard cell are such that this current can be taken
for many hours without significant change in the standard cell voltage.
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