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(assume the CURRENT control, Ig, and VOLTAGE
control, Eg, are set at the maximum rating of the
suppiy).
As can be seen, up to the maximum high
voltage range current rating, the supply operates
as a constant voltage source furnishing the high
range output voltage as specified by the setting of
the VOLTAGE control.
As load resistance decreases
and output current reaches the maximum high volt¬
age range current rating (point A), the supply goes
into the gross current limit region in which the out¬
put current is maintained up to approximately 130%
(at nominal line voltage) of the high voltage range
current rating.
If the load resistance continues to
decrease and attempts to draw more current, the
output current will increase slightly while the out¬
put voltage decreases rapidly (since the gross cur¬
rent limit circuit supplies a near constant current
output, a decrease in load resistance results in a
decrease in output voltage).
When the output volt¬
age reaches a point approximately 130% (again, at
nominal line) of the low voltage range rating (point
B), the gross current limit circuit turns off and the
output current increases up to the programmed val¬
ue.
At this point, the supply enters the constant
current mode in which, as described above, the
output voltage is varied to maintain the output cur¬
rent at the setting of the CURRENT control.
Notice,
then, that under these circumstances the VOLTAGE
control of the supply is overriden by the gross cur¬
rent limit circuit and the supply is automatically
switched to the low voltage range.
Since the gross
current limit circuit is not as precise as the con¬
stant vdltage and current comparators, the maxi¬
mum voltage and current outputs of the supply dur-
Figure 4-3.
Operating Locus of Supply When
Operated Out of Voltage/Current Range Limits
ing gross current limit is slightly above the maxi¬
mum ratings of the supply thus allowing for a mar¬
gin of safety in that the supply will provide up to
approximately 130% of high voltage range current
rating and, conversely, it will supply 130%' of low
voltage range voltage while supplying maximum
current (if operated in this region, however, power
supply specifications such as output ripple are ad¬
versely affected).
4-9
The reference supply provides stable refer¬
ence voltages used by the constant voltage compar¬
ator, the constant current comparator, and the dri¬
ver.
In addition, unregulated +28V is supplied to
front panel indicator circuits.
Less critical opera¬
ting voltages are provided by the bias supply.
4-10 'The overvoltage protection crowbar monitors
the output of the supply, and if it exceeds a pre¬
set (adjustable) threshold, fires an SCR which ap¬
plies a virtual short circuit across the supply, thus
reducing the output voltage to approximately zero.
4-11 The meter circuit provides a continuous indi¬
cation of the output voltage and/or current of the
supply (two meters are provided in the 6104A and
6105A models).
Output voltage is sensed directly
across the sensing terminals, while output current
is sensed by monitoring the IR drop across the cur¬
rent sampling resistor.
442 DETAILED CIRCUIT ANALYSIS
4-13 GENERAL
4-14
Except for differences due to the different
voltage/current ratings of the power supplies and
the different VOLTAGE controls and meter circuits
employed, all of the supplies operate in a similar
fashion.
Since the supplies are similar, two schem¬
atics have been provided, one for the dual-meter,
ten-turn VOLTAGE control supplies (the 6104A and
6105A) and one for the single meter, pushbutton
VOLTAGE control supplies (the 6114A and 6115A).
Generally, then, the following discussions apply
to the 6104A, 6114A, 6105A, or 6115A supplies.
For clarity, however, certain circuits (i.e. , the
series regulator) are discussed for specific supplies
(the 6104A/6114A).
Except for the different voltage
and current values, these discussions apply equal¬
ly to the other supplies.
Further, where specific
differences in circuit operation exist between power
supply models, they are discussed for each supply.
4-15 VOLTAGE FEEDBACK LOOP
4-16 The voltage feedback loop functions to main¬
tain the output voltage of the supply constant.
For
purposes of this discussion, assume that the out¬
put voltage instantaneously rises (goes positive)
due to a change in the external load circuit.
The
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