HP Lab Series User Manual

•
•
•
comparator produces an amplified error signal which
is furthet amplified by the error amplifier and then
fed back to the series regulator in the correct phase
and amplitude to counteract the difference. In this
manner, the constant voltage comparator helps to
maintain a constant output voltage and also gener-
ates the q error" signals necessary to set the output
voltage at the level that is established by the
VOLTAGE controls.
4-7 During constant current operation, the con-
stant current comparator detects any difference be-
tween the voltage dropacross the current sampling
resistor and the voltage across the CURRENT con-
trols. The voltage drop across the current sampling
resistor is directly proportional to the output current
that flows through it. If the two inputs to the com-
parator are momentarily unequal, an error signal is
generated which alters the conduction of the series
regulator by the amount necessary to reduce the er-
ror voltage at the comparator input to zero. Hence,
the IR drop across the current sampling resistor,
and therefore, the output current, is maintained at
a constant value.
4-8 Since the constant voltage comparator tends
to achieve zero output impedance and alters the
output current whenever the load resistance changes,
while the constant current comparator causes the
output impedance to be infinite and changes the
output voltage in response to any load resistance
change, it is obvious that the two comparison am-
plifiers cannot operate simultaneously. For any
given value of load resistance, the power supply
must act either as a constant voltage source or as
a constant current source - it cannot be both.
4-9 Figure 4-2 shows the output characteristic of
a Constant Voltage/Constant Current power supply.
With no load attached (R1 = oo), lOUT = 0, and
EOUT =Es, the front panel voltage control setting.
When a load resistance is applied to the output
terminals of the power supply, the. output current
increases, while the output voltage remains con-
stant: point D thus represents a typical constant
voltage operating point. Further decreases in load
resistance are accompanied by further increases in
lOUT with no change in the output voltage until the
output current reaches Is, a value equal to the
front pa'nel current control setting. At this point
the supply automatically changes its mode of oper-
ation and becomes a constant current source; still
further decreases in the value of load resistance
are accompanied by a drop in the.supply output
voltage with no accompanying change in the output
current value. With a short circuit across the out-
put load terminals, IQUT = IS and EoUT = O.
4-10 The ncrossover" value of load resistance can
be defined as Re= Es /Is. Adjustment of the front
4-2
OPEN CIRCUlT LOAD
D
E
I
/
CONSTANT CURRENT
I
RL>Rc~ RL • Rc-;1
OPERATING REGION
l
I / RL<l!c
B
I / ..,,
I / ./
EOUT
/0.
I / / E
I / /
I / / fe-AI-1
I//
/// Rt•~
SHORT CJRCUJT I.Oo\O
ll. I
0
I
our-
Is
Es • FRONT PANEL VOLTAGE CONTROL SETTlNG
ls = FRONT PANEL CURRENT CONTROL SETTING
Re• ~" 'CRmCAL" OR 'CROSSOVER"
Is VALUE or LOP.O RESISTANCE.
Figure 4-2. Operating Locus of a CV/CC
Power Supply
panel voltage and current controls permits this
"crossover" resistance Re to be set to any desired
value from 0 to oo. If RL is greater than RC, the
supply is in constant voltage operation, while if
RL is less than Re. the supply is in constant cur-
rent operation.
4-11 The turn-on control circuit is a long time
constant network which achieves a gradual turn-on
characteristic. The slow turn-on feature protects
the preregulator SCR' s and the series regulator from
damage which might occur when power is first ap-
plied to the unit. At turn-on, the control circuit
and the series regulator (via the error and driver
amplifiers). A short time after the unit is in opera-
tion. the inhibit voltages are removed and the cir-
cuit no longer exercises any control over the opera-
tion of the supply.
4-12 The reference supply provides stable refer-
ence voltages which are used by the constant volt-
age and current eomparators in the main power sup-
ply. Less critical operating voltages are obtained
from the bias supply.
4-13 DETAILED CIRCUIT ANALYSIS {Refer to
Schematic
at Rear of Manual.)
4-14 PREREGULATOR AND CONTROL CIRCUIT
4-15 The preregulator minimizes changes in the
power dissipated by the series regulator due to
output voltage, load current, or input line voltage
changes. Preregulation is accomplished by means
of a phase control circuit utilizing an SCR (CR35)

HP Lab Series Specifications

General

Brand	HP
Model	Lab Series
Category	Power Supply
Language	English