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Typical Series Regulated Power Supply
Figure 3 shows the basic feedback circuit principle used in Agilent series regulated power supplies. The ac
input, after passing through a power transformer, is rectified and filtered. By feedback action, the series
regulator alters its voltage drop to keep the regulated dc output voltage constant despite variations in the ac line,
the load, or the ambient temperature.
The comparison amplifier continuously monitors the difference between the voltage across the voltage control
resistor, RP, and the output voltage. If these voltages are not equal, the comparison amplifier produces an
amplified difference (error) signal. This signal is of the magnitude and polarity necessary to change the
conduction of the series regulator which, in turn, changes the current through the load resistor until the output
voltage equals the voltage (EP) across the voltage control.
Figure 3. Series Regulated Constant Voltage Power Supply
Since the net difference between the two voltage inputs to the comparison amplifier is kept at zero by feedback
action, the voltage across resistor R
R
is also held equal to the reference voltage E
R
. Thus the programming
current I
P
flowing through R
R
is constant and equal to E
R
/R
R
. The input impedance of the comparison amplifier
is very high, so essentially all of the current I
P
flowing through R
R
also flows through R
P
. Because this
programming current I
P
is constant, E
P
(and hence the output voltage) is variable and directly proportional to R
P
.
Thus, the output voltage becomes zero if R
P
is reduced to zero ohms.
Of course, not all series regulated supplies are continuously adjustable down to zero volts. Those of the OEM
modular type are used in system applications and, thus, their output voltage adjustment is restricted to a narrow
range or slot. However, operation of these supplies is virtually identical to that just described.
Pros and Cons of Series Regulated Supplies
Series regulated supplies have many advantages and usually provide the simplest most effective means of
satisfying high performance and low power requirements.
In terms of performance, series regulated supplies have very precise regulation properties and respond quickly