HP 6268B Operating And Service Manual

the stability of the feedback loop seriously enough to cause
oscillation. If remote sensing disturbs the supply's stability,
try these two corrective measures:
a. Adjust the equalization control R47 until the oscilla¬
tion stops. To achieve the best possible transient response
for a given remote sensing installation, measure the transient
response using the procedure given in paragraph 5-27 and
adjust R47 while observing the transient response wave¬
forms.
b. If adjusting R47 does not eliminate the instability,
it may be beneficial to disconnect output capacitor A3C3
from the circuit and connect a similar capacitor directly
across the load. To gain access to capacitor A3C3, the
A2 RFI Assembly must first be removed. Follow steps (a)
through (c) of paragraph 5-65 to remove the A2 assembly.
Then unsolder the heavy wire from the A3 circuit board
that connects the positive terminal of A3C3 to the positive
output bus bar. (This heavy red-insulated wire is identified
in Figure 7-2 -)
NOTE
Do not unsolder the capacitor's negative lead.
The negative lead to A3C3 carries collector
current for transistor A4Q101 and would
disable the power supply if disconnected.
Tape the free end of the disconnected wire, replace the A2
assembly, and replace the bottom cover of the supply. The
substitute capacitor should have approximately the same
capacitance, an equal or greater voltage rating, and good
high frequency characteristics. Connect it directly across
the load using the shortest possible leads. Readjust equali¬
zation control R47 as in step (a) above after installing the
substitute output capacitor.
3-34 Remote Programming
3-35 The output voltage or current of these power
supplies can be remotely controlled by connecting an
external resistor or applying an external voltage to rear
panel terminals. If resistance programming is used, a
variable resistor can control the output over its entire
range. Or, a variable resistor connected in series with a
fixed resistor can have its control restricted to a limited
portion of the output range. Alternately, a switch can be
used to select fixed values of programming resistance to
obtain a set of discrete voltages or currents. (The switch
must have make-before-break contacts to avoid producing
the output voltage transients that momentarily opening
the programming terminals would cause.) To maintain
the temperature and stability specifications of the supply,
programming resistors must be stable, low noise resistors
with a temperature coefficient of less than 30ppm per C
and a power rating at least 30 times what they will actually
dissipate.
3-36 Both voltage and current outputs can also be
controlled through a voltage input. When voltage program¬
ming the output voltage, the choice can be made between
using a connection that produces a unity gain relationship
between input and output (paragraph 3-41) or another
connection that produces variable voltage gains (paragraph
3-42). Similarly, the output current can be programmed
using a connection that produces a fixed gain (paragraph
3-47) or a variable gain (paragraph 3-48).
3-37 Connecting a supply for remote voltage or current
programming disables the corresponding front panel controls.
3-38 The following paragraphs discuss in greater detail
the methods of remotely programming the output voltage
or current using either a resistance or a voltage input.
Whichever method is used, the wires connecting the pro¬
gramming terminals of the supply to the remote program¬
ming device must be shielded to reduce noise pickup. The
outer shield of the cable should not be used as a conductor
but should be connected to ground at one end only.
3-39 Constant Voltage Output, Resistance Input. The
rear panel connections shown in Figure 3-4 allow the out¬
put voltage to be varied by using an external resistor to
program the supply. The supply's constant voltage program¬
ming current determines its programming coefficient. In
the supplies covered by this manual, this programming
current is factory adjusted to within 1% of 5mA, resulting
in a programming coefficient of 200 ohms per volt. If a
greater programming accuracy is required, it can be obtained
either by changing resistor R3 as discussed in paragraph 5-86
or, if the instrument is equipped with Options 020 or 022,
by adjusting potentiometer R112 as discussed in paragraph
5 87.
3-40 With the programming terminals shorted (terminals
A2 to -S), the no-load output voltage of the supply should
be —1 5mV ±5mV. If a minimum output voltage is required
that is closer to zero than this, it can be obtained either
Figure 3-4. Resistance Programming of Output Voltage
3-5

HP 6268B Specifications

General

Brand	HP
Model	6268B
Category	Power Supply
Language	English