HP 6177B Operating And Service Manual page 14

voltmeter, ?s shown in Figure 3-7. Notice that
one end of the meter is connected to the guard ter-
mina 1 (AO). This effective ly isolates the ffi€ter
from the main power supply', preventing the per-
formance ciegradation that would
occu~
i: the meter
were connected directly across the
+
anc - output
terminals.
VM
101
I
- A4 A5 AS
Figure 3-7. External Voltage Monitoring
NOTE
The external voltmeter must not draw
more than 2mA from the auxiliary
source that provides the guard volt-
age. A current drain in excesS of 2mA
will
serious
ly impair the operation of
the power supply.
3-31 EXTERNAL AC MODULATION
3-32 Figure 3-8 shows a method of superimposing
an ac component on top of the adjustable de output
current of the supply, which is operating as a con-
stant current source. The dc current level is con-
trolled in the normal fashion from the front panel
while the ac component of the output current is
determined by the modulation percentage. The
percentage of modulation is affected by the ampli-
tude of the external voltage input and the value of
the series resistance, as follows: % Modulation : ; ; ; ;
Esource (pk-pk)/RX (in
J:..1.
Using the above for-
mula, the user would require an external resistance
of 2K.n. and a 2 Volts peak-to-peak"±nput signal
from the external source to modulate a de current
level of lOOmA by 100%.
In this case, the output
.::urrent would swing between -200mA and zero Am-
3-5
peres. The output current should never be allowed
to swing beyond the rating of the supply (SOOmA,
p-p, fo:- Model 61778 or 2S0mA, p-p, for Model
61818) or clipping of the output and possible in-
ternal damage will result.
+
AO Ai A2 A3
j
idJ!81!i;i
A4 A5 A6
Figure 3-8. External AC Modulation
3-33 The coupling capacitor (C e ) should be
chosen so that its reactance is at least ten-times
smaller than RX at the frequency of interest. For
input frequencies up to 100Hz the output of the
supply can be modulated 100%. Above 100Hz the
modulation capability decreases linearly to approx-
imately 10% at 1000Hz.
3-34 Notice that it is possible to remotely program
the dc output current of the supply in the external
modulation mode of operation. This can be accom-
p lished by removing the strap between Al and A2
and connecting the programming source across
these terminals.
3-35 SPECIAL OPERATING CONSIDERATIONS
3-36 PULSE LOADING
3-37 The power supply will automatically cross
over from constant current to voltage limiting oper-
ation in response to an increase (over the preset
limit) in the output voltage. Although the preset
limit may be set higher than the average output
voltage high peak voltages (as occur in pulse
loading) may exceed the preset limit and cause
crossover to occur. If this crossover limiting is
not desired, set the preset limit for the peak re-
quirement and not the average.
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