HP 740B Operating And Service Manual page 19

Section ill
Model 740B
voltage and the setting of the CURRENT LIMIT control.
With the CURRENT LIMIT control set to maximum
(fully cw), the maximum output current varies from
50 rnA at voltage outputs less than 1 V to 20 rnA for a
1000 V output (Figure 3 -6). When the CURRENT LIMIT
control is set to minimum (fully ccw) the maximum
output current varies from 5 rnA (less than 1 V output)
to 2 rnA (1000 V output). These figures and the values
represented in Figure 3 -6 should be regarded only as
typical, since the current limit circuit is affected by
temperature and the setting of the maximum current
limit adjustment (A10R8).
-------..,,, OUTPUT VOLTAGE
r.------·
Figure 3-6. Maximum Output Current
Characteristics
3-41. DIFFERENTIAL VOLTMETER.
3-42. DESCRIPTION.
3-44. High input impedance is maintained at all times
regardless of null condition. An off -null condition
does not load the voltage source being measured.
3 -43. In the Differential Voltmeter mode, the 740B
measures the input voltage by nulling feedback voltage
from the main loop amplifier with an internally gen-
erated reference voltage. The value of the reference
voltage is controlled by a voltage divider operated by
the VOLTAGE SET controls. The meter indicates the
difference between the reference voltage and the feed-
back voltage. The SENSITIVITY pushbuttons control
the meter sensitivity; and mOVing left to right de-
pressing each successive pushbutton, increases the
meter sensitivity by a factor of 10. The ZERO con-
trol sets the input zero reference. The OVERLOAD
indicator lights whenever the input voltage exceeds the
selected range and may also light temporarily when-
ever RANGE, FUNCTION or VOLTAGE SET is
changed. The input voltage is indicated directly on
the digital readout tubes (first five digits) and the
meter (sixth digit). The decimal point is automatically
placed.
3-45. OPERATING PROCEDURE.
I
BOOV IOOOV
900V
I
400V 600V
500V 700V
I
200V
IOOV 300V
~AX.ISET~ING~-
-
"""-
--
CURRENT lIMI
. . . . r----
iCONTROL
- f - -
"f-
--
r--
-MIN. SETTING,
r--
-
CURRENT t,.IMIT
t
f - -
j/CONTROL
I
-I I
I
60MA-
40MA-
50MA-
J
140B-RO
MAX.
OUTPUT
30MA-
CURRENT
l
20MA-
IOMA-
OMA-
I
OV
3-40. Since the setting of the CURRENT LIMIT con-
trol corresponds to different current values at different
voltage outputs, accurate setting of the CURRENT
LIMIT control can be accomplished only at the desired
voltage output. The following procedure can be used
to set the CURRENT LIMIT control to limit output
current to a specific value.
3 -46. To make a differential measurement, proceed
as follows:
a. Set rear panel LINE VOLTAGE switch to cor-
rect position for available line voltage.
b. Connect ac power cord to ac power connector;
connect cord plug to ac line.
a. Calculate the load resistance value needed to
draw the desired output current at the desired
output voltage:
c. Turn on the Model 740B by depressing the
POWER pushbutton (indicator glows). Allow
the instrument to warm up for one hour.
E
R
=
out
load Idesired
d. Set FUNCTION to 6.VM.
e. Check OUTPUT indicator. If glowing, depress
to turn Output off.
b. Calculate the load resistance power rating:
Power rating
=
Idesired x E out
f. Connect Input Cable Assembly (-hp-ll054A)
to INPUT receptacle. Set INPUT Z switch
to
00 •
c. Connect a resistor meeting the requirements
of a and b across the Model 740B
+
and -
OUTPUT terminals.
g. Set RANGE to the range nearest above voltage
to be measured.
d. Turn CURRENT LIMIT control fully ccw.
h. Set VOLTAGE SET controls to zero (all fully
ccw).
e. Set Model 740B for the desired output voltage.
Turn Output on. (OVERLOAD light will glow. )
i. Connect a shorting wire from
+
to - INPUT
terminals.
f. Rotate the CURRENT LIMIT control slowly
cw until the OVERLOAD light just goes out.
The Model 740B will now limit current atthe
desired current value.
j.
Depress X104 SENSITIVITY. (If 1 mV range
is selected, depress X103 SENSITIVITY.)
3 -8
01794-1