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Input Connections
A
DC Current Measurements
The INPUT EXT-INT pushbutton selects front panel or
rear Interface input.
Three input connectors provide front panel measure-
ment connections. The YOLTS/Tr and LOW input connec-
tors are used for ac or dc voltage or resistance
measurements.
The
mA and LOW input connectors are
used for
ac
or dc current measurements. Rear interface
pins 28B (HI) and T8A (LO) are used for rear interface
voltage and resistance measurements. N o r m l rneasure-
rnent conditions are with the LOW terminal ungrounded. A
connection between the LOW input connector and the
ground terminal may be made t o referencetheinput tothe
DM 505 chassis ground. Use caution,
as
the Lowterminal
is then connected
ta
earth ground through the power
module three-wire power cord. False readings may be
obtained due t o ground loops.
The maximum input voltage is
I
kV
peak
at
the
front
panel
connectors
and 200 Y peak
at
the
rear
inter-
face
connectors.
The front
panel VOLTSIfl
and
LOW connectors
may be floated
at
I kVmaximum
above
ground.
fhe rear input
connector
200
V.
Sine-wave Response
The
DM
505 responds to the average value of an ac or
dc current or voltage and the readout displays the
sinusoidal rms value. The effective or rms value of
a
sine-
wave is 0.707 times the peak voltage or current.
The
average value is
0
636 of the peak value. The scale factor of
the DM 505 is 0.636/0.107 or 0.9. To obtain the average
value of a sinusoidal input voltage or current, multiply the
DM 505 readout
by 0.9.
'DC
Voltage
Measurements
Press the VOLTS DC pushbutton and an appropriate
range button. Apply the voltage to be measured
to the
LOW and VOLTS/n input connectors.
0
bserve
the
max-
imum input voltage ratings as indicated on the front panel.
The readout displays a
f
if the input t o the VOLTS/R
connector is positive with respect t o the LOW input
connector. A
-
is displayed if the input at the
LOW input
connector is the more positive. With the LOW and
VOLTS/n input connectors shorted, the display reads
zero
+
one count.
Press
the
mA DC pushbutton
and
an appropriaterange
button. Connect the dc current to be measured tathe LOW
and
mA
input connectors. Conventional current flowing
into the mA connector and out of the LOW connector
(or
electron flow into the
LOW
connector and out of the mA
connector) indicates
a
+
on the display. The input
resistance in the current mode is listed in Table 2-T.
Table 2-1
AC Voltage and Current Measurements
CURRENT MODE INPUT RESISTANCE
For ac voltage measurements, press the VOLTS AC
pushbutton and an appropriate range button. Connect the
unknown voltage
to
the LOWand VOLTS/R input connec-
tors.
Range
200 PA
2
mA
20 mA
200 mA
2000 mA
To measure ac current, press the rnA AC pushbutton
and an appropriate range button. Connect the unknown
ac current to be measured t o the
mA
and
LOW
input
connectors. The input resistance in
the
current mode
is
listed in Table 2-1.
Approximate
Resistance
1001 R
100.4
R
10.25 R
1.2 R
0.25
R
Resistance Measurements
Press the k f l pushbutton and an appropriate range
button. Also, press the
HI
n-LO
fl
button for
a maximum
full-scale probe-tip voltage of 0.2
V.
Release the HI n-LO
R button
(out
position) for
a
maximum full-scale probe-tip
voltage of 2 V (except on the 200
fl
range which is 0.2 V).
The low probe-tip voltage is useful for making in-circuit
measurements without forward-biasi ng silicon diodes or
transistor junctions.
The
kR
mode provides an accurate constant current at
the
LOW
and VOLTS/n input connectors. Refer to Table
2-1
for the value
of
current and maximum voltages across
the input terminals for full scale display readings (instru-
ment not over-ranged). The maximum (open circuit)
voltage available from the YOLTS/fl terminal t o the LOW
terminal
in the
resistance mode is about +6 V.
REV A OCT
1979
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