Keithley 2750, User Manual

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3-16 Basic DMM Operation Model 2750 Multimeter/Switch System User’s Manual
AMPS fuse replacement (front panel AMPS input)
WARNING Make sure the instrument is disconnected from the power line and
other equipment before replacing the AMPS fuse.
1. Turn off the power and disconnect the power line and test leads.
2. From the front panel, gently push in the AMPS fuse holder with a flat blade screw-
driver and rotate the fuse holder one-quarter turn counterclockwise.
3. Remove the fuse and replace it with the same type (3A, 250V, fast-blow,
5
×
20mm). The Keithley part number is FU-99-1.
CAUTION Do not use a fuse with a higher current rating than specified or instru-
ment damage may occur. If the instrument repeatedly blows fuses,
locate and correct the cause of the trouble before replacing the fuse.
4. Install the new fuse by reversing the procedure above.
NOTE For the Model 7700 switching module and other similar modules that support
the amps function, there are solder mount amps fuses. See the Model 2750 Ser-
vice Manual for fuse replacement information.
Resistance measurements (
Ω
2 and
Ω
4)
The Model 2750 uses the constant-current method to measure resistance from 1
Ω
to 1M
Ω
.
The Model 2750 sources a constant current (I) to the resistance and measures the voltage
(V). Resistance (R) is then calculated (and displayed) using the known current and mea-
sured voltage (R = V/I). For the 10M
Ω
and 100M
Ω
ranges, the ratiometric method is used
to measure resistance.
Standard resistance measurements — The Model 2750 can make resistance measure-
ments from 1µ
Ω
to 120M
Ω
. For resistances >1k
Ω
, the 2-wire (
Ω
2) method is typically used
for measurements. For resistances
≤1
k
Ω
, the 4-wire (
Ω
4) measurement method should be
used to cancel the effect of test lead (and channel path) resistances.
Offset compensated ohms (OCOMP) — The presence of thermal EMFs (voltages) can
adversely affect low-resistance measurement accuracy. To overcome these unwanted off-
set voltages, you can use offset-compensated ohms on the 1
Ω,
10
Ω,
100
Ω,
1k
Ω,
and
10k
Ω
ranges for the
Ω
4 function.
Dry circuit testing (DRYCKT) — For low-resistance measurements that require low
open-circuit voltage (20mV), dry circuit ohms can be used on the 1
Ω,
10
Ω,
100
Ω,
and
1k
Ω
ranges for the
Ω
4 function.
2750-900-01.book Page 16 Wednesday, August 3, 2011 7:56 AM