6-2 Model 2790 SourceMeter
®
Switch System User’s Manual
Introduction
As a general rule for testing inflators, the programmable I-source of the 7751/7752/7753
modules should be used to measure low resistance (bridgwires and shunt bars). The pro-
grammable V-source of the 7751/7753 module should be used to measure insulation resis-
tance. For other resistance measurements (wiring harnesses, fuse blocks, switches/relays,
etc.), an ohms function (Ω2 or Ω4) of the Model 2790 can be used.
Low ohms measurements
The ohms functions of the Model 2790 provide better accuracy than the I-source method
used by the 7751/7752/7753 module. However, the test current used by the Model 2790 is
fixed. For the 100Ω and 1kΩ ranges, the test current is fixed at 1mA.
Low-ohms accuracy comparison:
• When using the current source of the 7751, 7752, or 7753 module to perform 4-wire,
low ohms measurements, accuracy is 0.09% (@50mA).
•For the Model 2790, when using the Ω4 function (4-wire ohms), 100Ω range, and
offset compensation, basic resistance measurement accuracy is 0.01% (1 year spec).
Fuse resistance considerations
Two fuses (F3 and F4) are in series with the output of the on-card current source to insure
that the test current cannot exceed 62mA. Fuses F1 and F2 are in series with the backplane
connection to the meter INPUT HI. For DMM resistance measurements, these fuses are in
series with the mainframe current source; similar to F3 and F4, these fuses limit the main-
frame current output to a maximum of 62mA. Because fuses F1 and F2 are in series with
the DMM output, they appear as “lead resistance” in a 2-wire measurement. As a result,
the measured value of a short circuit connected to channels 1, 2, 4, 5, 7, 8, 10, or 11 and
measured with the DMM in 2-wire ohms will basically equal the resistance of these fuses
plus any additional trace and lead resistance.
The nominal cold resistance of a single fuse is about 5.5Ω. Therefore, when you measure a
short through the 7751/7752/7753 modules using the DMM 2-wire ohms function (for
example, contact checks), you will actually read two times the fuse resistance plus any
trace and other lead resistance. The resistance of these fuses is fairly temperature sensitive.
Typically, a short measured right at the channel terminals reads between 12Ω (cold) to
approximately 15Ω (hot). Although not usually a problem, you can correct for this offset
using the REL function of the DMM, subtract the offset from the readings in your test pro-
gram, or you can simply adjust pass/fail limits.
NOTE Fuse resistance does not affect 4-wire measurements, which sense around the
lead resistance.
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