Metek EX1200A SERIES, Руководство пользователя

Страница: 92 / 163

VTI Instruments Corp.
92 EX1200A Triggering
measurements with motors and other devices with a high starting current, be certain that the
current rush does exceed the fuse’s rating.
The shunt resistance value applicable for each current range varies, making the selection of the
proper range crucial for DMM safety. For example, if a 2 A current input signal is being measured
while the DMM’s current range is manually set to 100 mA can seriously damage the DMM
circuitry.
When current is passed through a switch multiplexer, the path dedicated for current measurements
must be used. For example, the EX1200-3048 has three dedicated channels for current
measurement. While making voltage measurements, the DMM is connected in parallel with the
DUT measurement point and, hence, will not interrupt DUT operation. Current measurements,
however, need the DMM to be placed in series with a wire, in which the current needs to be
measured. If current measurements are required on more than one channel (wire) without
disturbing/interrupting the current path, an MBB (Make-Before-Break) switch topology must be
selected.
The previous discussions regarding DCV measurements and ACV measurements, such as ac filter
settings, aperture, PLC count, integration time, etc., are applicable to the respective current
measurements as well.
R
ESISTANCE
M
EASUREMENTS
The EX1200A DMM offers two methods for measuring an unknown resistance: 2-wire and
4-wire. These methods are discussed below.
2-Wire Resistance Measurements
When a 2-wire resistance measurement is made, the EX1200A DMM sends a known dc current,
I
TEST
(also referred to as an excitation or test current), through the unknown resistance (R
DUT
) and
then measures the resulting dc voltage (V
MEAS
).
 
LEAD LEAD DUT TEST MEAS
R R R I V
  
This calculation assumes the that the leads have the same resistance. The DMM then calculates the
resistance by taking the ratio of the measured voltage over the supplied current. In this method, the
test current is passed through the DMM’s HI and LO terminals and the voltage developed between
these terminals is measured. However, since the connecting wires/leads are in series with
resistance being measured, the lead resistance (R
LEAD
) is added to the resistance measured. This
lead resistance error is inherent to 2-wire measurements. When the lead resistance small compared
to the R
DUT
, this error it typically negligible.
Sense HI
Input HI
Input LO
Sense LO
Resistance Source
I
TEST
EX1200 DMM
-V
MEAS
+V
MEAS
R
LEAD
R
LEAD
R
DUT
F
IGURE
4-12: DMM C
ONNECTIONS FOR
2-W
IRE
R
ESISTANCE
M
EASUREMENTS