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5
Using Load Cards and Loads
Current Monitor
Selecting and Loading Flyback Protection
* LEM USA, Inc. 6643 West Mill Road, Milwaukee, WI, 53218. (414) 353-0711
5-58
Each channel on the Agilent E6178 uses a LEM
Current Transducers with a 75
ppm/°C) resistor for sensing the current through each channel.
The sense relays are switched independently from the channel
load relays.
The LEM current transducer module has a current gain of
1000:1 (primary to secondary, i.e., for a 1 amp primary current
the secondary current is 1mA). The secondary of the LEM
current transducer is connected across a precision 75 resistor,
see
Figure
5-39. The primary current is calculated by making a
voltage measurement across this precision resistor and is
determined by the equation:
V
Isense
I
=
---------------- -
ch
0.075
The current monitor specification is with respect to the channel
primary current which is being measured:
Gain Error:±1.0% Maximum over the temperature range
Offset Error:±0.3 Amps Typical over the temperature range
±0.7 Amps Maximum over the temperature range
The error due to the offset component can be reduced by
measuring the zero current offset voltage (channel relay open)
after the test system has warmed up and using this offset to
correct for all subsequent measurements. This should be done
for each channel individually. Using this technique, the errors
due to offset may be reduced to:
Offset Error:±0.1 Amps over the temperature range
Coils used as loads may have large flyback voltages which have
the potential to damage other electronic components. Generally
the UUTs are equipped with flyback protection, so flyback
protection on the load cards is redundant but provides backup
protection in case a defective UUT is tested. The system
integrator is responsible for ensuring the flyback protection
devices are installed on the load cards.
*
model LA 55-TP
0.01%, 0.3W low TC (±2.5
E6198B Switch/Load Unit User Manual
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