Lake Shore Model 370 AC Resistance Bridge User’s Manual
Current Excitation Mode (Continued)
To change the current excitation use the
Current Excitation
VW
keys. Available ranges are listed in Table 1-1. As the
range table indicates, not every combination of current excitation and resistance range is possible. The following
messages are displayed in the excitation display to indicate limits.
Maximum
Maximum current excitation has already been chosen
Minimum
Minimum current excitation has already been chosen
Decrease Resistance
Resistance range must be decreased before changing current excitation
Increase Resistance
Resistance range must be increased before changing current excitation
Resistance ranges have approximately 20% over range but it is recommended that the range be increased when the
reading is at full scale for the range to guarantee enough overhead to account for noise voltages. Better measurement
resolution can be achieved by decreasing the resistance range when the reading is below 20% of full scale.
To change the resistance range
Resistance Range
VW
keys. The following messages are displayed to indicate range
limits.
Maximum
Maximum resistance range has already been chosen
Minimum
Minimum resistance range has already been chosen
Decrease Excitation
Current excitation must be decreased before changing resistance range
Increase Excitation
Current excitation must be increased before changing resistance range
The autorange feature allows the instrument to automatically choose an appropriate resistance range. The instrument will
increase the resistance range (decrease input gain) if the measurement reaches full scale. Normally no over range error is
created because of the approximate 20% over range capability of the input. Over range errors are possible when
resistance changes very quickly or when resistance increases beyond the limit of the current excitation selection. The
autorange algorithm will not change the excitation setting to accommodate larger resistances. The instrument will
decrease the resistance range (increase input gain) if the measurement drops below 20% of full scale. Autorange is not
recommended when resistance changes are very fast, measurements are very noisy or when data acquisition cannot
tolerate a range change delay in the middle of a data set.
To turn autorange on or off press the
Autorange
key. The LED annunciator above the key lights to indicate autorange is
on.
4.5.3
Turning Excitation Off
The Model 370 offers many excitation levels with the lowest being very low in power. It is still sometimes necessary to
turn excitation off. Excitation should be turned off any time the current source leads are open circuited and then attached
to a cold resistor with the instrument turned on. This condition may occur when the input is scanned or multiplexed
externally. Excitation is turned off automatically for channel changes when the Model 3716 Scanner is being used.
When excitation is turned off, the instrument sets its current source to the lowest output possible and then shunts across
the output with a mechanical relay. The relay provides a low resistance path for any remaining leakage current and
prevents the current source from climbing to its compliance voltage. When excitation is turned back on, the process is
reversed to prevent transients from heating the measured resistor.
To turn excitation off press the
Voltage Mode
and
Current Mode
keys at the same time for one second. Both excitation
mode LED annunciators will turn off and the excitation display location will show Off.
To restore excitation press any of the range or excitation keys. The first key press will return the instrument to its
previous excitation without making a change.
4.6 INPUT
SETUP
There are several parameters besides resistance range and excitation that directly affect the measurement input. The
parameters grouped under
Input Setup
are specific to individual scanner channels if a scanner is installed. Channel
selection, scan pause time and channel enable are skipped if a scanner is not installed. Change pause and curve or
temperature coefficient should be set even if the Model 370 is configured for only one channel. The parameters grouped
under
Instrument Setup
(Paragraph 4.7) apply to all channels whether or not a scanner is installed.
4-8
Operation
