Lakeshore Learning Materials 642 - Manual (Page 45)

Lake Shore Model 642 Electromagnet Power Supply User’s Manual
Operation 4-5
Setting Output Current (Continued)
To change the output current setting, press the Output Setting key. The output current setting value on the normal
display will be highlighted to prompt for the new output current setting value.
Set: + 0.0000 A
Use the data entry keys to enter an output current setting value between –70.0000 and + 70.0000. Press Enter to accept
the new value. Press Escape to restart the setting sequence and enter a different value. Press Escape again to leave the
setting sequence.
NOTE: The output current setting value can be set as high as ±70.1000 A. This can be used to compensate for variances
in calibration. The output current is guaranteed to reach a minimum of 70 A into a 0.5 ohm load but may not be
able to reach 70.1 A in all circumstances.
The output current setting is not allowed to change if the instrument is setup so that the output current is programmed
solely by an external voltage. Refer to Paragraph 4.15 to setup the external current program mode.
Change not allowed while
in external current program mode
4.7 CURRENT RAMP RATE
The output current of the Model 642 will always ramp from one current setting to another. There is no way to turn off the
current ramping function, but if a very fast ramp rate is desired, a ramp rate as high as 99.999 A/s can be entered.
To change the current ramp rate press the Ramp Rate key. The ramp rate value on the normal display will be
highlighted to prompt for the new ramp rate value.
Rate: 1.0000 A/s
Use the data entry keys to enter the ramp rate value between 0.0001 and 99.999 A/s. Press Enter to accept the new
value. Press Escape to restart the setting sequence and enter a different value. Press Escape again to leave the setting
sequence.
4.8 RAMP SEGMENTS
The magnetic field produced by electromagnets is not linear with current setting. The best way to compensate for this is
to use closed loop field control but if absolute accuracy is not necessary than some of this nonlinearity can be corrected
by the use of the ramp segments feature. The ramp segments feature can be used to increase the current ramp rate as the
magnet saturates in an attempt to maintain the same field ramp rate. This feature can change the output current ramp rate
based on the output current setting. As the output setting ramps through the segment boundary, the new ramp rate will be
used although it will still be limited by the maximum ramp rate setting. Refer to Paragraph 4.11.2 to set the maximum
ramp rate.
To use the ramp segment feature, the ramp segments must be enabled and the ramp segment table must be setup to
specify which ramp rate to use for each current setting. The table should be setup in order of increasing current.
A current entry of 0 A indicates the end of the table and the instrument will not search any higher in the table.