Lakeshore Learning Materials 642 - Manual (Page 110)

Lake Shore Model 642 Electromagnet Power Supply User’s Manual
7-18 Service
7.13.3.3 Calibrate Output Voltage Reading Zero
This step assumes that the previous two steps are successful and the current through the shunt resistor is quite low
resulting in virtually no voltage across the Model 642 output terminals.
1. Send “CALZ 5, 0” to set the current reading offset constant to 0.
2. Get the Model 642 output voltage reading (by front panel or interface).
3. Calculate zero offset constant: – (output voltage reading).
4. Send “CALZ 5, zero offset constant”.
5. Verify the Model 642 output voltage reading to be 0 ±0.001 V.
6. Send “CALSAVE” to write this calibration to non-volatile memory.
7.11.3.4 Calibrate External Programming Voltage Reading Zero
1. Short the external current programming input lines (pins 3 and 11 of the Analog I/O connector).
2. Send “CALZ 7, 0”. To set the external programming voltage reading constant to 0.
3. Get the Model 642 external programming voltage reading.
NOTE: To get this reading from the Model 642, press and hold the Status key on the front panel until the display
goes dark ( ≈ 3 seconds). When the key is then released, a diagnostics display will be seen. The upper right
reading, “EXT PROG”, is the reading needed for this step.
4. Calculate zero offset constant: – (external programming voltage reading).
5. Send “CALZ 7, zero offset constant”.
6. Verify the Model 642 external programming voltage reading to be 0 ±0.0001 V.
7. Send “CALSAVE” to write this calibration to non-volatile memory.
7.11.3.5 Calibrate Output Current Gain (Span)
This calibration is the most difficult and most important of the procedure. It first requires measuring the span of the trim
range (which changes from unit to unit within 1 – 2%) to determine the trim adjustment range that corresponds to a
100% calibration trim change. In other words, we adjust the calibration in % from –100% to +100% but the size of the
calibration trim range changes slightly for each unit. We first measure the span of the calibration trim range and then
measure the full span of the instrument with the calibration trim set to 0%. We have adopted the use of % as the unit
name for the trim calibration since it corresponds to only the percent of trim range and not the full span of the
instrument.
The 65 A set point for calibration is chosen because the maximum trim range at 70 A forces the current past the 70.1 A
internal analog limit and therefore creates significant error. The shunt resistor remains connected in series with the
magnet load for this procedure.
1. Send “CALG 10, 0”. To set the output current gain trim constant to 0.
2. Set the Model 642 output current to +65 A (ramp rate 30 A/s nominal).
3. Wait 30 seconds for settling.
4. Measure the actual voltage across the shunt and record (V
shunt
).
5. Calculate and record (I
max
): V
shunt
/R
shunt
.
6. Send “CALG 10, –100”. To set the output current gain trim constant to maximum.
7. Wait 10 seconds.
8. Measure the actual voltage across the shunt and record (V
shunt
).
9. Calculate and record (I
maxpostrim
): V
shunt
/R
shunt
.
10. Send “CALG 10, +100”. To set the output current gain trim constant to minimum.
11. Wait 10 seconds.