Lake Shore 625, User Manual

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Lake Shore Model 625 Superconducting MPS User’s Manual
Service 7-17
Calibrate Output Current Gain (Continued)
NOTE: The Model 625 has an internal, 25

resistor in parallel with the output terminals.
The “V
measured
/25” factor accounts for the current that is diverted from the external
shunt that flows through the internal resistor. See Section 7.7 for further details.
33. Set the Model 625 output current to 55 A (ramp rate 20 A/s nominal).
34. Wait 30 seconds.
35. Measure the actual voltage across the shunt and record (V
shunt
).
36. Calculate and record (I
out
+): V
shunt
/R
shunt
.
37. Calculate and record (I
verify
): 55 – (V
measured
/25) NOTE: V
measured
is from Step 4 of this section.
38. Verify I
out
+ = I
verify
±0.008 A (0.015%).
39. Set the Model 625 output current to –55 A (ramp rate 20 A/s nominal).
40. Wait 30 seconds.
41. Calculate and record (I
out
–): V
shunt
/R
shunt
.
42. Verify I
out
– = –I
verify
±0.008 A (0.015%).
43. Set the Model 625 output current to 0 A.
44. Send “CALSAVE” to write this calibration to non-volatile memory.
7.13.3.7 Calibrate Current Reading Gain
1. Send “CALCOMP 0” (this is a math computation that takes into account the current that flows through the 25

damping resistor that is across the output of the Model 625. “CALCOMP 0” turns the compensation ON so that the
reading reflects only the current going through the load by subtracting the current going through the resistor).
2. Send “CALG 0, 1”. To set the output current reading gain constant to 1.
3. Set the Model 625 output current to –60 A. (Ramp rate 20 A/s nominal).
4. Wait 30 seconds.
5. Measure the actual voltage across the shunt and record (V
shunt
).
6. Calculate and record (I
measuredneg
): V
shunt
/R
shunt
.
7. Get the Model 625 output current reading (by front panel or interface) and record (I
readingneg
).
8. Set the Model 625 output current to +60 A (Ramp rate 20 A/s nominal).
9. Wait 30 seconds.
10. Measure the actual voltage across the shunt and record (V
shunt
).
11. Calculate and record (I
measuredpos
): V
shunt
/R
shunt
.
12. Get the Model 625 output current reading (by front panel or interface) and record (I
readingpos
).
13. Calculate gain constant per Equation 3.
Equation 3
14. Verify gain factor to be 1, ±0.02.
15. Send “CALG 0, gain constant”.
16. Verify the Model 625 output current reading to equal I
measuredpos
, ±0.008 A.
17. Set the Model 625 output current to 0 A.
18. Send “CALSAVE” to write this calibration to non-volatile memory.
Again, this method is somewhat lengthy, but takes into account the contribution of the damping resistor to the current
reading with resistance in the high current cabling even when cable resistance is low. It also averages the differences
between positive (+) and negative (–) excursions.
 
 
measuredpos measuredneg
readingpos readingneg
I I
Current Reading Gain Constant
I I


