Lake Shore 625, User Manual

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Lake Shore Model 625 Superconducting MPS User’s Manual
7-14 Service
7.13.2 Calibration Equipment
1. Shunt Resistor. The output current of the Model 625 must be measured externally as the primary reference for
calibration. When current is measured, it is the result of the current through the known resistance of the calibration
resistor. This resistor must be accurately measured and its actual value
( R Shunt) used to determine the actual
current flow. For example, if the resistor is measured at 0.99661 m

, the actual current flowing is calculated by
Equation 1.
Equation 1
This resistor must withstand the full current of the Model 625 and do so with a minimum of heating that can easily
change the resistance and therefore the current measurement. At 60 A, this resistor only dissipates 3.6 W. Even so, it
is highly recommended to mount the resistor on a heat sink with forced air-cooling to minimize temperature rise.
(Alpha PSBWR0010F is suggested)
2. Load Resistor. This resistor is placed in series with the 1 m

resistor to create a compliance voltage to calibrate the
voltage reading of the Model 625. This resistor value should be in a range from 0.075

to 5

The actual value of
this resistor is not critical since only a voltage measurement is made to calibrate a voltage reading on the Model 625.
To determine the amount of power the resistor will be required to handle, reference the formula in Section 7.13.3.8
step 4. This specifies the amount of current through the resistor. Note the output voltage is compliance limited to
5 V. For example, a 0.075

resistor will develop 270 W and a 5

resistor will develop 5 W.
3. PSH Resistor. This is simply a 1 W, 100

resistor used to develop a voltage from the PSH output current to drive
the Analog Current Programming Input for its gain calibration. During the procedure, this resistor will dissipate only
0.25 W.
4. DC Voltmeter (DVM). The voltmeter must measure VDC accurately to 10’s of µV if resolution to 10’s of mA
(from the Model 625) is to be assured. The HP34401 or better is suggested.
7.13.3 Calibration Procedure
The following calibration steps should be performed exactly in the order provided.
7.13.3.1 Calibrate Current Output Zero
The 1 m

unt resistor is wired directly across the Model 625 output terminals with #4 AWG wire. Cable length is
relatively unimportant but should be less than 5 feet to the shunt resistor. Voltage across the shunt resistor is to be
monitored by the DVM.
1. Send “CALZ 10, 0” to set the output offset constant to 0.
2. Set the Model 625 output current to 0 A.
3. Measure the actual voltage across the shunt and record ( V
shunt
).
4. Calculate the zero offset constant: ( V
shunt
/R
shunt
).
5. Send “CALZ 10, zero offset constant”.
6. Reset the Model 625 output current to 0 A (loads the new offset setting).
7. Verify the actual output current to be less than ±1 mA.
8. Send “CALSAVE” to write this calibration to non-volatile memory.
7.13.3.2 Calibrate Current Reading Zero
1. Send “CALZ 0, 0” to set the current reading offset constant to 0.
2. Measure the actual voltage across the shunt and record (V
shunt
).
3. Get the Model 625 output current reading (by front panel or interface) and record ( I reading ) .
4. Calculate the zero offset constant:
–
(I
reading
– (V
shunt
/R
shunt
) ) .
5. Send “CALZ 0, zero offset constant”.
6. Verify the Model 625 output current reading to match the actual output current within ±0.0005 mA.
7. Send “CALSAVE” to write this calibration to non-volatile memory.
3
0.99661 10
shunt
shunt
V
I


