Lakeshore Learning Materials 622 User Manual
Page 33
Lake Shore Model 620/622/623/647 Magnet Power Supply User’s Manual
Operation
3-7
I
MAX
SET: 0.00 A to +50.00 A (620), +125.00 A (622), +155.00 A (623), or +72.00 A (647)
Current Maximum Setting. MPS current cannot exceed this setting. The software allows no setting above
these limits. The rated maximum magnet current is the current above which magnet damage may occur.
Never exceed the rated maximum current.
I
MAX
is the maximum output current. I
SET
is the constant output current. The value for I
SET
cannot exceed the
value for IMAX. For example, if the user sets I
MAX
to 1 A, the MPS limits any constant output current in excess
of 1 A to 1 A. If the user increases I
MAX
to 10 A, and sets I
SET
to 1 A, the MPS still limits constant output
current to 1 A, not 10 A. Changing I
SET
to 10 A allows a constant output current of 10 A, and so on.
To set IMAX, move to the IMAX SET line and enter the maximum current allowed for the magnet.
COMPLIANCE: 0.00 V to ± 5.00 V (620), ± 30.00 V (622 & 623), or ± 32.00 V (647)
Compliance Voltage Limit. The MPS cannot exceed the Compliance Voltage Limit of the given model.
Set the compliance voltage lower than the rated voltage of both the magnet and, if present, the magnet
protection diodes. Determine these values from magnet manufacturer information. Once the user enters a
Compliance voltage, the MPS retains it in non-volatile memory as the default value.
Some magnets come equipped with diodes across the magnet leads to protect the magnet in the event of a
magnet quench or a ramp rate that produces a charging voltage that exceeds the rated voltage of the magnet.
Set the MPS voltage limit to below the rated voltage of the protection diodes.
CAUTION: Never set the MPS voltage limit above the rated voltage of the magnet's built-in protection
diodes. Doing so may result in damage to the magnet and/or its diodes.
If the magnet voltage reaches the MPS compliance voltage during charging, the MPS limits the ramp rate and
holds the voltage at the compliance until the end of charging or until the ramp rate falls below:
V
L
set
where
V
set
= MPS compliance voltage, and
L = magnet inductance in henrys.
The MPS automatically switches between operation as a current source and limitation by the compliance
voltage. The MPS operates most smoothly as a current source. As such, current ramp rate is limited by:
di
dt
<
V
L
set
where
di
dt
= max magnet current ramp rate in amperes/second,
V
set
= MPS compliance voltage,
and
L = magnet inductance in henrys.
In a multiple MPS configuration, each MPS must operate in current mode. If one unit operates in compliance
mode while the others operate in current mode, uneven distribution of the current load results.
Though compliance voltage is user-defined, if operation above safe diode voltage is a possibility, Lake Shore
can limit the hardware output voltage range and OVP to a safe level for the magnet. Example: ±3 volts
maximum output voltage limit with an OVP setting of ±7 to 9 volts (assuming this voltage is below the diode
forward voltage at 4.2 K). Return the MPS to the factory for this hardware modification.
To set the compliance voltage, move to the COMPLIANCE line and enter a compliance voltage less than the
maximum voltage for both the magnet and the magnet protection diodes.
Software also automatically limits MPS output power to 1000 VA (622 & 623) or 2000 VA (647) by adjusting
the voltage. For example, if the user sets current to 0 amps and compliance to 30 volts, then changes current
to 100 amps, the MPS software automatically sets compliance to 10 volts.
3.6.2
Function Menu 2 Screen
To display Function Menu 2 shown on the right,
press the Function Menu key to display Menu 1,
then the Next Menu key. The 2/3 in the upper right
corner indicates Menu 2 of 3. Without a PSH Option
Card, a “2/2” appears in the upper right corner
indicating menu 2 of 2. With all options installed,
Function Menu 2 has four possible selections:
OUTPUTS ONLY, LHe LEVEL, FIELD and PSH SETUP . <NP> indicates the function is “Not Present” and the
associated function key is ignored.
2/3
OUTPUTS ONLY
LHe LEVEL
FIELD
PSH SETUP