Lake Shore 625, Руководство пользователя

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Lake Shore Model 625 Superconducting MPS User’s Manual
Installation 3-7
3.8 INSTRUMENT GROUNDING AND ISOLATION
Chassis grounded circuits should have a low impedance path to Earth ground for both performance and safety. This is
achieved by using a 3-conductor power cord in a properly grounded outlet. All instrumentation that is connected to the
analog outputs or computer interface (or use Earth ground as measurement common) should have similar low impedance
to the same ground. Good grounding of all chassis in a group of instruments prevents voltage differences between their
chassis (usually at line frequency) from inducing noise on the measurement. If good quality Earth ground is not
available, the chassis of line powered instruments should be strapped together with heavy gauge copper wire or ground
strap. Signal leads in communication cables should not be relied on for ground strapping.
The high current output stage in the Model 625 uses a bridging output technology and is optically isolated from power
line ground. Optical isolation of the output stage provides noise immunity from power line (Earth) related ground loops
with outside measurement equipment.
A 100-ohm internal resistor ties the power line ground to the internal output stage common to provide a reference to the
output stage in instances where the output is allowed to float. This is to prevent electrostatic build up between the
isolated sections of the Model 625. If the magnet coil is left ungrounded, this is the only path that keeps the common-
mode voltage of the magnet at or near earth ground. In many instances, it is expected that one of current output terminals
will be attached to power line ground. In this instance, it is recommended that the negative terminal be attached to power
line ground since tying the positive terminal to ground will result in a small current reading error of 30
–
70 mA. Either
output terminal can be connected to power line ground without damage.
The computer interfaces are still referenced to power line ground. The Output Voltage Monitor and Output Current
Monitor outputs are sourced from the isolated output stage but are electronically balanced and have a common-mode
voltage limit of 10 V. This limit is more than adequate for any equipment that is referenced to power line ground.
Similarly, the External Current Programming and Remote Voltage inputs are differential and allow easy interface to
power line grounded equipment.
3.9 CONNECTING MULTIPLE UNITS IN PARALLEL
Up to two Model 625’s can be connected in parallel to provide up to 120 A of current at a maximum compliance voltage
of 5 V. When the units are connected, they are still operated individually. The total amount of current supplied to the
load is the sum of the currents that each supply is delivering. If both supplies are ramping current at the same time, then
the total ramp rate is equal to the sum of both the ramp rates as long as neither of the supplies reaches its compliance
voltage limit. If the compliance voltage limits of the two supplies are not the same, then the total compliance voltage is
limited to the lower setting of the two supplies.
To connect two power supplies together, the +OUTPUT and –OUTPUT terminals of one supply should be connected to
the +OUTPUT and –OUTPUT terminals of the other supply respectively. Make these leads as short as possible to
minimize output potential differences between the two power supplies and use a heavy enough gauge wire to carry at
least 60 A. Refer to Table 3-1 for wire gauge properties. Connect the +OUTPUT and –OUTPUT terminals of one of the
supplies to the load using a heavy enough gauge wire to carry 120 A. On the Digital I/O connector, connect the Fault
Output of each supply to the Remote Inhibit of the other supply. Be sure to secure the Digital I/O connectors to the
power supplies with the connector screws. When all connections are made, power up both supplies at the same time. It is
possible that one or both of the supplies will power up with a Remote Inhibit fault detected if both supplies are not
powered up at the same time. If this occurs, wait until both units have powered up and then clear the error using the
Status key. Refer to Paragraph 7.6 for information on clearing errors. Lake Shore sells a Dual Supply Interconnect Cable
Kit with all of the necessary cables to connect two supplies in parallel. Refer to Paragraph 6.2 for ordering accessories.