Safety Precautions 19
1.4 Safety Grounding Considerations
There are potentially lethal voltages generated within the Model 8742 Picomotor
Controller/Driver and presented on the pins of the “Motor” connectors EVEN
WHEN NO PICOMOTOR IS INSTALLED. (These voltages are not present when
power is not applied to the unit. They are not present when the unit is switched
OFF.) The User must manage certain system-level aspects to ensure safe operation.
One of these is grounding.
The Model 8742’s external DC power supply provides 12VDC. The negative output
from this supply is typically isolated from MAINS earth ground, but this isolation
should not be assumed. This same negative output is connected directly to the
Model 8742’s external case within the Model 8742. This same “ground” is at the
same potential as the MOTOR connectors’ return signal pins. This means that the
MOTOR connectors’ drive signal pins are, at times, at VERY HIGH VOLTAGES
relative to the Model 8742 case, the USB connector shell, and the DC input/loop-
through connectors’ negative terminals.
An 8-32 screw is provided on the unit and marked with the “Ground” symbol shown
in Paragraph 1.1.8, above. While it is not necessary to connect this terminal to a
building “earth” ground, New Focus recommends doing so because it will simplify
system-level compliance with most regulatory requirements. Once this connection
is made, the user must ensure that all other system components are appropriately
grounded. Do not depend upon the physical mounting hardware to accomplish this
grounding.
If the other components are not grounded, a damaged Picomotor (or its wire) could
result in a dangerously high voltage being present between anything that is grounded
(like the 8742 and a computer attached to it via USB) and the Picomotor case. Note
that the simplest way to ground a Picomotor is via its threaded mounting bushing or
other mounting hardware. DO NOT ATTEMPT TO OPEN A PICOMOTOR TO
ACHIEVE THIS GROUNDING.
If system requirements prevent the Picomotor from being grounded, then the user
must ensure that the Model 8742 and all other system components (including any
USB-connected computers) are properly isolated from earth ground. This is because
a short of the high-voltage drive signal to an earth ground may result in potentially
lethal voltages existing between the case of the 8742 and anything that is earth
grounded, such as a floor. The User is advised to consult all applicable regulations
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