Model 8742 Series Quick Start Guide 90066735 Rev. E Page 13 of 16
1.3
Safety Grounding Considerations
There are potentially lethal voltages generated with 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 also 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 1.1.9, 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. 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 because this insulation will
probably need to have a breakdown voltage of several thousand volts due to
lightening concerns.
In summary, New Focus strongly recommends grounding the Model 8742’s
case.
