20-02245 REV E
III
5.
ELECTRICAL NOISE
Electrical noise can be problematic if it causes visual interference in monitors,
video cameras, computers, and other devices. Electrical noise compounds when
several devices share the same conductors for their power source. Electrical noise is
generally caused by motors, switching power supplies, lamps, computers, ignition
systems, alternators, etc.
ROS pan & tilts use motors, and the SmartLight emulates a switching power
supply; therefore, these devices intrinsically create some electrical noise. ROS has
gone to great lengths to design our products with state-of-the-art electrical noise filtering
circuits. Even with these circuits, some residual noise may be generated.
Any video camera that shares power conductors with other devices should be
considered the most susceptible to electrical noise; in this configuration, lines or
patterns that are not associated to the viewed scene may be displayed on the video
monitor.
6.
POWER SOURCE
All ROS RS-485 products are designed to be powered with DC voltage only. It is
essential that power delivered to any VDC device be free of electrical noise. Switching
power supplies are among the most common power source devices, due to their size
and efficiency. Unfortunately, these switching power supplies can be the source of
electrical noise and may mislead the system designer or user to inappropriately attribute
the noise to a device attached to the power conductors. It is recommended to ensure
that the ripple noise of the power supply’s specification sheet not be greater than
250mVpk-pk.
The system designer must calculate the maximum power consumption of the
entire network, and that the power supply exceeds such power. For example: if the
entire network draws 50W then it is recommended that the power supply exceeds the
power drop by at least 10% (55W). When the power source exceeds the network power
consumption it guarantees power to each device.
7.
CABLES AND WIRES
When selecting a cable for both the system power and RS-485 communication,
the system designer must consider: cable length, wire gauge, power drop, and power
source.
As described in section 6, the power source must exceed the maximum power
required by the network. Also note that cable length effects system voltage drop; the
system designer must compensate for this voltage drop. For example: an RS-485 pan &
tilt can be attached to one end of a 100’ cable, with a 24 VDC power supply on the
other. By specification, the minimum voltage required by the pan & tilt is 20 VDC. If the
wires within the cable are 26AWG, there could be a 5 VDC voltage drop over the
cable’s length. In that case, the pan & tilt would only receive 19 VDC at its connector,
which is less than the minimum voltage required by the device.
