Watlow Electric Micro-Based Autotuning Control SERIES 980/985 - Manual (Page 7)

Noise Sources
Switches and relay contacts operating inductive loads such as motor, coils,
solenoids, and relays, etc.
Thyristors or other semiconductor devices which are not zero crossover-fired
(randomly-fired or phase angle-fired devices).
All welding machinery.
Heavy current carrying conductors.
Fluorescent and neon lights.
How To Decrease Noise Sensitivity
Physical separatlon and wire routing must be given careful consideration in
planning the layout of the system. For example, A.C. power supply lines
should be bundled together and physically kept separate from input signal
lines (sensor lines). A 12 inch minimum separatlon is usually effective. Keep
all switched output signal lines (high power level) separate from input signal
lines (sensor lines). Cross other wiring at 90° angles whenever crossing lines
is unavoidable.
Another important practice is to look at the system layout and identify electri-
cal noise sources such as solenoids, relay contacts, motors, etc., and where
they are physically located. Then route the wire bundles and cables as far
away as possible from these noise sources. Don’t mount relays or switching
devices close to a microprocessor control. Don’t have phase angle-fired
devices in the same electrical enclosure or on the same power line with the
control.
Shielded cables should be used for all low power signal lines to protect from
magnetic and electrostatic coupling of noise. Some simple pointers are:
• Whenever possible, low level signal lines should be run unbroken from
signal source to the control circuit.
• Connect the shield to the control circuit common at the control end only.
Never leave the shield unconnected at both ends. Never connect both
shield ends to a common or ground.
• If the shield is broken at some termination point and then continued on, the
shield must be connected to maintain shield continuity.
• If the shield is used as a signal return, no electrostatic shieldlng should be
assumed. If this must be done, use a triaxed cable (electrostatically
shielded coaxial cable).
Twisted pair wire should be used any time control circuit signals must travel
over two feet or when they are bundled in parallel with other wires.
The size or gauge of wire should be selected by calculating the maximum
circuit current and choosing the gauge meeting that requirement. Using
greatly larger wire sizes than required generally will increase the likelihood of
electrostatic (capacitance) coupling of noise.
Install and Wire, Chapter 2
WATLOW Series 980/985 User's Manual
7