Appendix B
Reducing Electrical Noise
Noise-related difficulties can range in severity from minor positioning errors to damaged
equipment from runaway loads crashing blindly through limit switches. In microprocessor-
controlled equipment such as the APEX615n, the processor constantly retrieves instructions
from memory in a controlled sequence. If an electrical disturbance occurs, it may cause the
processor to misinterpret an instruction or access the wrong data. This can be catastrophic to
the program and force you to reset the processor.
Sources of Noise
Being invisible, electrical noise can be very mysterious, but it invariably comes from the
following sources:
• Power line noise
• Externally conducted noise
• Transmitted noise
• Ground loops
The following electrical devices are notorious for generating unwanted electrical noise
conditions:
• Coil-driven devices: conducted and power line noise
• SCR-fired heaters: transmitted and power line noise
• Valves, motors & motor drives: transmitted and power line noise
• Welders (electric): transmitted and power line noise
Power Line Noise
Power line noise is usually easy to resolve due to the wide availability of line filtering
equipment for the industry. Only the most severe situations call for an isolation transformer.
Line filtering equipment is required when other devices connected to the local power line are
switching large amounts of current, especially if the switching occurs at a high frequency.
Any device having coils is likely to disrupt the power line when it is switched off. Surge
suppressers, such as metal oxide varistors (MOVs) are capable of limiting this type of
electrical noise. A series resistor/capacitor (RC) network across the coil is also effective
(resistance: 500 to 1,000 Ω; capacitance: 0.1 to 0.2 µF). Coil-driven devices (inductive
loads) include relays, solenoids, contactors, clutches, brakes, and motor starters.
MOV
AC or DC
R
C
Typical RC Network
AC or DC Inductive
Load
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