Section 9 – Operation
OPERATION
International Thermal Reseach 9-7
controllers that use electronic logic, especially those using
integrated circuits, are more sensitive to noise. A typical noise
source is any piece of equipment that can cause or produce very
rapid or large amplitude changes in voltage or current when turned
ON and OFF. A single side band transmitter is an example of this
type of equipment.
Noise sources:
• Loose connections
• Switches and relay contacts operating inductive loads, such as
motors, coils, solenoids, and relays etc.
• All welding machinery
• Heavy current carrying conductors
• Fluorescent and neon lights
The sensor input and power output lines, as well as the power
source line, all have the potential to couple or link the control circuit
to a noise source.
Common Impedance Coupling occurs when two circuits share a
common conductor. An example would be operating multiple,
separate loads and the return lines from all are connected together
and run back to the power source with one conductor. The best way
to prevent this is to use independent leads for each return circuit
and terminate them all at the same physical point.
Magnetic (Inductive) Coupling generally appears where there are
wires running parallel or in close vicinity to each other. This is
especially true when the wires from several different circuits are
bundled together in order to make the system wiring appear neat.
Electrostatic (Capacitive) Coupling is a function of the distance the
wires run parallel with each other, the distance between the wires
and the diameter of the wire. The best way to eliminate these is to
run separate leads from separate circuits in separate bundles,
taking special care to keep AC* (high power lead) wires separated
from DC (low power level) wires. If it is at all possible, twisted lead
pairs and shielded cables should be used.
Electromagnetic (Radiation) Coupling occurs when the control
circuit is very close to a high-energy source that is capable of
magnetic or electrostatic induction of a voltage. A common source
of such radiation is an inverter, alternator, generator, motor
transformers, fluorescent lights, radio, TV, and navigation
equipment.
*Note that special attention should be given to the AC power line
because it is a source of unusual types of noise-related problems in
control circuits.
To Next Page