Appendix A. Reducing Electrical Noise
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Appendix A: Reducing Electrical Noise
Noise-related difficulties can range in severity from minor
positioning errors to damaged equipment from runaway
motors crashing blindly through limit switches. In
microprocessor-controlled equipment such as the OEM-
AT6400, the processor constantly retrieves instructions
from the PC-AT and from on-board 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
•
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 suppressors, 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
µ
F). Coil-driven
devices (inductive loads) include relays, solenoids,
contractors, clutches, brakes, and motor starters.
MOV
AC or DC
R
C
Typical RC Network
AC or DC
Inductive
Load
Externally Conducted Noise
Externally-conducted noise is similar to power line noise,
but the disturbances are created on signal and ground
wires that are connected to the OEM-AT6400. This kind
of noise can get into logic circuit ground or into the
processor power supply and scramble the program. The
problem here is that control equipment often shares a
common DC ground wire that may be connected to
several devices, such as a DC power supply,
programmable controller, remote switches, etc. When a
noisy device like a relay or solenoid is attached to the DC
ground, it may cause disturbances within the OEM-
AT6400.
To solve a noise problem caused by DC mechanical relays
and solenoids, you can connect a diode backwards
across the coil to clamp the induced voltage
kick that the
coil will produce. The diode should be rated at 4 times the
coil voltage and 10 times the coil current. Using solid
state relays is another way to eliminate this problem.
Diode
DC
To eliminate ground loops, multiple devices on the same
circuit should be grounded together at a single point.
Furthermore, power supplies and programmable
controllers often have DC common tied to Earth (AC
power ground). As a rule, it is preferable to have the
OEM-AT6400 signal ground or DC common floating with
respect to Earth. This prevents noisy equipment which is
grounded to Earth from sending noise into the OEM-
AT6400. When floating the signal ground is not possible,
you should make the Earth ground connection at only one
point.
In many cases, optical isolation may be required to
completely eliminate electrical contact between the OEM-
AT6400 and a noisy environment. Solid state relays
provide this type of isolation.
Transmitted Noise
Transmitted noise is picked up by external connections
to the OEM-AT6400, and in severe cases can attack the
OEM-AT6400 when there are no external connections.
The sheet metal enclosure of your computer will typically
shield the electronics from this, but openings in the
enclosure for connections and front panel controls may
leak.
When high current contacts open, they draw an arc,
producing a burst of broad spectrum radio frequency
noise that can be picked up on a limit switch or other
wiring. High-current and high-voltage wires have an
electrical field around them and may induce noise on
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