P
AGE
10
2.5.1.5 NOISE SUPPRESSION AT THE SOURCE
Usually when good wiring practices are followed, no further noise protection is necessary.
Sometimes in severe electrical environments, the amount of noise is so great that it has to be
suppressed at the source. Many manufacturers of relays, contactors, etc. supply "surge
suppressors" which mount on the noise source.
For those devices thta do not have surge suppressors supplied, RC (resistance-capacitance)
networks and/or MOV (metal oxide varistors) may be added.
Inductive Coils - MOV's are recommended for transient suppression in inductive coils con-
nected in parallel and as close as possible to the coil. See Figure 2-2. Additional protection
may be provided by adding an RC network across the MOV.
Contacts - Arcing may occur across contacts when the contact opens and closes. This results
in electrical noise as well as damage to the contacts. Connecting a RC network properly
sized can eliminate this arc.
For circuits up to 3 amps, a combination of a 47 ohm resistor and 0.1 microfarad capacitor
(1000 volts) is recommended. For circuits from 3 to 5 amps, connect 2 of these in parallel.
See Figure 2-3.
FIGURE 2-2
FIGURE 2-3
Coil
0.5
mfd
1000V
220
ohms
115V 1/4W
230V 1W
Inductive
Load
R
C
MOV