Watlow CLS - Manual (Page 32)

24 CLS User’s Guide
Installation
Wiring: Noise Suppression
If the CLS's outputs control dry contact electromechanical relays with
inductive loads--like alarm horns and motor starters
-- you may get
Electro-magnetic Interference (EMI, or “noise”) The following section
explains how to avoid noise problems; read it before you wire the CLS.
Symptoms of RFI/EMI
If your controller displays the following symptoms, suspect EMI.
• The CLS's display blanks out and then reenergizes as if power had
been turned off for a moment.
• The process value does not display correctly.
EMI may also damage the digital output circuit--so digital outputs will
not energize. If the digital output circuit is damaged, return the
controller to Watlow Anafaze for repair.
Avoiding Noise Problems
To avoid noise problems:
Where possible, use solid state relays (SSRs) instead of
electromechanical (EM) relays. If you must use EM relays, try to avoid
mounting them in the same panel as the CLS equipment.
Separate the 120 Vac power leads from the low level input and output
leads connected to the CLS. Don't run the digital output or PID control
output leads in bundles with 120 Vac wires. (Never run input leads in
bundles with high power leads. See the General Wiring section.)
If you must use EM relays and you must place them in a panel with CLS
equipment, use a .01 microfarad capacitor rated at 1000 Vac (or higher)
in series with a 47 ohm,
½ watt resistor across the NO contacts of the
relay load. This network is known as an arc suppressor or snubber
network.
You can use other voltage suppression devices, but they are not usually
required. For instance, you can place a metal oxide varistor (MOV)
rated at 130 Vac for 120 Vac control circuits across the load, which
limits the peak AC voltage to about 180 Vac (Watlow Anafaze P/N 26-
130210-00). You can also place a transorb (back to back zener diodes)
across the digital output, which limits the digital output loop to 5 Vdc.
(All the parts mentioned here are available from Watlow Anafaze).
The above steps will eliminate most noise problems. If you have further
problems or questions, please contact Watlow Anafaze.