Watlow CLS200 - Manual (Page 39)

CLS200 Series User’s Guide Chapter 2: Installation
Doc.# 0600-3050-2000 Watlow Anafaze 23
Avoiding RFI/EMI
• To avoid or eliminate most RFI/EMI noise problems:
• Connect the CLS200 case to earth ground. The
CLS200 system includes noise suppression circuitry.
This circuitry requires proper grounding.
• Separate the 120 or 240V Å (ac) power leads from the
low-level input and output leads connected to the
CLS200 series controller. Do not run the digital I/O or
control output leads in bundles with ac wires.
• Where possible, use solid-state relays (SSRs) instead
of electromechanical relays. If you must use electro-
mechanical relays, try to avoid mounting them in the
same panel as the CLS200 series equipment.
• If you must use electromechanical relays and you
must place them in a panel with CLS200 series equip-
ment, use a 0.01 microfarad capacitor rated at
1000V
Å (ac) (or higher) in series with a 47 Ω , 0.5 watt
resistor across the N.O. contacts of the relay load. This
is known as a snubber network and can reduce the
amount of electrical noise.
• 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 130V
Å for
120V Å (ac) control circuits across the load, which lim-
its the peak ac voltage to about 180V Å (ac) (Watlow
Anafaze part number 26-130210-00). You can also
place a transorb (back-to-back zener diodes) across the
digital output, which limits the digital output voltage.
Additional Recommendations for a Noise Immune System
It is strongly recommended that you:
• Isolate outputs through solid-state relays, where pos-
sible.
• Isolate RTDs or “bridge” type inputs from ground.
• Isolate digital inputs from ground through solid state
relays. If this is not possible, then make sure the digi-
tal input is the only connection to earth ground other
than the chassis ground.
• If you are using EIA/TIA-232 from a non-isolated host,
either (1) do not connect any other power common
point to earth ground, or (2) use an optical isolator in
the communications line.