Watlow CPC400 - Manual (Page 36)

Chapter 2: Installation CPC400 Series User’s Guide
22 Watlow Anafaze Doc. 0600-2900-2000
Avoiding Noise
To avoid or eliminate most RFI/EMI noise problems:
• Connect the CPC400 case to earth ground. The
CPC400 system includes noise suppression circuitry.
This circuitry requires proper grounding.
• Separate the 120V Å ( ac) and higher power leads from
the low-level input and output leads connected to the
CPC400 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, avoid mounting them in the same
panel as the CPC400 series equipment.
• If you must use electromechanical relays and you
must place them in a panel with CPC400 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 normally-open 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 (ac)
for 120V Å (ac) control circuits across the load, which
limits 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
We strongly recommended the following:
• Isolate outputs through solid-state relays, where pos-
sible.
• 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.