D8 Series User’s Guide
Chapter 2: Installation
27
Symptoms of Noise
If your controller displays the following symptoms, suspect noise:
• The display screen blanks out and then reenergizes as if power had been turned off for
a moment.
• The process variable value is incorrect on the controller display.
Noise may also damage the digital output circuit such that the digital outputs will not turn on. If the
digital output circuit is damaged, return the controller to Watlow for repair.
Avoiding Noise
To avoid or eliminate most RFI/EMI noise problems:
• Connect the D8 case to earth ground. The D8 system includes noise suppression circuitry.
This circuitry requires proper grounding.
• Separate the 120VAC and higher power leads from the low-level input and output leads
connected to the D8 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 electromechanical relays, avoid mounting them in the same panel as the
D8 series equipment.
• If you must use electromechanical relays and you must place them in a panel with D8 series
equipment, use a 0.01 microfarad capacitor rated at 1000VAC (or higher) in series with a
47Ω, 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 130VAC for 120VAC control
circuits across the load, which limits the peak ac voltage to about 180VAC. 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 possible.
• 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 digital input is the only connection to earth ground other than the chassis ground.
Ground Loops
Ground loops occur when current passes from the process through the controller to ground. This
can cause instrument errors or malfunctions.
A ground loop may follow one of these paths, among others:
• From one sensor to another.
• From a sensor to the dc power supply.
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