IAI America PCON-ABU - Manual (Page 32)

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[2] Wiring precautions
Use a twisted cable for connecting the 24-VDC power supply.
Wire the controller cables separately from high-current lines of power circuits, etc. (Do not bundle them
together or place them in the same cable duct.)
(2) Noise sources and how to prevent noise
There are many noise sources, but the ones you should pay greatest attention to are solenoid valves,
magnet switches and relays. Noise from these parts can be prevented by the methods specified below.
[1] AC solenoid valves, magnet switches and relays
Action --- Install a surge absorber in parallel with the coil.
[2] DC solenoid valves, magnet switches and relays
Action --- Install a diode in parallel with the coil, or use the built-in diode.
Surge absorber
Install the surge absorber at the closest possible
location to the coil.
If the surge absorber is installed on a terminal block
or otherwise away from the coil, the surge absorber
cannot eliminate noise effectively.
With DC circuits, connecting the diode in wrong
polarities may damage the diode, internal controller
parts, and/or DC power supply. Pay careful attention
to the diode polarities.