IAI America PSEL - Manual (Page 41)

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Part 1 Installation
(3) Noise sources and noise elimination
There are many noise sources, but solenoid valves, magnet switches and relays are of particular
concern when building a system. Noise from these parts can be eliminated using the measures
specified below:
[1] AC solenoid valve, magnet switch, relay
Measure --- Install a surge killer in parallel with the coil.
[2] DC solenoid valve, magnet switch, relay
Measure --- Install a diode in parallel with the coil. Determine the diode capacity in accordance with
the load capacity.
In a DC circuit, connecting a diode in reversed polarity will
damage the diode, internal parts of the controller and DC
power supply. Exercise due caution.
The above noise elimination measures are particularly important when a 24-VDC relay is driven directly by
a controller output and there is also a 100-VAC solenoid valve, etc.
Surge killer
 Point
Wire from each coil over the shortest distance.
Installing a surge killer on the terminal block, etc.,
will be less effective because of a longer distance
from the coil.
Diode