Installing Your
Programmable Controller
Chapter 4
4-5
All category-2 conductors must be properly shielded, where applicable,
and routed in a separate raceway.
If a category-2 conductor must cross power feed lines, it should do so at
right angles.
Route category-2 conductors at least 1 foot from 120V ac power lines, 2
feet from 240V ac power lines, and 3 feet from 480V ac power lines.
Route category-2 conductors at least 3 feet from any electric motors,
transformers, rectifiers, generators, arc welders, induction furnaces, or
sources of microwave radiation.
If a category-2 conductor is in a metal raceway or conduit, that raceway
or conduit must be well grounded along its entire length.
All category-3 conductors should be routed external to all raceways or
in a raceway separate from any category-1 or category-2 conductors.
Power Distribution
In many applications, you can connect the processor power supply directly
to the secondary of a transformer (Figure 4.1 and Figure 4.2). The
transformer can provide dc isolation from other equipment not connected
to that transformer secondary. Connect the transformer primary to the ac
source; connect the high side of the transformer secondary to the L1
terminal of the power supply; connect the low side of the transformer
secondary to the neutral (common) terminal of the power supply.
Sizing the Transformer
Note that the external-transformer rating (in VA) of each supply is 2.5
times its input power requirements (in Watts). This is necessary because,
converting ac to dc draws power only from the peak of the ac voltage
wave-form.
If the transformer’s rating is too small, it will clip the peak of the sine
wave. When the input voltage is still above the lower voltage limit, the
power supply will sense this clipped wave form as a low voltage and shut
down the processor prematurely. If the transformer is too large, it will not
provide as much isolation as a transformer of proper size because a larger
noise spike on the primary can pass through to the secondary.
