Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 48)

Installing Your
Programmable Controller
Chapter 4
4-9
Second Transformer
Allen-Bradley power supplies have circuits which suppress
electromagnetic interference from other equipment. However, it is useful
to isolate output circuits from power supplies and input circuits to guard
against output transients from being induced into inputs and power
supplies. Therefore, in many applications, power is provided to the input
circuits and power supplies through a second transformer as in Figure 4.3.
In some applications, a special kind of transformer is used for the
second transformer.
Isolation Transformer
For installations near particularly excessive electrical noise generators, an
isolation transformer (for the second transformer) will provide further
suppression of electromagnetic interference from other equipment. The
output devices being controlled should draw power from the same ac
source as the isolation transformer, but not from the secondary of the
isolation transformer (Figure 4.3).
ConstantĆVoltage Transformer
In applications where the ac power source is especially “soft” and subject
to unusual variations, a constant-voltage transformer can stabilize the ac
power source to the controller, thereby minimizing shutdowns. However,
the constant-voltage transformer must provide a sinusoidal output.
If the controller power supply receives its ac power through a
constant-voltage transformer, the input devices connected to the I/O
chassis must also receive their ac power through the same constant-voltage
transformer. If the inputs receive their ac power through another
transformer, the ac source voltage could go low enough that erroneous
input data enters memory while the constant-voltage transformer prevents
the power supply from shutting down the controller.
The output devices being controlled should draw power from the same ac
source as the constant-voltage transformer, but not from the secondary of
the constant-voltage transformer (Figure 4.3).