Allen-Bradley micrologix 1500, User Manual

Page: 51 / 149

Publication 1764-UM001A-US-P
Wiring Your Controller 3-15
Controller I/O Wiring
Minimizing Electrical Noise
Because of the variety of applications and environments where controllers are
installed and operating, it is impossible to ensure that all environmental noise will
be removed by input filters. To help reduce the effects of environmental noise,
install the MicroLogix 1500 system in a properly rated (i.e. NEMA) enclosure.
Make sure that the MicroLogix 1500 system is properly grounded.
A system may malfunction may occur due to a change in the operating
environment after a period of time. We recommend periodically checking system
operation, particularly when new machinery or other noise sources are installed
near the Micrologix 1500 system.
Transistor Output Transient Pulses
The transient energy is dissipated in the load, and the pulse duration is longer for
loads with high impedance. The graph below illustrates the relation between pulse
duration and load current. Power-up transients will not exceed the times shown in
the graph. For most applications the pulse energy is not sufficient to energize the
load.
To reduce the possibility of inadvertent operation of devices connected to transistor
outputs, consider adding an external resistor in parallel to the load to increase the
on-state load current. The duration of the transient pulse is reduced when the
on-state load current is increased or the load impedance is decreased.
ATTENTION
!
A brief transient current pulse may flow through transistor
outputs if the external supply voltage is suddenly applied at the
V dc and V dc com terminals (e.g. via the master control relay). It
is a fast rate-of-change of voltage at the terminals that causes the
pulse. This condition is inherent in transistor outputs and is
common to solid state devices. The transient pulses may occur
regardless of whether the controller is powered or running.
0.4
1.0
0.9
0.8
0.7
0.6
0.5
0.3
0.2
0.1
0.0
1000 900 800 700 600 500 400 300 200 100 1
On-State Load Current (mA)
Ti
m
e - Dur
at
ion o
f T
rans
ie
nt
(m
s)
Transient Pulse Duration as a
Function of Load Current
AB Spares