Cut Sheet

Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com V8-T12-33
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Sensor Output Circuits
As we learned earlier,
sensors interface to other
control circuits through the
output circuit. The control
voltage type is a determining
factor when considering
output type. Control voltage
types, whether AC, DC or
AC/DC, can be categorized as
either load-powered sensor
or line-powered sensor.
Load-Powered—Two-Wire
Sensors
Load-powered devices are
similar to limit switches. They
are connected in series with
the controlled load. These
devices have two
connection points to the
circuit and are often
referred to as two-wire
switches. The operating
current is drawn through
the load.
Load Powered/Two-Wire
Circuit
When the switch is not
operated, it must draw a
minimum operating current
referred to as off-state
leakage current. Off-state
leakage current is also
sometimes referred to as
residual current. This current
is used to keep the sensor
electronics active while it
“looks” for a target. Residual
current is not a problem for
loads such as relays, motor
starters, and so on (with low
impedance). However, loads
such as programmable logic
controllers (with high
impedance) require a leakage
current of lower than 2 mA.
Otherwise, an input like a
PLC (Programmable Logic
Controller) might see the
voltage as being an ON
signal. Most sensors are 1.7
mA. If a particular PLC
requires less than 1.7 mA, a
loading resistor is added in
parallel to the input to the
PLC load. The resistor lowers
the current seen by the PLC
so it doesn’t false trigger.
The current required to
maintain the sensor when
the target is present, is
called the minimum load or
holding current. This current
is about 5 mA depending on
the sensor specification. If
the current drawn by the load
is not high enough, then the
sensor cannot operate.
Sensors with a 5 mA or less
minimum holding current can
be used with PLCs without
concern.
Line-Powered—Three-Wire
Sensors
Line-powered switches
derive their power from the
line and not through the load.
They have three connection
points to the circuit, and
are often referred to as
three-wire switches.
Load Powered/Three-Wire
Circuit
The operating current the
switch pulls from the line,
is called the burden current.
This is typically 20 mA.
Because the operating
current doesn’t pass through
the load, it is not a major
concern for circuit design.
Sensor
Load
Load
Two-Wire Sensors
Most sensors are three-wire
devices, but some
manufacturers offer two-wire
devices. They are designed
to be easy replacements for
limit switches without the
need to change wiring and
logic.
Because two-wire switches
“steal” their operating
power from the load circuit,
there is a voltage drop across
the switch when it is on
(about 7–9 volts in AC
powered devices).
If more than one two-wire
switch is wired in series with
the load, there is a cumulative
voltage drop across the
switch. When more than one
two-wire switch is connected
in parallel with a load, there is
a cumulative effect on the
leakage current. This
increased off-state leakage
current could cause a PLC to
receive a false ON signal.
Output Types
There are three output types
available—Relay, Triac and
Transistor.
A relay is a mechanical
device that can handle load
currents at higher voltages.
This allows the sensor to
directly interface with
motors, solenoids and other
inductive loads. They can
switch either AC or DC
loads.
Relays are subject to contact
wear and resistance build up,
but contact life depends on
the load current and the
frequency of operations. Due
to contact bounce, they can
produce erratic results with
counters, programmable logic
controllers and other such
devices, unless the input to
those devices are filtered.
Because relays are
mechanical, they can add
10 to 25 milliseconds to a
sensor’s response time.
For the majority of
applications, these limitations
cause no problems, or can
easily be minimized. Relay
outputs are very commonly
used in sensors.
A triac is a solid-state
device designed to control
AC current. A triac switch
turns on in less than a
microsecond when its gate
(control leg) is energized, and
shuts off at the zero
crossing of the AC power
cycle.
Because a triac is a solid-
state device, it is not subject
to the mechanical limitations
of a relay. Switching
response time is limited only
to the time it takes the 60 Hz
AC power to go through one-
half cycle.
Triac Output Circuit
Inductive
or Capacitive
Control Circuit
Opto-Coupler
Triac Driver
SnubberTriac
G
To AC
Load
Circuit