Programmable logic controller(PLC)

The purpose of a PLC was to directly replace and substitute electromechanical relays as logic elements, substituting instead a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks.

A PLC has many "input" terminals, through which it explain"high" and "low" logical states
from sensors and switches. It also has many output terminals, through which it outputs "high"
and "low" signals to power lights, contactors,solenoids, relays, small motors, and other devices lending
themselves to on/off control. In an effort to make PLCs easy to program, their programming
language was designed to resemble ladder logic diagrams. Thus, an industrial electrician or electrical
engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC
to perform the same control functions.

PLCs are industrial computers, and as such their input and output signals are typically 120 volts
AC, just like the electromechanical control relays they were designed to replace. Although some
PLCs have the ability to input and output low-level DC voltage signals of the magnitude used in
logic gate circuits, this is the exception and not the rule.

Signal connection and programming standards vary somewhat between different models of PLC,
but they are similar enough to allow a "generic" introduction to PLC programming here. The
following illustration shows a simple PLC, as it might appear from a front view. Two screw terminals
provide connection to 120 volts AC for powering the PLC's internal circuitry, labeled L1 and L2. Six
screw terminals on the left-hand side provide connection to input devices, each terminal representing
a different input "channel" with its own "X" label. The lower-left screw terminal is a "Common"
connection, which is generally connected to L2 (neutral) of the 120 VAC power source.

Inside the PLC housing, connected between each input terminal and the Common terminal, is an
op-to-isolator device (Light-Emitting Diode) that provides an electrically isolated "high" logic signal
to the computer's circuitry (a photo-transistor interprets the LED's light) when there is 120 VAC
power applied between the respective input terminal and the Common terminal. An indicating LED
on the front panel of the PLC gives visual indication of an "energized" input.

Output signals are generated by the PLC's computer circuitry activating a switching device
(transistor, TRIAC, or even an electromechanical relay), connecting the "Source" terminal to any
of the "Y-" labeled output terminals. The "Source" terminal, correspondingly, is usually connected
to the L1 side of the 120 VAC power source. As with each input, an indicating LED on the front
panel of the PLC gives visual indication of an "energized" output.

The actual logic of the control system is established inside the PLC by means of a computer pro-
gram. This program dictates which output gets energized under which input conditions. Although
the program itself appears to be a ladder logic diagram, with switch and relay symbols, there are
no actual switch contacts or relay coils operating inside the PLC to create the logical relationships
between input and output. These are imaginary contacts and coils, if you will. The program is
entered and viewed via a personal computer connected to the PLC's programming port.

Equally important to understand is that the personal computer used to display and edit the
PLC's program is not necessary for the PLC's continued operation. Once a program has been
loaded to the PLC from the personal computer, the personal computer may be unplugged from
the PLC, and the PLC will continue to follow the programmed commands. I include the personal
computer display in these illustrations for your sake only, in aiding to understand the relationship
between real-life conditions (switch closure and lamp status) and the program's status ("power"
through virtual contacts and virtual coils).


The true power and versatility of a PLC is revealed when we want to alter the behavior of a
control system. Since the PLC is a programmable device, we can alter its behavior by changing the
commands we give it, without having to recon figure the electrical components connected to it. For
example, suppose we wanted to make this switch-and-lamp circuit function in an inverted fashion:
push the button to make the lamp turn off, and release it to make it turn on. The "hardware"
solution would require that a normally-closed pushbutton switch be substituted for the normally open switch currently in place. The "software" solution is much easier: just alter the program so
that contact X1 is normally-closed rather than normally open.

SOLID-STATE RELAYS

As versatile as electromechanical relays can be, they do suffer many limitations. They can be expensive to build, have a limited contact cycle life, take up a lot of room, and switch slowly, compared
to modern semiconductor devices. These limitations are especially true for large power contactor
relays. To address these limitations, many relay manufacturers offer "solid-state" relays, which use
an SCR, TRIAC, or transistor output instead of mechanical contacts to switch the controlled power.
The output device (SCR, TRIAC, or transistor) is optically-coupled to an LED light source inside
the relay. The relay is turned on by energizing this LED, usually with low-voltage DC power. This
optical isolation between input to output rivals the best that electromechanical relays can offer.

Being solid-state devices, there are no moving parts to wear out, and they are able to switch on
and off much faster than any mechanical relay armature can move. There is no sparking between
contacts, and no problems with contact corrosion. However, solid-state relays are still too expensive
to build in very high current ratings, and so electromechanical contactors continue to dominate that
application in industry today.

One significant advantage of a solid-state SCR or TRIAC relay over an electromechanical device
is its natural tendency to open the AC circuit only at a point of zero load current. Because SCR's
and TRIAC's are thyristors, their inherent hysteresis maintains circuit continuity after the LED is
de-energized until the AC current falls below a threshold value (the holding current). In practical
terms what this means is the circuit will never be interrupted in the middle of a sine wave peak.

Such untimely interruptions in a circuit containing substantial inductance would normally produce
large voltage spikes due to the sudden magnetic field collapse around the inductance. This will not
happen in a circuit broken by an SCR or TRIAC. This feature is called zero-crossover switching.

One disadvantage of solid state relays is their tendency to fail "shorted" on their outputs, while
electromechanical relay contacts tend to fail "open." In either case, it is possible for a relay to
fail in the other mode, but these are the most common failures. Because a "fail-open" state is
generally considered safer than a "fail-closed" state, electromechanical relays are still favored over
their solid-state counterparts in many applications.

Protective relays

A special type of relay is one which monitors the current, voltage, frequency, or any other type of
electric power measurement either from a generating source or to a load for the purpose of triggering
a circuit breaker to open in the event of an abnormal condition. These relays are referred to in the
electrical power industry as protective relays.

The circuit breakers which are used to switch large quantities of electric power on and of are
actually electromechanical relays, themselves. Unlike the circuit breakers found in residential and
commercial use which determine when to trip (open) by means of a bimetallic strip inside that bends
when it gets too hot from over current, large industrial circuit breakers must be "told" by an external
device when to open. Such breakers have two electromagnetic coils inside: one to close the breaker
contacts and one to open them. The "trip" coil can be energized by one or more protective relays,
as well as by hand switches, connected to switch 125 Volt DC power. DC power is used because it
allows for a battery bank to supply close/trip power to the breaker control circuits in the event of
a complete (AC) power failure.

Protective relays can monitor large AC currents by means of current transformers (CT's), which
encircle the current-carrying conductors exiting a large circuit breaker, transformer, generator, or
other device. Current transformers step down the monitored current to a secondary (output) range
of 0 to 5 amps AC to power the protective relay. The current relay uses this 0-5 amp signal to power
its internal mechanism, closing a contact to switch 125 Volt DC power to the breaker's trip coil if
the monitored current becomes excessive.


Likewise, (protective) voltage relays can monitor high AC voltages by means of voltage, or
potential, transformers (PT's) which step down the monitored voltage to a secondary range of 0
to 120 Volts AC, typically. Like (protective) current relays, this voltage signal powers the internal
mechanism of the relay, closing a contact to switch 125 Volt DC power to the breaker's trip coil is
the monitored voltage becomes excessive.
There are many types of protective relays, some with highly specialized functions. Not all
monitor voltage or current, either. They all, however, share the common feature of outputting a
contact closure signal which can be used to switch power to a breaker trip coil, close coil, or operator
alarm panel. Most protective relay functions have been categorized into an ANSI standard number
code.