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Ladder diagrams are used to describe the
logic of electrical control systems. There are differences
in the way ladder logic was implemented in computerized form
as compared to hard wired so I will be talking about the old
way first. The basic component of the control system is the
control relay which is a solenoid that operates a number of
switches or contacts. The contacts come normally open and
normally closed, normal being when the relay is not
energized. Relays come in various breeds like time delay and
latching types. Other components of the control system are
the field devices such as push buttons, limit switches,
lights, and controlled devices like motor starters and
solenoid operated valves. As I said, ladder diagrams show
the logic of the controls but they are not used to build the
system, a wiring diagram is used for that. But the wiring
diagram wouldn�t be used to trouble shoot with or show
functionality, that�s where the ladder is most useful.
When viewing the pictorial version of the controls as in
Figure 1, one can see that the devices on a rung of the
ladder are in series reading horizontally and in parallel
reading vertically. Control voltage is supplied to the
vertical rails, L1 being hot and L2 being common or ground.
In industry it is common to see 120vac control and 480vac
power circuits. Anything less than 600 volts is considered
low voltage and virtually everything will have an insulation
rating of 600 volts. (My robot uses 5vdc control and 28vdc
power circuits.) In Figure 1 we see a normally closed (N.C.)
stop button and a normally open (N.O.) start button and a
motor starter. The circle with the M represents the coil of
a relay, not the actual motor. The M contact is physically
part of the starter and actuates with the coil. The contact
labeled O.L. (over load) is also part of the starter and is
a circuit breaker tripped by over current in the motor legs.
The M contact is called the seal contact. Without it, the
motor would run as long as someone held down the start
button and would stop when released. With it, the power is
allowed to flow through the start button to energize the
coil, which closes the M contact, maintaining the complete
circuit when the start button is released. To stop the
motor, any element in series with the coil can break the
circuit, in this case the stop button. True, this was a long
winded explanation but you now have the critical pieces, how
to turn something on, make it stay on, and how to shut it
off. It�s a truth table in disguise, ANDs and ORs, ONs and
OFFs. |
