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When solid-state logic was first introduced, the comparison of solid-state
logic devices and the traditional relay logic became a fierce competition
because it was thought that the newer solid-state logic would replace all
relay logic. It's important to understand that both the relay and the solid-state
logic devices have unique characteristics that may lead a designer to choose
one over the other. Even though the relay is much older and may seem out
of date, you will still find a wide variety of relays in industry today.
They are no longer used to produce large-scale logic circuits. Instead
two or three relays will be used in machine control circuits to provide
limited logic functions or to provide a means of connecting smaller control
voltages to motor starters that control 50-hp motors.
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It may be better to relate the comparison of relays to solid-state logic
circuits with the comparison of an automobile to a pickup truck. Both
the automobile and pickup truck have unique features that make them both
good at their particular jobs. Problems arise when you have decided to
use one instead of the other and then the conditions change. For instance,
if you need to haul lumber on Saturday morning, the pickup truck is ideal.
Problems start to arise as conditions change slightly because you have
to take your two children and large dog with you. The problem becomes more
complex when it starts to rain, and the large dog riding in the back
wants to be inside the truck with you and the two kids where it's dry. What
started out as a good choice of using the pickup truck to make a simple
trip now looks like a poor choice because you have to deal with a large
dog and two children inside a small truck cab. This is much the same problem
that arises when you have selected solid-state logic or relay logic to control
a large industrial circuit. By the time you are asked to troubleshoot the
circuit, many years have passed and the original conditions of the machine
have changed. It's best to understand the strengths and limitations of both
the solid-state logic devices and the relays so that notice potential problems
as the control system evolves.
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