Programmable Logic Controller Basics

EET3370 Lab 1

Name

SAFETY:

The industrial trainers are equipped with ground fault interrupters which are designed to detect small currents to ground and interrupt the circuit. These do not eliminate the possibility of electrical shock. While performing control wiring on the trainers, the circuit breaker should be turned OFF and only turned ON when you are ready to test the wiring. The back of the panel has exposed wiring and 120 volt ac potentials are present when the trainers are plugged in.

For wiring purposes the 120 volt ac line and the neutral are brought out to two busses on the lower portion of the trainer panel. Other busses labeled B1 thru B4 can be used as tie points.

At the end of the lab period remove power to the trainers by disconnecting the trainer from the supply receptacle. When working in groups, make sure that all members know when the trainers are energized.

RELAY LOGIC GATES:

1. With the power turned off and using an ohmmeter, determine the contacts and operation of the push buttons and selector switches on the industrial trainer.

2. The basic building block of a relay ladder diagram or relay logic is the relay. It consists of a coil which when energized causes a group of contacts to change states. For example, if a contact was opened it would close or if a contact was closed it would open. It is very important to realize that relay ladder diagrams are shown in the de-energized state. In other words the contacts are shown as if there was no power applied to the circuit. Contacts are called "normally opened" or "normally closed" depending on their state when the relay is de-energized.

We will examine the operation of relays by wiring the circuit per the ladder diagram attached. Do not put the dotted contact connections in at this time.

The contact on line 4 is a normally opened contact and the contact on line 5 is a normally closed contact. Fill in the table below by energizing and de-energizing the coil of CR1 and observing the lights in series with the contacts.

Line 4 Line 5

Relay CR1 N.O. Contact CR1 N.C. Contact CR1

De-Energized LIGHT IS _______ LIGHT IS _______

Energized LIGHT IS _______ LIGHT IS _______

Let’s next look at some relay logic:

A. The contacts on line 8 and 9 form an OR gate in junction with the green light
on line 8.

B. The contacts on line 10 form an AND gate in junction with the red light on
line 10.

C. The contacts on line 11 and 12 form an OR gate in which one input must be
de-energized.

D. The contacts on line 13 form an AND gate in which one input must be
de-energized.

E. The contacts on line 14 and 15 form an exclusive OR gate.

Using the attached form, make a truth table for each of the five gates.

NOTE: De-Energized (‘off’) is shown as 0. Energized (‘on’) is shown as 1.

3. Now make the dotted contact connections in the circuit. Don’t forget to turn off the circuit breaker while making the control wiring change. Observe what happens when you energize relay CR1.

What happened differently from before?

The added contact is commonly referred to as a "Seal in contact." Why is this so?

When would a seal in contact be desirable?

Answer questions and fill in lab as directed. When you have done this, disconnect your circuit and put the trainer away. Turn in your lab before you leave.