Industrial Electricity

Name ______

Lab 9: Wiring a Relay` DUE Friday, 3/09/18

Relays

A relay is a device that controls one electrical circuit by opening and closing contacts in another (different) electrical circuit. A small voltage applied to the “input” of a relay results in a large voltage being switched at its “output.” Basically, a relay is an electrically controlled switch.

There are two sections to a relay, the coil and the contacts. Energizing the coil in the control circuit generates an electromagnetic force field which can be used, as in the case of a solenoid, to move a plunger. In the case of a relay, the electromagnetic force causes contacts in the power circuit that are normally open (NO) contacts to close and/or normally closed (NC) contacts to open. The opening and closing of the contacts can be used to energize or de-energize load.

There are two basics types of relays: Electromechanical relays (EMR) and SolidState relays (SSR). Each type has advantages and disadvantages. EMRs often have several sets of contacts for opening and closing numerous circuits simultaneously. SSRs generally have only one set of contacts but they generally outlast EMRs because they contain no moving parts.

Equipment/Parts

Power Supply

12 volt battery pack

Protoboard

Pushbutton Switch

LED

Resistor

Relay

Relay Socket

Connecting Wires

Lamp

DMM

The schematic diagram below is of the relayyou will be using in this lab. Show how you would wire the relay to meet the following specifications:

NOTE:Please use the correct schematic symbolsand label all devices and components in your circuit.

Please have your work verified before continuing.

Have your lab instructor supply you with a relay and relay socket. Determine the proper numbering scheme for the socket’s screw terminals based on the schematic diagram on the previous page. Write the correct number for each screw terminal.

Measure the resistance of the relay coil. RCOIL = _____.

Measure the resistance of the incandescent lamp. RLAMP = _____.

Connect the 12 volt battery across the coil and make sure you can hear the relay contacts open and close. If not, you’ve either got a dead battery or the wrong terminals. Ask if you have questions.

Wirejust the control circuit using the schematic diagram that you developed on the previous page and the two correct screw terminals on the relay socket.

If you have this correct, pressing the pushbutton should cause the LED to light and the relay contacts to “click.”

Please have your work verified before continuing.

Temporarily disconnect the 12 volt battery from the circuit.

Connect up the power circuit using the schematic diagram that you developed on the previous page and the two correct screw terminals on the relay socket.

Adjust the power supply to 24 volts DC and then connect the power supply leads to the power circuit. Reconnect the 12 volt battery to the control circuit.

Prove the circuit’s operation.

Temporarily disconnect both power sources from your circuit. Read the following carefully:

In your original schematic you used a “pair” of pins to light the 24 volt lamp. You used either screw terminals 4 and 6 or screw terminals 3 and 5. Identify the “pair” you did not use. Unused pair: ______

Connect wires from each of these unused terminals across (in parallel with) the pushbutton.

Reconnect both power sources to the circuit. You should see a definite difference in the operation of the circuit. If not, ask! Comment on how the circuit functions now.

Remove the extra jumper wires from the previous step so the circuit functions in its original form.

With the pushbutton pressed (circuit energized), measured the following voltages.

LED / Resistor / Relay Coil / Lamp / Thermal Fuse

Trip the thermal fuse. Measure the voltage across the thermal fuse (circuit energized). VFUSE = _____. Compare this with the voltage across the fuse when it is not tripped. Please explain these readings.

Use the voltage measured across the resistor to calculate the amount of current through it. How does this compare to the theoretical current you used to calculate the resistor value in the first place. What are some reasons why the two values may not be exactly the same?

Calculate the amount of current flowing through the relay coil. Show your steps clearly and completely.

Calculate the amount of current flowing through the lamp. Show your steps clearly and completely.

Use the DMM to measure the voltage across the relay contacts (either 3 and 5 or 4 and 6, whichever you used), when:

a) The control circuit is de-energized. V = _____.

b) The control circuit is energized. V = _____.

Please explain these readings.