Lesson Plan Industry Sector

Lesson Plan Originally Created By: Mike Monahan

Operation and Testing of Relays (updated) by Mike Monahan

Part of Unit: Fundamentals of Electrical and Electronic Systems

Lesson Plan Overview / Details

In this lesson, the student will be introduced to the Bosch style relay. Terminology, circuit identification and a variety of testing for this device will be covered.

Materials Needed

  1. A variety of old relays: some functioning, others not.
  2. 10 foot of 12 gauge wire
  3. Wire cutters
  4. Connectors for relay terminals
  5. Electric tape
  6. A test board or head lights with additional 10 foot of 12 gauge wire
  7. Electrical connectors for light connections and battery connectors

Lesson Time

2 Class Periods
100 Minutes

At the end of this lesson, the student will:

  • Understand how to operate and check relays
  • Diagnose and repair these relays
  • Diagnose, service and repair lighting systems and also repair, diagnose, and service horns, wipers/washers, and other accessories that have relays in their circuits
  • Learn the following terminology: Relay, Terminal, Wire Gauge, Fuse/Breaker, Magnet, Coil and Contact set
  • Identify the parts of the relay and the circuit

Activities in this Lesson

  • Do this demonstration using actual headlights on high beams.

    - Using a test board, show a long wire approximately 15 foot long that runs the head lights.

    - Then, show a relay in the circuit .  More current to the head lights= brighter lights

    - To make this more dramatic, use a longer wire or put on more lights.

    - Less voltage drop = more current going where needed.

    Resources and Materials

    • headlight diagram [ Download ] This diagram without relay and long wire - showing voltage drop (duller lights)
    • headlight-relays.gif [ View Image ] [ Download Original ] A drawing of a head light relay
  • Discuss the methods for testing relays: the power up method and the ohm meter reading accross the coil.

    Show / demonstrate the 9 volt battery trick across the terminals.

    Resources and Materials

    • Headlight upgrades Relays.pdf [ Download ] Explanation- with drawings with and without relays
  • Relay Circuit 1 Lab / Shop

    Diagram of Bosch style relay - Answer the questions at the bottom of the sheet.

    Resources and Materials

  • Relay Circuit 2 Lab / Shop

    Answer questions at the bottom of page.

    Resources and Materials

    • Relays serve several purposes.doc [ Download ] Relay Description
  • Have a quick recap of the testing and function of relays.  Share a global perspective for nuclear and power generation - switching stations and the grid.

  • video on relays Demo / Modeling

    Terminal identification of a relay - How to check relays in the car and on the bench.

    Resources and Materials


Assessment Types:
Demonstrations, Teacher-Made Test,

A box of relays test all of them and select the ones that are good. Show both ways for testing. Bench and live test.