BASIC ELECTRICAL FOR THE HVAC TECHNICIAN

Trace the Circuit You Just Built

Shop Project — Use your meter to prove what the circuit is doing.

In the previous lessons you built a simple circuit,
learned Ohm’s Law, and learned how to use a meter.
Now you are going to put those skills together.

Safety First

This was originally a supervised shop exercise using
the electrical training board.

Do not reproduce this exercise on household line voltage
as an unsupervised experiment.

Voltage measurements require an energized circuit.
Resistance measurements require the circuit to be
completely de-energized.

If you are following this material independently,
use an appropriate low-voltage training system unless
you have suitable electrical training, equipment and
safety procedures.

What You Will Do

By the end of this exercise you should be able to:

1

Measure voltage between specific points in a circuit.

2

Measure resistance with the circuit de-energized.

3

Compare readings with what you expect the circuit to do.

4

Explain why the readings change when SW1 opens or closes.

The Circuit You Are Testing

This is the same basic circuit you built in the earlier
shop project.

Source
→

Path
→

SW1
→

B1
→

Return to Source

Training board circuit showing service switch SS, circuit switch SW1, light bulb B1, line conductor L1, neutral N, and marked test points. — **Figure 10.**
Basic training-board circuit used for the voltage and
resistance measurements in this exercise.

Using Your Meter

Voltage Measurements

Set the meter for the correct AC voltage range.

Voltage is measured between two points while the
circuit is energized.

Resistance Measurements

Turn the power OFF and disconnect the training board
before measuring resistance.

Set the meter to ohms (Ω).

Before every measurement, ask:

What should I expect the meter to read?

Then compare the actual reading with your expectation.

PART 1

Voltage Measurements — SW1 Open

With SW1 open, measure the voltage between each pair
of points and enter your reading in the right-hand column.

Point 1	Point 2	Your Voltage Reading
1	2	Voltage between points 1 and 2 with SW1 open V
1	3	Voltage between points 1 and 3 with SW1 open V
1	4	Voltage between points 1 and 4 with SW1 open V
1	5	Voltage between points 1 and 5 with SW1 open V
5	4	V
5	3	V
5	2	V
5	1	V

PART 2

Voltage Measurements — SW1 Closed

Close SW1 and repeat the same voltage measurements.

Point 1	Point 2	Your Voltage Reading
1	2	V
1	3	V
1	4	V
1	5	V
5	4	V
5	3	V
5	2	V
5	1	V

PART 3

Resistance Measurements — SW1 Open

Power OFF and unplug the training board.

Never measure resistance on an energized circuit.

Open SW1 and measure the resistance between the following points.

Point 1	Point 2	Your Resistance Reading
1	2	Ω
1	3	Ω
1	4	Ω
1	5	Ω

PART 4

Resistance Measurements — SW1 Closed

Keep the circuit OFF and unplugged. Close SW1 and repeat
the resistance measurements.

Point 1	Point 2	Your Resistance Reading
1	2	Ω
1	3	Ω
1	4	Ω
1	5	Ω

Now explain your readings.

Can you match each reading to the rules you learned
in the earlier lessons?

What changed when SW1 changed position?

Which measurements show an open circuit?

Which measurements show a complete conductive path?

If a reading does not make sense, do not simply write it
down and continue. Check your meter setup, test points,
circuit wiring and what you expected to measure.

Student Notes

Use this space to record anything unusual about your readings
or questions you want to review.

About These Entry Fields

The measurement fields on this page are currently provided
as a worksheet only.

Nothing is submitted or saved to the website yet.
They have been structured as form fields so that future
versions of the course can add saving, checking or
instructor-review functions without rebuilding the lesson.

What You Should Have Learned

1

Voltage is measured between two points in an energized circuit.

2

Resistance is measured only on a de-energized circuit.

3

Opening and closing SW1 changes the electrical condition
of different points in the circuit.

4

Meter readings are useful only when you understand
what you expected to measure.

Original Course Sign-Off

In the original classroom version of this exercise,
the instructor reviewed the student’s measurements,
observed proper meter use, and asked the student to
explain the readings before signing off on the project.

If you are studying independently, you should be able
to explain why each measurement changes—or does not
change—when SW1 is opened or closed.