BASIC ELECTRICAL FOR THE HVAC TECHNICIAN

Build a Series Circuit

Shop Project — Build, measure and explain a two-load series circuit.

In the previous lesson you learned how series and parallel
circuits behave. In this shop project you will build a
series circuit with two lamp loads, then use your meter
to verify the rules you just learned.

Safety First

This exercise was originally performed on a supervised
electrical training board.

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

Keep the circuit de-energized while wiring it.
Resistance measurements must also be made with the
circuit completely de-energized.

When measuring current, the meter must be inserted
in series with the circuit. Never place
an ammeter directly across the source.

What You Will Do

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

1

Build a circuit containing two lamp loads in series.

2

Verify that the same current flows through the entire series path.

3

Measure the voltage drop across each lamp.

4

Verify that the voltage drops add up approximately
to the source voltage.

5

Explain what happens if any part of the series path opens.

6

Compare your actual meter readings with what you predicted.

The Circuit You Are Building

Figure 17 shows the training-board circuit for this project.
The service switch, circuit switch, and both lamp loads are
all in the same current path.

Series training-board circuit showing L1 feeding service switch SS, circuit switch SW1, lamp B1 and lamp B2 in series before returning to neutral N. — **Figure 17.**
Series training-board circuit with SS, SW1, B1 and B2
all connected in one current path.

Remember the Series Circuit Rules

One Path

Current has only one route through the circuit.

Same Current

The same current flows through every component
in the series path.

Voltage Divides

The source voltage is divided among the series loads.

Drops Add Up

The individual voltage drops add to approximately
the source voltage.

One Open Stops Everything

If any component opens, current stops through
the entire series path.

Instructional build-series-circuit diagram showing a source, switch, two lamps in series, measurement points, expected behaviors and worksheet guidance. — Build and measurement reference for the two-lamp
series-circuit exercise.

Build the Circuit

Keep the source disconnected while making the wiring connections.

Connect the Source to SS

Connect the line side of the training-board source
to the service switch SS.

Connect SS to SW1

Connect the output of SS to the circuit switch
SW1.

Connect SW1 to B1

Connect the output of SW1 to one side of lamp
B1.

Connect B1 to B2

Connect the other side of B1 directly to one side
of lamp B2.

Complete the Return Path

Connect the remaining side of B2 back to the
neutral or return side of the source.

Inspect the Circuit

Trace the complete path:

L1 → SS → SW1 → B1 → B2 → N

Verify that all connections match Figure 17
before applying power.

Predict Before You Measure

Before energizing the circuit, think about what you expect
your meter to show.

Will the current through B1 be the same as the current
through B2?

Will each lamp have the full source voltage across it?

What should happen to the circuit if either lamp opens?

PART 1

Voltage Measurements

With the circuit energized and both switches closed,
measure the source voltage and the voltage drop across
each lamp.

Measurement	Where to Measure	Expected Result	Your Reading
Source Voltage	Across the source	Full source voltage	V
Voltage Across B1	Across lamp B1	Portion of source voltage	V
Voltage Across B2	Across lamp B2	Portion of source voltage	V
Combined Voltage Drop	Add B1 voltage + B2 voltage	Approximately equal to source voltage	V

PART 2

Current Measurements

Current measurements are made differently from voltage measurements.

The ammeter must be inserted in series with the circuit.
Do not place the meter in current mode directly across
the source or a load.

Measure current at several locations in the same series path.

Measurement	Meter Location	Expected Result	Your Reading
Current Before B1	Insert meter in series before B1	Series current	A
Current Between B1 and B2	Insert meter in series between B1 and B2	Same series current	A
Current After B2	Insert meter in series after B2	Same series current	A

PART 3

Resistance Measurements

Power must be OFF.

Disconnect the source before making resistance measurements.

Measurement	Where to Measure	Expected Result	Your Reading
B1 Resistance	Across B1 with power OFF	Depends on lamp type and temperature	Ω
B2 Resistance	Across B2 with power OFF	Depends on lamp type and temperature	Ω
Total Series Resistance	Across B1 + B2 path with both included	Approximately R_B1 + R_B2	Ω

Observations and Conclusions

Is the current approximately the same at every point
in the series circuit? Explain.

How do the voltage drops across B1 and B2 compare
with the source voltage?

What happens to B2 if B1 opens or is removed?
Why?

What happens to B1 if B2 opens or is removed?
Why?

What did this exercise teach you about series circuits?

About the Worksheet Fields

These entry fields are currently provided only as an
interactive worksheet.

Nothing is saved or submitted to the website yet.
They are structured as form fields so that saving,
checking, printing, instructor review, or other
functionality can be added later without rebuilding
the lesson.

What You Should Have Learned

1

A series circuit has only one current path.

2

The same current flows through B1 and B2.

3

Source voltage is divided between the series loads.

4

The voltage drops across B1 and B2 add to approximately
the source voltage.

5

Series resistances add together.

6

If one component opens, current stops through
the entire series path.

Do not stop at “both lamps came on.”

The important part of the exercise is understanding why
the current is the same through both lamps and why the
source voltage is divided between them.

When the measurements make sense to you before you take
them, you are beginning to troubleshoot instead of guess.