BASIC ELECTRICAL FOR THE HVAC TECHNICIAN

12. Shop Project – Build a Circuit with a Transformer

Use a 24 VAC control circuit to operate loads in a separate
120 VAC circuit.

In the previous lesson you learned that the primary winding
of a transformer acts as a load on its supplying circuit,
while the secondary becomes the power source for a separate
low-voltage circuit.

In this project you will combine a transformer, relay,
switch and line-voltage loads into one operating system.

Safety First

This project was designed for a supervised electrical
training board.

Do not make or change wiring connections while the
trainer is energized.

The circuit contains both 120 VAC line voltage and
24 VAC control voltage. Keep the two circuits clearly
identified while wiring and troubleshooting.

Have the completed circuit inspected before power is applied.

What You Will Do

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

1

Identify the 120 VAC primary circuit and the separate
24 VAC secondary circuit.

2

Recognize the transformer primary as a load on the
line-voltage circuit.

3

Recognize the transformer secondary as the power
source for the low-voltage circuit.

4

Use a 24 VAC switch circuit to energize a relay coil.

5

Explain how the relay contacts control the
120 VAC loads.

6

Predict voltage readings before checking them with a meter.

The Circuit You Are Building

Figure 24 contains two electrically separate circuits that
interact through the transformer and relay.

The upper portion operates from 120 VAC. The transformer
primary is connected to this circuit and acts as a load.

The transformer secondary supplies the separate 24 VAC
circuit. In that circuit, the secondary is the
power source.

Figure 24 transformer and relay shop project ladder diagram with numbered test points 1 through 13. — **Figure 24.**
Transformer and relay shop-project circuit.

Before You Wire It: Identify the Two Circuits

120 VAC

Line-Voltage Circuit

L and N provide the source for the line-voltage circuit.

The transformer primary is connected across this
source and therefore acts as a
load on the 120 VAC circuit.

The C1 relay contacts also control the line-voltage loads.

24 VAC

Control Circuit

The transformer secondary produces the voltage for
this circuit.

The secondary is therefore the
power source for the 24 VAC circuit.

SW controls the C1 relay coil from this secondary source.

Certification Point

Do not call the transformer secondary a load.

The primary is a load on its supplying circuit.
The secondary is the source for the circuit
connected to it.

Predict the Sequence of Operation

Study Figure 24 before wiring the circuit.

Do this before using the trainer.

Your goal is to understand what should happen before
the meter tells you what happened.

What should happen when the circuit is first plugged in
with SW open?

What should happen when SW is closed?

Build the Circuit

Identify the Transformer

Verify which terminals belong to the
120 VAC primary and which belong to the
24 VAC secondary.

Identify the Relay

Make sure you are using the relay with the
24 VAC coil.

Do not substitute the 120 VAC coil relay used
in the previous shop project.

Wire the 120 VAC Circuit

Wire the line-voltage portion of Figure 24.

Follow the trainer convention of black and white
conductors for the line-voltage circuit.

Wire the 24 VAC Circuit

Wire the transformer secondary, SW and the
24 VAC C1 relay coil.

Use different conductor colors for the low-voltage
control circuit. The original trainer exercise uses
orange and yellow.

Trace Both Circuits

Trace the 120 VAC circuit from L to N.

Then separately trace the 24 VAC circuit from
one side of the transformer secondary, through
SW and C1, and back to the other side of the secondary.

Inspect Before Energizing

Check every connection against Figure 24 and
verify that there are no unintended connections
between L and N.

If you are completing the exercise in a classroom,
have the instructor inspect the circuit before
applying power.

Operate the Circuit

After the circuit has been inspected, apply power with
SW open. Observe the loads.

Then close SW and observe what changes.

Record what actually happens as SW is opened and closed.

MEASUREMENT EXERCISE

Measure the Circuit Voltages

Measure the voltage between each pair of test points shown
below. Take each measurement twice:
once with SW open and again with SW closed.

Predict before measuring.

Before placing the meter leads, decide approximately
what voltage you expect to see and why.

Point 1	Point 2	SW Open	SW Closed	What Does This Measurement Tell You?
1	6
1	2
1	3
1	4
1	5
1	8
1	7
10	9
10	12
10	11
10	13
9	12
9	13
9	11
6	2
9	3

What Are You Proving?

Primary Voltage

The primary should have approximately its rated
line voltage across it.

Secondary Voltage

The secondary should produce approximately its
rated low voltage.

Open Switch

An open switch interrupts the 24 VAC path to the
C1 relay coil.

Closed Switch

Closing SW completes the secondary circuit and
energizes C1.

C1 Contacts

Energizing C1 causes every contact labeled C1 to
change from its normal position.

Two Voltage Levels

The 120 VAC circuit and the 24 VAC circuit remain
electrically separate even though they interact.

Compare Your Prediction with the Actual Circuit

Describe the actual sequence of operation from the moment
the trainer is plugged in through closing SW.

Did the actual operation match your original prediction?
If not, explain what you misunderstood.

Student Notes

Record additional observations, troubleshooting notes,
or questions from the exercise.

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What You Should Have Learned

1

The transformer primary is a load on the
line-voltage circuit.

2

The transformer secondary is the power source
for the low-voltage circuit.

3

The primary and secondary circuits are electrically
isolated from one another.

4

A 24 VAC control circuit can operate a relay that
switches a separate 120 VAC circuit.

5

Relay contacts are shown in their normal position
with the relay coil de-energized.

6

You should be able to predict normal voltage readings
from the schematic before using the meter.

Knowing the correct voltage is not enough.

You should know approximately what voltage to expect
before placing your meter leads.

When troubleshooting equipment, a meter reading becomes
useful only when you can compare it with what a properly
operating circuit should be doing.