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newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Electrical Controls for HVAC/R Systems › Lesson 5
BLD-HVAC07-03 · Lesson 5 · 13.5 clock hours · lab

Control Transformers and Low-Voltage Thermostat Circuits

Objective. Students will wire a 24 VAC control circuit from a step-down transformer through a thermostat sub-base to a relay coil, verify correct voltage at each point, and explain the function of the R, C, G, Y, and W thermostat terminals.

Control Transformers and Low-Voltage Thermostat Circuits

Why Low Voltage?

Most residential and light-commercial HVAC/R systems use a 24 VAC control circuit to operate thermostats, relay coils, and contactor coils. Running low voltage to a wall thermostat is far safer for occupants than running line voltage, and it allows long wire runs with inexpensive 18 AWG thermostat wire. The control transformer is the device that converts 120 VAC (or 240 VAC) line power down to 24 VAC for the control circuit.

The Control Transformer

A control transformer is a step-down transformer with two windings wound around a common iron core:

Transformers work on electromagnetic induction. Alternating current in the primary creates a changing magnetic field that induces a voltage in the secondary. The ratio of primary turns to secondary turns determines the voltage ratio.

VA Rating – Transformers are rated in volt-amperes (VA), not watts, because control loads are often inductive (coils). A typical residential system uses a 40 VA transformer. To select the correct VA rating, add up the VA draw of every load the secondary must power simultaneously (thermostat electronics, relay coils, contactor coils, damper actuators, etc.) and choose a transformer whose rating equals or exceeds that total. Using an undersized transformer causes the secondary voltage to sag under load, leading to chatter or failure to pull in contactors.

Sizing formula:

> VA required = Σ (volts × amperes) of all simultaneous secondary loads

Because the output is 24 VAC, you can also write: VA = 24 V × total secondary amperes.

Thermostat Terminal Designations

The low-voltage thermostat sub-base is the control hub of the circuit. Each terminal letter has a standard meaning recognized across the industry:

| Terminal | Function |

|----------|----------|

| R | 24 VAC hot (from transformer secondary) |

| C | Common – the return path to the transformer secondary |

| G | Fan relay (indoor blower – fan only) |

| Y | Compressor contactor (cooling) |

| W | Heat relay or gas valve (heating) |

Some sub-bases split R into Rh (heat) and Rc (cool) with a factory jumper that can be removed when a system has two separate transformers.

The R and C terminals form the two legs of the 24 VAC circuit. When the thermostat closes a switch between R and Y, for example, current flows from R through the switch to Y, through the contactor coil, and back to C – completing the circuit and energizing the coil.

Tracing the Circuit

Follow the circuit from source to load:

1. Transformer primary – 120 VAC supply → primary winding.

2. Transformer secondary – 24 VAC output → one lead to terminal R, other lead to terminal C.

3. Thermostat switch (Y terminal closed) → 24 VAC passes through Y to the contactor coil.

4. Other side of contactor coil → returns to C.

5. Coil energized → contactor pulls in → compressor starts.

The G and W terminals work identically; the thermostat simply closes a different internal switch to route 24 VAC through G to the fan relay coil, or through W to the heat relay or gas valve.

Safety Rules for This Lab

Personal Protective Equipment (PPE): Safety glasses are required whenever working with live circuits. Although 24 VAC is low voltage, the 120 VAC primary side carries lethal potential.

Lockout/Tagout (LOTO): Before making or changing any wiring connection, de-energize the training board and apply your personal lockout device per your shop's LOTO procedure. Never work on a live primary side.

Inspect before energizing: Have the instructor visually check all wiring before applying power. A miswired transformer primary can damage equipment or create a shock hazard.

Multimeter range selection: Set the meter to an AC voltage range higher than the expected reading before probing. Confirm meter leads are in the correct jacks (V and COM) before touching probes to terminals.

No unapproved substitutions: Use only the 40 VA, 120/24 VAC transformer specified for this lab. Substituting a higher primary-voltage transformer on a 120 VAC supply can destroy the secondary winding.

The Half-Split Diagnostic Method

When a circuit does not operate, half-splitting finds the fault efficiently:

1. Identify the two ends of the suspect circuit.

2. Measure voltage at the midpoint. If voltage is present past the midpoint, the fault is in the second half; if absent, the fault is in the first half.

3. Repeat, halving the remaining suspect section each time.

4. This method finds the fault in the fewest possible measurements.

Document each measurement in your lab log: the point tested, the reading, and your conclusion. This written record is the professional standard for diagnostic work.

Key terms9

Step-down transformerA transformer whose secondary voltage is lower than its primary voltage, used in HVAC/R to convert 120 VAC line power to 24 VAC for control circuits.
VA ratingVolt-ampere rating; the maximum apparent power a transformer secondary can deliver continuously without overheating. Calculated by summing the VA draw of all simultaneous loads.
R terminalThe 24 VAC hot leg terminal on a thermostat sub-base; connected to one lead of the transformer secondary.
C terminalThe common (return) terminal on a thermostat sub-base; connected to the other lead of the transformer secondary to complete the control circuit.
Y terminalThe compressor or cooling terminal on a thermostat sub-base; closing the Y circuit energizes the contactor coil to start the compressor.
W terminalThe heating terminal on a thermostat sub-base; closing the W circuit energizes the heat relay coil or gas valve.
G terminalThe fan terminal on a thermostat sub-base; closing the G circuit energizes the indoor blower relay for fan-only operation.
Half-split methodA systematic troubleshooting technique in which the technician measures at the midpoint of a suspect circuit, then halves the remaining suspect section repeatedly until the fault is isolated.
Contactor coilThe electromagnetic coil inside a contactor that, when energized by 24 VAC from the control circuit, pulls in the power contacts to connect line voltage to the compressor motor.

Worked examples4

1. A residential air conditioner has the following 24 VAC loads: one contactor coil drawing 0.5 A, one fan relay coil drawing 0.3 A, and a thermostat that draws 0.1 A. What minimum VA-rated transformer is needed?
  1. List each load current: contactor coil = 0.5 A, fan relay = 0.3 A, thermostat = 0.1 A.
  2. Sum the currents: 0.5 + 0.3 + 0.1 = 0.9 A total.
  3. Apply the formula: VA = 24 V × 0.9 A = 21.6 VA.
  4. Select the next standard transformer size that equals or exceeds 21.6 VA. Common sizes are 20 VA, 40 VA, 75 VA. Choose 40 VA.
Answer: A 40 VA transformer is the correct minimum standard size for this application.
2. During the lab, you measure 24.3 VAC at the R terminal and 0.1 VAC at the C terminal (referenced to the equipment ground). Is the transformer secondary output within the acceptable ±1 V tolerance of 24 VAC?
  1. The voltage across the secondary is measured from R to C: 24.3 VAC − 0.1 VAC ≈ 24.3 VAC (the C terminal is the reference, so the R-to-C reading is approximately 24.3 VAC).
  2. Acceptable range: 24 V − 1 V = 23 V minimum; 24 V + 1 V = 25 V maximum.
  3. 24.3 VAC is between 23 V and 25 V.
Answer: Yes, 24.3 VAC is within the ±1 V tolerance and the circuit passes this checkpoint.
3. You are half-splitting a dead Y circuit. The circuit path is: R terminal → wire A → junction point M → wire B → Y terminal → contactor coil → C terminal. You measure 24 VAC at junction point M referenced to C. Where is the fault?
  1. Voltage is present at junction point M, which is the midpoint. This means the first half of the circuit (R to M, including wire A) is good.
  2. No current is reaching the contactor coil, so the fault must be in the second half: wire B from M to Y, the Y terminal connection, or the contactor coil wiring to C.
  3. Measure at the Y terminal next. If 0 VAC at Y, the fault is in wire B or the Y terminal connection. If 24 VAC at Y but coil does not pull in, the fault is the coil or its return to C.
Answer: The fault is in the second half of the circuit — between junction point M and the C return through the contactor coil.
4. A replacement transformer is labeled 40 VA, 120/24 VAC. The technician needs to confirm the turns ratio is correct. What turns ratio (primary:secondary) should the transformer have?
  1. Turns ratio = Primary voltage ÷ Secondary voltage.
  2. Turns ratio = 120 V ÷ 24 V = 5.
  3. The primary winding has 5 times as many turns as the secondary winding.
Answer: The turns ratio is 5:1 (primary to secondary).

The math in this lesson2

Learners total 24 VAC load amps against transformer VA and find the 120:24 turns ratio.
6.RP.3A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates transformer primary/secondary wiring and explains VA rating selection for a typical residential application; students record the formula for transformer sizing in their lab logs. · Students wire a low-voltage control circuit on a training board: transformer primary to 120 VAC, secondary to thermostat sub-base terminals R and C, then Y terminal through a contactor coil to C, verifying 24 VAC at each terminal with a multimeter. · Fault-insertion exercise: instructor opens one wire in the student's completed circuit; student uses a multimeter to half-split the circuit, locates the open, corrects it, and documents the diagnosis in the lab log.
Materials: Control transformers (40 VA, 120/24 VAC), thermostat sub-bases, contactor coils (24 VAC), training wiring boards, terminal strips, 18 AWG thermostat wire, wire strippers, digital multimeters, lab log forms
Check: Circuit operates correctly (relay pulls in when Y terminal is jumpered), voltage readings within ±1 V of nominal at all checkpoints, and fault diagnosis documented with correct half-split logic = pass. Wiring must pass instructor visual inspection before energizing.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

HVAC Excellence – Electrical Systems
1. What is the purpose of the C terminal on a thermostat sub-base?
NATE – Core; HVAC Excellence – Electrical Systems
2. Which thermostat terminal, when closed to R, energizes the indoor blower relay for fan-only operation?
NATE – Core; NCCER HVAC – Electrical
3. A technician measures 0 VAC across a contactor coil even though the thermostat Y switch is closed. Using the half-split method, the technician first measures voltage at the midpoint of the Y circuit and finds 24 VAC. What does this result indicate?
HVAC Excellence – Electrical Systems; NCCER HVAC – Electrical
4. A residential system has 24 VAC control loads totaling 1.2 amperes. What is the minimum VA capacity the transformer secondary must supply?
OSHA / Safety – all credential exams
5. Before changing any wiring connection on an energized training board, the correct safety action is to:
NATE – Core; HVAC Excellence – Electrical Systems
6. A 120/24 VAC step-down transformer has a turns ratio of 5:1. If the primary voltage drops to 108 VAC, what is the approximate secondary output voltage?
NCCER HVAC – Electrical; HVAC Excellence – Electrical Systems
7. When sizing a control transformer, why is the load expressed in volt-amperes (VA) rather than watts (W)?
NATE – Core; NCCER HVAC – Electrical
8. During the lab pass/fail check, the instructor jumpers the Y terminal to R but the contactor coil does not pull in. The technician measures 24 VAC at R-to-C but 0 VAC at Y-to-C with the jumper in place. The MOST likely cause is:
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