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Lesson plan

Transformers in practice lesson plan

The iron box between the wall and your project, measured: turns ratio, VA ratings, center taps, regulation under load, isolation, and the 24 V control transformer in every furnace.

7 chapters, 7 lessons, about 5 hours, no board. Every lesson runs in the browser, with nothing to install.

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The unit

Objectives and materials

By the end, students can

  • Volts down, amps up, and why DC gets nothing through.
  • Half load, full load and overload on a 12 VA transformer.
  • Two halves and two diodes make full-wave DC.
  • Why a 12 V transformer reads 13 V, and what a brownout adds.
  • A floating secondary, and what it does not protect against.
  • R, C, W and G, the VA budget, Class 2 and the fuse.
  • A 24 VAC control supply with a regulated 12 V rail.

Materials for the whole unit

  • Capacitor
  • Diode x 2
  • Filament lamp
  • Mains transformer, 230 V or 120 V to 12 V
  • Multimeter x 3
  • Resistor x 2
  • Signal generator
  • Voltage regulator

Every part is already on the simulator’s bench, so there is nothing to buy or hand out. The counts are the most any one lesson uses, if you build it on real breadboards too.

Chapter 1. About 40 min

Turns ratio

Volts down, amps up, and why DC gets nothing through.

LessonTimeWhat students do
Turns ratio: volts down, amps upTwo coils on one iron core: the turns ratio sets the volts, the amps trade the other way, and DC gets nothing through at all.40 min
  1. Run it and read the secondary
  2. The primary barely sips
  3. Predict: half the mains
  4. Volts down means amps up
  5. Predict: a 24 V transformer
  6. DC gets nothing through

Materials

  • Mains transformer, 230 V or 120 V to 12 V
  • Multimeter x 2
  • Resistor
  • Signal generator

Check for understanding

Each student finishes "Turns ratio: volts down, amps up" on their own, without opening the hints. The simulator checks 6 of its 6 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "DC gets nothing through".

Chapter 2. About 40 min

VA rating

Half load, full load and overload on a 12 VA transformer.

LessonTimeWhat students do
VA rating: how much it can giveVolts times amps is the number on the label. Load a 12 VA transformer to half, to full and past it, and watch the volts sag and the copper heat.40 min
  1. Half load
  2. Read the VA
  3. Full load
  4. Predict: overload it
  5. Sag and heat
  6. Design with a margin

Materials

  • Mains transformer, 230 V or 120 V to 12 V
  • Multimeter
  • Resistor
  • Signal generator

Check for understanding

Each student finishes "VA rating: how much it can give" on their own, without opening the hints. The simulator checks 6 of its 6 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Design with a margin".

Chapter 3. About 50 min

Center taps

Two halves and two diodes make full-wave DC.

LessonTimeWhat students do
Center taps and full-wave DCA tap in the middle of the secondary gives two equal halves: with two diodes, a full-wave rectifier with half the ripple of one.50 min
  1. Two halves
  2. Half-wave DC
  3. Measure the ripple
  4. Wire the other half
  5. Full-wave DC
  6. Predict: the ripple
  7. End to end

Materials

  • Capacitor
  • Diode x 2
  • Mains transformer, 230 V or 120 V to 12 V
  • Multimeter x 3
  • Resistor
  • Signal generator

Check for understanding

Each student finishes "Center taps and full-wave DC" on their own, without opening the hints. The simulator checks 7 of its 7 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "End to end".

Chapter 4. About 40 min

Regulation under load

Why a 12 V transformer reads 13 V, and what a brownout adds.

LessonTimeWhat students do
Regulation under loadWhy a 12 V transformer reads 13 V on the bench: no-load versus full-load voltage, regulation as a percentage, and what a brownout adds.40 min
  1. Light load
  2. Too bright
  3. Full load
  4. The lamp settles
  5. Predict: half load
  6. Brownout

Materials

  • Filament lamp
  • Mains transformer, 230 V or 120 V to 12 V
  • Multimeter
  • Resistor
  • Signal generator

Check for understanding

Each student finishes "Regulation under load" on their own, without opening the hints. The simulator checks 6 of its 6 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Brownout".

Chapter 5. About 40 min

Isolation

A floating secondary, and what it does not protect against.

LessonTimeWhat students do
Isolation: the secondary floatsNo wire joins the windings, so a floating secondary has no path to earth. What that protects you from, and what it never does.40 min
  1. A secondary with no reference
  2. Touch one side
  3. Predict: ground the secondary
  4. Now S1 has a voltage to earth
  5. Predict: a 24 V secondary
  6. What isolation does not do

Materials

  • Mains transformer, 230 V or 120 V to 12 V
  • Multimeter x 2
  • Resistor x 2
  • Signal generator

Check for understanding

Each student finishes "Isolation: the secondary floats" on their own, without opening the hints. The simulator checks 6 of its 6 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "What isolation does not do".

Chapter 6. About 40 min

24 V control transformers

R, C, W and G, the VA budget, Class 2 and the fuse.

LessonTimeWhat students do
24 V control transformersThe transformer in every furnace and air handler: R and C, the thermostat's W and G, a VA budget, and the fuse on the secondary.40 min
  1. R and C
  2. Call for heat
  3. Fan on too
  4. Predict: blow the VA budget
  5. The sag on the R wire
  6. Class 2 and the fuse

Materials

  • Mains transformer, 230 V or 120 V to 12 V
  • Multimeter x 2
  • Resistor x 2
  • Signal generator

Check for understanding

Each student finishes "24 V control transformers" on their own, without opening the hints. The simulator checks 6 of its 6 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Class 2 and the fuse".

Chapter 7. About 50 min

Boss project

A 24 VAC control supply with a regulated 12 V rail.

LessonTimeWhat students do
Boss: a 24 VAC control supplyOne 24 V transformer runs a thermostat loop and a regulated 12 V DC rail: center tap, full-wave, VA budget and regulation in one build you then change.50 min
  1. Run the supply
  2. The 12 V rail
  3. The display lights
  4. Call for heat
  5. Your change: a two-stage valve
  6. Still in spec
  7. Write up your supply

Materials

  • Capacitor
  • Diode x 2
  • Filament lamp
  • Mains transformer, 230 V or 120 V to 12 V
  • Multimeter x 2
  • Resistor
  • Signal generator
  • Voltage regulator

Check for understanding

Each student finishes "Boss: a 24 VAC control supply" on their own, without opening the hints. The simulator checks 7 of its 7 steps as they go.

Then ask each one to explain, in a sentence or two, the last thing it asked of them: "Write up your supply".

Your class

Teach this unit as a class

Start a class from this course and its 7 lessons are assigned in one go, in order. Students join with a six-character code or printed login cards, and each one’s work comes back to you.

The class tools are free for 30 days for one class of up to 20 students, with no card. The lessons themselves are free either way.

Start a class from this courseSee the demo class
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