Transformer (24 V / 12 V, EI30) · Exterior view
  1. 1EI core (silicon steel) · EI-30
  2. 2Secondary winding (thick wire) · 12 V
  3. 3Bobbin (former)
  4. 4Primary winding (thin wire, many turns) · 24 V
  5. 5Pins (primary / secondary)
EI lamination core, bobbin, separate primary and secondary leads
EI30 core width ≈ 30 mm; overall body and pin spacing depend on the manufacturer
Inductors and transformers

Transformer (24 V / 12 V, EI30)

In simple terms
What is this?

A translator made of two coils side by side. It passes electricity from one side to the other not through a wire but through a magnetic field.

What is it for?

It steps a high voltage down to a safe low voltage and separates the two sides from each other.

Where do you see it?

Inside old, heavy adapters and the doorbell transformer.

How do you spot it?

A square body of stacked iron layers; two terminals on one side for high voltage and two on the other for low voltage, four in total.

Caution:

Do not touch the high-voltage side; never feed the transformer DC like a battery, its winding will burn.

Technical definition

A component in which two separate windings transfer energy through a common magnetic core. AC current in the primary creates a changing magnetic flux; a voltage is induced in the secondary according to the turns ratio. This example is not an autotransformer; its windings are electrically isolated. The Lab uses only low voltage; the 230 V primary is not for a real connection experiment. That is why the part in the Lab is a safe 24 V / 12 V test transformer.

Primary (Lab)24 V RMS AC
Secondary (Lab)12 V RMS AC
Turns ratio (Lab)2:1
Schematic symbol Same in IEC and ANSI

IEC 60617: the drawing style used in Türkiye and Europe.

Pins 4
1Primary p1First end of the primary winding (Lab: 24 V~); do not connect it to real mainsp1
2Primary p2Other end of the primary; isolated from the secondaryp2
3Secondary s1First end of the 12 V~ low-voltage windings1
4Secondary s2Secondary return; no fixed plus/minuss2
Features typical
Primary (Lab)
24 V RMS AC
Secondary (Lab)
12 V RMS AC
Turns ratio (Lab)
2:1
Primary current limit (Lab)
Overload above 150 mA
Winding resistance (Lab)
Primary 4 Ω, secondary 1.5 Ω
Isolation
Separate primary and secondary; test voltage depends on the series
No-load voltage
May be higher than the rated value; depends on regulation
Real example: primary
230 V AC, 50 Hz (mains transformer; not in the Lab)
Real example: power
1.5–2 VA; secondary 12 V RMS, ≈125–167 mA
Real example: ratio
≈19.2:1 nominal; the turns ratio may differ with regulation
Lab limit
Low-voltage model only, no real primary connection
Example part
Lab: EI30, 24 V / 12 V test transformer; real example: EI30, 230 V / 12 V, 1.5–2 VA isolation transformer
Common mistakes
Applying DCIt does not convert continuous DC voltage; the winding resistance limits the current and excessive heating can occur.
Mistaking 12 V for DCThe secondary gives AC. For DC you need a rectifier and a filter; the rectified no-load voltage can be higher than the rated value.
Experimenting with the primary230 V is lethally dangerous. Use an enclosed, safe low-voltage adapter; make no mains connection in the Lab.

Test with a multimeter

Test whether this part is good with the multimeter on the measurement screen.

Take a measurement
Where it is used
Isolated low-voltage AC supplySource ahead of the bridge rectifierExamining the turns ratio in the model
Related lessons
Rectifying with a diode
Module 4 · Diode and transistorsoonFiltering the power supply
Module 3 · Capacitor and timesoonMeasuring voltage
Module 2 · Measuring with a multimetersoon
Try it in the Lab