OHMLENS // EDA

Passives

Transformer

Two coils wound on one iron core. AC in the first coil makes a changing magnetic field that makes a voltage in the second. The ratio of turns sets the ratio of voltages, and the two sides are electrically isolated.

Think of it as: Two gears of different sizes: speed (voltage) goes one way, force (current) the other.

Pins

Primary
: The input winding, for example connected to mains.
Secondary
: The output winding. It shares no wire with the primary.

Mains transformers are heavy blocks of laminated iron with copper windings. Switch-mode power supplies use tiny ferrite transformers running at high frequency instead.

What to look for inside

  • AC on the primary (10 turns) slowly swings positive and negative.
  • That current makes a magnetic flux (purple) that runs round the iron core and keeps reversing.
  • Every turn of the secondary (4 turns) picks up the same changing flux, so it gets 4/10 of the voltage.
  • With steady DC the flux stops changing and the secondary gives nothing. That is why transformers only work with AC.

The numbers that matter

SpecTypical
Turns / voltage ratio
Sets the output voltage.
e.g. 230 V : 12 V
VA rating
Volts × amps it can deliver without overheating.
1 VA to 1000 VA
Frequency
A mains transformer at the wrong frequency overheats; higher frequency allows a much smaller core.
50/60 Hz (mains) · 20 to 500 kHz (switch-mode)
Isolation voltage
How much voltage the gap between windings can withstand: a safety rating.
1.5 kV to 4 kV

Where you’ll use it

  • Old-style power adapters (mains → low-voltage AC, then rectified).
  • Inside every switch-mode charger, for isolation and voltage change.
  • Audio and signal isolation.

Common mistakes

  • Connecting DC to a transformer. The coil is just a low resistance and burns.
  • Working on the mains side without knowing what you are doing. The primary is at lethal voltage.

Go deeper

U2U1=N2N1,I2I1=N1N2\tfrac{U_2}{U_1} = \tfrac{N_2}{N_1},\quad \tfrac{I_2}{I_1} = \tfrac{N_1}{N_2}

Voltage goes with the turns ratio, current the opposite way, so power stays (nearly) the same.

U=N dΦdtU = N\,\tfrac{d\Phi}{dt}

Faraday’s law: only a changing flux makes a voltage.

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