Transformer and direct current

Curricula: GCSE, Sek I/II

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Aim

Check whether a transformer works on direct current, and see that a voltage appears in the secondary winding only when the current in the primary changes.

The bench

A 6 V DC voltage source is connected through a switch and an ammeter to the primary winding of the transformer (2000 turns, resistance 40 Ω). A 3.5 V / 0.5 W lamp and a DC voltmeter are connected to the secondary winding (1400 turns). The switch is open.

Procedure

1. Close the switch. Watch the lamp and the ammeter.
2. Wait a few seconds. Record the readings of the ammeter and the voltmeter.
3. Open the switch. Watch the lamp.
4. Repeat the experiment with a lower simulation speed to see the flashes better.

Expected result

When the switch is closed, the current in the primary winding rises gradually, over a fraction of a second, and settles at about 0.15 A: it is limited by the resistance of the winding. The flux changes slowly, the induced voltage is small, and the lamp barely lights — the filament only just turns red, which is noticeable only in the dark. While the current is constant, the voltmeter on the secondary winding reads 0 and the lamp is off. When the switch is opened, the current breaks off sharply and the lamp flashes brightly — twice as bright as at its rating — and goes out within a quarter of a second: the induced current is about 0.2 A, 1.4 times its rated current. A voltage in the secondary winding is induced only by a changing magnetic flux, so a transformer does not work on direct current.

The current in the primary winding is not limited by accident. The core of a real transformer can saturate on direct current, and a strong current at opening would pass into the secondary circuit in the same proportion as the turns and burn out the lamp.

Questions

1. Why does the lamp flash brighter when the switch is opened than when it is closed?
2. Why does the current in the primary winding not rise immediately after closing?
3. Why are transformers used only in AC circuits?

Answers

1. When the switch is closed, the current and the magnetic flux grow gradually, and a slow change of flux induces only a small voltage. When it is opened, the primary current of 0.15 A stops almost at once, and the energy stored in the magnetic field of the core goes into the secondary circuit: the flux keeps a current of about 0.15 A · 2000 / 1400 ≈ 0.2 A flowing through the lamp, 1.4 times its rated current.
2. The winding has 2000 turns on a steel core and therefore a large inductance. The growing current builds up a growing magnetic flux, and this flux induces in the winding a voltage that opposes the growth of the current (self-induction), so the current approaches 6 V / 40 Ω ≈ 0.15 A gradually.
3. A transformer transfers energy only through a changing magnetic flux; a direct current makes a constant flux, and the secondary winding gives no voltage. On direct current the primary current is also limited only by the resistance of the winding: with the default 500-turn primary (2.5 Ω) the 6 V source would drive about 1.9 A through it, and a real winding would overheat.