PhysicsUK Interactive electricity laboratory

Change · predict · trace · explain

Transformer Lab

Build a step-up or step-down transformer, watch both coil voltages live, then switch to DC and catch the transient that explains why transformers need alternating current.

AQA GCSE Physics · 4.7.3.4 OCR GCSE · transformer principles Higher tier · turns ratio and power

Live apparatus

Primary → core → secondary

Step-down transformer
POWER SUPPLY
12.0 VAC rms · 50 Hz
PRIMARY AMMETERAC RMS
0.139 Aη 90% · input current
Primary scope5 V/div
Vp12.0 V rms5 ms/div
Adjustable laminated-core transformer A primary copper winding and secondary copper winding surround opposite sides of a laminated iron core. Animated arrows show changing magnetic flux. PRIMARY SECONDARY LAMINATED IRON CORE
Np600
Ns300
Alternating flux links both coils
VARIABLE LOADConnected
24 Ω1.50 W delivered
Secondary scope2 V/div
Vs6.00 V rms5 ms/div

The primary ammeter shows the extra input current needed when efficiency falls. The output voltage remains fixed by the turns ratio in this model.

The primary oscilloscope shows a sinusoidal alternating voltage with a 12 volt rms value at 50 hertz.

The secondary oscilloscope shows a sinusoidal alternating voltage with a 6 volt rms value at 50 hertz.

Turns ratio Ns/Np0.500
Primary voltage Vp12.0 V rms
Secondary voltage Vs6.00 V rms
Primary current Ip0.139 A
Secondary current Is0.250 A
Input power1.67 W
Useful output power1.50 W
Power loss0.167 W

Live model

What sets the output?

Vs/Vp = Ns/Np6.00 / 12.0 = 300 / 600

The secondary has half as many turns, so its rms voltage is halved. At 90% efficiency, 10% of the input power is transferred to the surroundings.

Predict · test · explain

Transformer challenges

1 of 4

Challenge 01 · Turns and direction

With the current windings, what type of transformer have you built?

Predict first, then check against the apparatus.

Choose a prediction before checking.

Model limits and oscilloscope convention

The AC voltage ratio is ideal. Efficiency is applied as a black-box power loss once a load is connected. A real transformer's winding resistance and leakage can cause some terminal-voltage drop under load, but that regulation effect, magnetising current and detailed equivalent-circuit losses are outside this model. The DC pulse is an illustrative switching transient, not a quantitative L/R prediction.