PHY079 · Energy & Renewable · Classes 10–12
Transformer ratio
Objective: To verify the transformer turns-ratio equation using a 12 V AC lab supply.
🧰 Apparatus
- Demountable transformer: laminated iron core and coils of 50–1000 turns
- 12 V AC lab supply
- Two AC voltmeters
- Connecting leads
📐 Formulas and SI units
V_s / V_p = N_s / N_p
ideal: V_p I_p = V_s I_s
💡 Theory
An alternating current in the primary coil makes a changing magnetic flux in the iron core, which induces an EMF in the secondary coil. The voltages are in the ratio of the turns: V_s/V_p = N_s/N_p. More secondary turns step the voltage up; fewer step it down. An ideal transformer conserves power: V_pI_p = V_sI_s.
🪜 Procedure
- Assemble the core with a 200-turn primary and a 50-turn secondary.
- Connect the primary to the 12 V AC lab supply.
- Measure V_p and V_s.
- Change the secondary coil and repeat.
- Check V_s/V_p = N_s/N_p.
📋 Observations Simulated values
✅ Result and conclusion
12 V on 200 turns gives 3 V on a 50-turn secondary (step-down).
🔬 In a real lab
Flux leakage and core losses make V_s a few per cent lower than ideal; transformers do not work on DC.
⚠️ Safety
Use only the 12 V AC lab supply, never mains. Step-up coils can give higher voltages: do not touch the terminals while on.
❓ Quick check
1. N_s > N_p makes a…
2. Why do transformers need AC?
3. Why is the core laminated?
🏋️ Practice
EasyV_p = 12 V, N_p = 100, N_s = 400. V_s?
Answer: 48 V
MediumTurns ratio to step 230 V down to 12 V?
Answer: ≈ 19 : 1
ChallengeWhy is power sent at high voltage?
Answer: Lower current, less I²R loss