Calculate transformer secondary voltages from turns
Use Vp/Vs = Np/Ns with primary and secondary roles fixed and consistent voltage units.
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Multiply and divide ratios.
- Use primary and secondary terms.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
For consistent voltage ratios, should 0.24 kV be treated as 0.24 V?
What you will learn
- Use Vp/Vs = Np/Ns with primary and secondary roles fixed and consistent voltage units.
- Calculate corresponding transformer voltages and check ratio direction and unit consistency.
Keep corresponding values together
For the stated simple transformer voltage-ratio model, Vp/Vs = Np/Ns. Rearranging gives Vs = Vp × Ns/Np. Voltage and turn ratios correspond to the same primary and secondary roles. Turn counts are dimensionless whole counts; use consistent voltage units when comparing voltages.
For supplied primary 120 V, 600 turns and secondary 150 turns, Vs = 120 × 150/600 = 30 V. The lower output turn count gives a lower voltage, which checks the step-down sense. If a calculation increases voltage here, inspect whether the turn ratio was inverted.
Retain the stated approximation
The ratio is the simple linked-winding voltage model used by this syllabus. Real loading, winding resistance, leakage and other departures require additional information. State the assumed simple behaviour rather than treating a number as certified output under every load.
Keep voltage transformation separate from power creation or current calculation. A supplied ratio alone does not establish an actual secondary current. Identify input and output values before substituting, especially when a question changes which winding receives the source.
Pause and explain
Primary 230 V/1150 turns and secondary 60 turns give which secondary voltage?
Worked example
Find secondary voltage for primary 120 V/600 turns and secondary 150 turns in the supplied simple model.
Show the worked solution
- Use Vs = Vp × Ns/Np.
- Substitute 120 × 150/600.
- The result is 30 V, consistent with step-down.
Answer 30 V.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Calculate Vs for Vp = 120 V, Np = 600 and Ns = 150.
Give me a hint
Use the output-to-input turn multiplier.
Compare my reasoning
- Ns/Np = 150/600 = 1/4.
- Multiply 120 V by 1/4.
- The secondary voltage is 30 V.
30 V.
Look for these in your work
- Use corresponding turns.
- Check the smaller output voltage.
Calculate Vs for Vp = 230 V, Np = 1150 and Ns = 60.
Give me a hint
Check that fewer secondary turns reduce voltage.
Compare my reasoning
- Use Vs = 230 × 60/1150.
- The turn ratio is 6/115.
- The result is 12 V.
12 V.
Look for these in your work
- Substitute the correct ratio.
- Retain volts.
A stated suitable arrangement uses primary 30 V/150 turns and secondary 600 turns. Find Vs and classify the change.
Give me a hint
Use the newly assigned source role.
Compare my reasoning
- The output-to-input turn ratio is 600/150 = 4.
- Vs = 30 × 4 = 120 V.
- The greater output voltage gives step-up action.
120 V; step-up.
Look for these in your work
- Identify the new primary.
- Separate voltage change from a current claim.
Common mistakes
- Match p values and s values.
- Check the step-up/down sense.
- Do not infer load current from voltage alone.
What can you explain now?
What rearrangement finds secondary voltage?
Compare with the explanation
Vs = Vp × Ns/Np
Output voltage scales with the secondary-to-primary turn ratio in the supplied simple model.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Calculate corresponding transformer voltages and check ratio direction and unit consistency.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.