Transformer Turns Ratio Calculator Formula
Understand the math behind the transformer turns ratio calculator. Each variable explained with a worked example.
Formulas Used
Turns Ratio (Np/Ns)
turns_ratio = v_primary / v_secondarySecondary Current
secondary_current = v_primary * primary_current_a / v_secondaryApparent Power
power_va = v_primary * primary_current_aVariables
| Variable | Description | Default |
|---|---|---|
v_primary | Primary Voltage(V) | 120 |
v_secondary | Secondary Voltage(V) | 12 |
primary_current_a | Primary Current(A) | 0.5 |
How It Works
Transformer Turns Ratio
Formulas
Np/Ns = Vp / Vs
Is = Ip x Np/Ns (ideal transformer)
Power = Vp x Ip = Vs x Is
An ideal transformer conserves power. Real transformers have 90-98% efficiency due to copper and core losses.
Worked Example
120 V primary to 12 V secondary, 0.5 A primary current.
v_primary = 120v_secondary = 12primary_current_a = 0.5
- 01Turns ratio = 120 / 12 = 10:1
- 02Secondary current = 0.5 x 10 = 5 A
- 03Power = 120 x 0.5 = 60 VA
Frequently Asked Questions
What is VA rating?
Volt-amperes: the apparent power rating of a transformer. For resistive loads, VA equals watts.
Step-up vs step-down?
Step-down has ratio > 1 (higher primary). Step-up has ratio < 1 (higher secondary). Same transformer works both ways.
Why does current increase when voltage decreases?
Power is conserved. Lower voltage means higher current to deliver the same power.
Ready to run the numbers?
Open Transformer Turns Ratio Calculator