Turns Ratio
10.00:1
Sec. Turns
100
VA
50 VA
Turns Ratio (Np:Ns)
10.00:1
Secondary Turns
100
Primary Current
0.417 A
Secondary Current
4.167 A
Power
50 VA
Inputs
Result
Ratio = 120/12 = 10. Ns = 1000/10 = 100. Is = 50/12 = 4.167A.
Inputs
Result
Ratio = 240/12 = 20. Ns = 1000/20 = 50. Ip = 50/240 = 0.208A.
Turns ratio = Vp/Vs = Np/Ns. A 120V to 12V transformer has a 10:1 ratio. If the primary has 1000 turns, the secondary has 100 turns.
| Primary V | Secondary V | Turns Ratio | Type |
|---|---|---|---|
| 120V | 12V | 10:1 | Step-down |
| 120V | 24V | 5:1 | Step-down |
| 120V | 240V | 1:2 | Step-up |
| 240V | 12V | 20:1 | Step-down |
VA = secondary voltage × max secondary current. A 12V transformer supplying 4A needs at least 48VA. Add 20% margin: use a 60VA transformer.
Transformers handle both resistive and reactive loads. VA (volt-amperes) accounts for apparent power including reactive components. Watts = VA × power factor.
Small transformers (10-50VA): 80-90% efficient. Large power transformers: 95-99%. Losses come from copper resistance (I²R) and core hysteresis/eddy currents.
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Last Updated: Mar 20, 2026
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