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Turns Ratio Effect on Voltage
The turns ratio has an effect on the voltage induced across the secondary windings by the electromagnetic field generated by the current passing through the primary windings. The voltage across the secondary windings is equal to the voltage across the primary windings, divided by the turns ratio. For example, a transformer with a turns ratio of 1:2 that has 120 Volts AC applied across the primary windings has a voltage of 240 Volts AC across the secondary windings (120 / 0.5 = 240).
Turns Ratio Effect on Current
The turns ratio has the opposite effect on current as it does on voltage. To solve for the current across a secondary coil, multiply the turns ratio by the input current. For example, a transformer with a turns ratio of 1:2 that has 10 Amps AC applied through its primary coil pushes only 5 Amps AC through its secondary coil. Note however that the voltage is still doubled.
Low Voltage
There are several common ratios for commercially available potential transformers designed to operate at low voltages. Low voltages, as defined by the Institute of Electrical and Electronics Engineers, are any circuit that operates at fewer than 1,000 Volts AC. To put that in perspective, ordinary household current is 110 -- 120 Volts AC. The common turns ratios for potential transformers in this class are 1:1, 2:1, 2.5:1, 4:1 and 5:1.
High Voltage
A high-voltage potential transformer is one that is designed to operate at over 1,000 Volts AC. The common turns ratios for commercially available potential transformers designed to operate at high voltages are 20:1, 35:1, 40:1, 60:1, 70:1, 100:1, 120:1, 200:1 and 300:1.