- 1). Calculate the turns ratio. The turns ratio is a ratio of the number of turns in the primary and secondary coil windings. This ratio is also equal to the voltage ratios between the primary and secondary winding. If the primary voltage is 120 volts and the secondary voltage is 12 volts, then the turns ratio is 10 to 1. That means that for every 10 turns of the primary coil, there is 1 turn of the secondary side.
- 2). Determine the secondary wire size. The secondary wire size is a function of the maximum current that needs to flow on that size. For example, if the maximum current is 5 amps, then the wire size has to be 14 gauge or larger. The maximum current that a 14-gauge wire can carry is 5.9 amps.
- 3). Determine the number of windings on each coil. The trick here is that if the turns ratio is 10 to 1, then that ratio must stay the same to keep the output voltage at 12 volts. If we have space for 100 turns of wire on the secondary side, then the primary side must have 10 times that number, or 100 x 10 = 1000 turns.
- 4). Determine the primary coil wire size. The primary wire size is dependant on the maximum current needed on the primary side. Current inversely follows the the turns ratio, since this is a step-down tranformer. If the current on the secondary side is 5 amps, then the current on the primary side will be 1/10 that amount, or 0.5 amps.
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