Toroidal type 220vac 24vac transformer

The Transformer is a electrical device which is designed to change one AC Voltage to another by magnetic induction. It "Steps-up"or "Steps-down" voltage in order to match incoming supply voltage from the utility to the voltage required by the user's end products. Common international voltages include 110V, 220V, 380V, and 415V. Typical USA supply voltages are 120V, 208V, 240V, 277V, 480V (and 575V in Canada). 

Volt amps (VA) is the effective power rating for a transformer. In the perfect transformer, the voltage and current (combined) applied at the primary will exactly equal the voltage and current (combined) on the secondary(s). Efficiency is a measure of a transformer's ability to deliver the input power to the load. Efficiency is expressed as a percent by:
% = ( PO / PI ) x 100
where; PO = Output power, PI = Input power, % = Efficiency

1. Read the transformer's secondary winding rating (printed or tagged on the transformer case). This should give a rating such as 5 V , 500 mA. Multiply the voltage times the current to receive the VA. 

Example: 5 V times 500 mA (or 0.5 A) equals 2.5 VA

2. Convert the secondary winding VA value to the primary winding VA rating as the equation VA (primary) equals VA (secondary) or VA (p) = VA (s). For the example of a 2.5 VA secondary, if the primary voltage is 120 V the current will be A (p) = VA (s)/V(p). Giving A(p) = 2.5 / 120 = 0.020833 (roughly 21mA). This is the calculation for a single input winding/single output winding transformer.

3. Add the separate VA ratings for a transformer with multiple secondary windings.
For secondary winding 1 = 5 V at 500 mA
Secondary winding 2 = 24 V at 100 mA
Total secondary VA = ( 5 x 0.5 ) + ( 24 x 0.1) = 4.9 VA
Primary VA will still equal the total secondary VA.

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