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).
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Things you need
- Digital Multimeter (DMM)
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. Ex: 5 V times 500 ma (or 0.5 A) equals 2.5 VA
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. Verify using DMM to read input voltage and current versus voltage at a given output current.
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.
Tips and warnings
- Although the actual VA for the primary will be slightly greater than the actual VA of the secondary, due to power loss in heat, coupling losses and such, the values will be close.
- VA is calculated at the maximum safe rating for the secondary windings to allow for worst case conditions when planning heat dissipation and overall power requirements.
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