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Technical Notes

Guide To Transformers - Power Rating
Transformers are rated in Volt Amps (VA), which is the product of rms AC voltage and rms AC current for a predominantly resistive load.

Example 1: 
A heating element requires 4 Amps at 
24 volts AC and is to be driven from a 115 or 230v, 60 Hz mains supply.

A 115 + 115v to 24v step down transformer is therefore required, with a VA rating of 4 x 24 = 96 VA (100 VA will therefore be suitable).

Example 2: 
Two 50 watt Halogen Lamps are to be connected in parallel, and require 12v AC to obtain full brightness. A 115v fan is also required to be run, which draws 182 milliamps. This combination is to be driven from 115v 60 Hz supply.

A 115v step down transformer with 2 separate secondary windings is therefore required.

Secondary 1 should be rated at 12v, 100 VA (8.33 Amps)
Secondary 2 should be rated at 115 x 0.182 = 20 VA 
The transformer total rating is 100 + 20 = 120 VA 

A transformer supplying reactive or rectifier loads needs to be rated according to the load characteristic. Our engineers are pleased to offer guidance regarding ratings for any application (see also rectifier transformers).


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