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). |