Selection of Toroidal Transformer

Toroidal Transformers offer many advantages over standard laminated power

transformers. Toroidals provide quiet, efficient operation with very low stray

magnetic fields. Their small size and weight support a package that is easy to

design into any application.

 Advantages of Toroidal Transformer:

1. A toroidal transformer weighs about half as much as conventional transformer with the

same capacity and heat rise. The main materials used in transformer are copper and steel,

both of which are getting scarcer with cost increases as a consequence.

2.No impregnation is needed to keep the laminates quiet so this potential environmental

hazard is avoided.

3.The toroidal transformer fits into a smaller space, which allows chassis plates and

enclosures to be smaller with less material used.

How to find VA for Transformers

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 measure of a transformer’s ability to delivery 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)

VA Calculation :

1. Read the transformer’s secondary winding rating (printed or tagged on the transformer

case). This should give a rating such as 5V, 500mA.

Multiply the voltage times the current to receive the VA.

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

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 example of a 2.5VA secondary, if the primary voltage is 120V 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 = 5VA at 500mA

Secondary winding 2 = 24V at 100mA

Total Secondary VA = ( 5 X 0.5 ) + ( 24 X 0.1 ) = 4.9VA

Primary VA will still equal the total secondary VA.