# Can you calulate a transformer secondary resistance by using load regulation %

1. Nov 29, 2012

### mp3streetpart

If I had a step down transformer of 300 VA in size at 45-0-45 Volts@3.3 Amps with a regulation of 10% could I do the following to calculate the internal resistance of the secondary winding for my power supply circuit?

Voltage * Regulation 10% to work out the voltage difference between noload and full load.

45 Volts* .1 = 4.5 volts

Secondary series resistance per winding
4.5 Volts/3.3 amps = 1.36 ohms

I know there is inductance as well but at the moment I am ignoring it.
I need to put values into circuit maker student edition to reflect the real world.

2. Nov 29, 2012

### The Electrician

You need to take the primary resistance into account. It is reflected by the square of the turns ratio to the secondary, and is a quite significant contributor to regulation.

If a transformer is well designed, typically the reflected value of the primary resistance on the secondary side will be the same as the secondary intrinsic resistance (equal copper losses in both windings being the goal). Often this is close to the truth, but not always. However, in the absence of any possibility of measuring both windings' DC resistance, the best you can do is to take them to be equal.

Since both the secondary resistance AND the reflected primary resistance will contribute equally to the regulation (if the copper losses both windings are equal), you should take the secondary resistance to be half of your calculated value.

3. Nov 29, 2012

### mp3streetpart

Ok I forgot about the primary resistance thanks for that.