Note! Transformer secondary AC voltage is usually reported as the voltage with maximum load. The source resistance is often not given in the datasheet. To find out the unloaded voltage, measure it yourself. To find out the source resistance, load the transformer with a certain load resistance and then measure the loaded secondary voltage. The source resistance can be calculated using the resistor divider calculator where U1 is the unloaded voltage, U2 is the loaded voltage, R2 is the load resistance and Iout=0. Compute R1, which will be the source resistance.