The voltage and current ratings for the HT winding can be calculated from the dc voltage and current using the formula below appropriate to your rectifier and smoothing circuit. The power rating for determining the size of the transformer is shown below as Pac (Watts).
The formulae below allowances for the voltage difference due to the wave form factor. The voltage drop across the diodes and the resistance of the choke must be allowed for. We specify the Vac on full load so you do not need to allow for resistance of the transformer windings
CAPACITOR INPUT FILTER FULL WAVE 
FULL WAVE
Vac = Vdc x 1.41
Iac = Idc
Pac = Pdc x 1.41
CAPACITOR INPUT FILTER BRIDGE 
BRIDGE
Vac = Vdc x 0.71
Iac = Idc x 1.61
Pac = Pdc x 1.14
CHOKE INPUT FILTER(Special choke must be used) FULL WAVE 
FULL WAVE
Vac = Vdc x 2.22
Iac = Idc x 0.65
Pac = Pdc x 1.44
CHOKE INPUT FILTER(Special choke must be used) BRIDGE 
BRIDGE
Vac = Vdc x 1.11
Iac = Idc x 1.06
Pac = Pdc x 1.18
