Hi all, was hoping someone could confirm that my calculations for sizing transformers for small dc loads is correct.
Im building a dc power supply with three available voltages, 5, 12 and -12 and have used the following equation
Vac= vdc+vreg+vrec+vrip/0.9*vnom/vlow*1/√2
Vdc= output of voltage regulator
Vreg= voltage drop of regulator
Vrec= voltage drop across diodes on rectifier
Vrip= ripple voltage @ 10% of vdc
0.9= rectifier efficiency
Vnom= 230v in uk
Vlow= 190 in uk
Iac= Idc*1.8conversion factor
For 5v dc @ 0.5a
Vac= 5+2+1.25+0.5=8.75/0.9= 9.7*230/190*1/√2= 8.3vac
Iac= 0.25*1.8= 0.45vac
Transformer va= 8.3*0.45*2= 7.47va
Note I will be using a dual rail PCB mount transformer, this is reason for *2 on va calculation. Plan to connect output rails in parallel to obtain Max current output of around 0.5a
For +12 and -12vdc @ 0.5a
Vac= 12+2+1.25+1.2= 16.5/0.9= 18.3*230/190*1/√2= 15.6v
Iac= 0.5*1.8= 0.9a
Transformer va= 15.6*0.9*2= 28.08va
Going by my calculations I have decided to select two transformers rated @ 10 and 30va
Appreciate if anyone could confirm that this is the best method to select transformers for a dc power supply using l78 and l79 voltage regulators, also that my results are correct, thanks.
Im building a dc power supply with three available voltages, 5, 12 and -12 and have used the following equation
Vac= vdc+vreg+vrec+vrip/0.9*vnom/vlow*1/√2
Vdc= output of voltage regulator
Vreg= voltage drop of regulator
Vrec= voltage drop across diodes on rectifier
Vrip= ripple voltage @ 10% of vdc
0.9= rectifier efficiency
Vnom= 230v in uk
Vlow= 190 in uk
Iac= Idc*1.8conversion factor
For 5v dc @ 0.5a
Vac= 5+2+1.25+0.5=8.75/0.9= 9.7*230/190*1/√2= 8.3vac
Iac= 0.25*1.8= 0.45vac
Transformer va= 8.3*0.45*2= 7.47va
Note I will be using a dual rail PCB mount transformer, this is reason for *2 on va calculation. Plan to connect output rails in parallel to obtain Max current output of around 0.5a
For +12 and -12vdc @ 0.5a
Vac= 12+2+1.25+1.2= 16.5/0.9= 18.3*230/190*1/√2= 15.6v
Iac= 0.5*1.8= 0.9a
Transformer va= 15.6*0.9*2= 28.08va
Going by my calculations I have decided to select two transformers rated @ 10 and 30va
Appreciate if anyone could confirm that this is the best method to select transformers for a dc power supply using l78 and l79 voltage regulators, also that my results are correct, thanks.