# Regulated DC supply - need help with calculations

I am in the process of making a regulated power supply to convert AC(220V, 60Hz) to DC(9V, 500ma). I have worked out all the calculations and would appreciate if someone could double check just to make sure they are alright.

My setup is as follows

AC(220,60Hz) -> Transformer(12V,500ma) -> Bridge Rectifier -> Voltage Regulator(7809)

1. the 78xx series requires at least 2V to work => i need at least 11 as rectifier output

2.the ac input might vary by 10% ..

peak transformer voltage = 12 $\cdot$ $\sqrt{2}$ = 16.97V

min transformer voltage = 16.97 $\cdot$ 0.9 = 15.27V

vout-bridge = vmin-transformer - (2 $\cdot$ 0.7) - Vripple
vripple = 15.27 - 1.4 - 11 = 2.87V
dt * load current / C = 2.87V
C = 8ms * 500mA / 2.87V
C = 1393 μf

can someone confirm this is correct ??

thank you for the help !

• @stevench ... Makes sense but then I am planning to connect a micro controller To t.. The current drawn by which would be max 200ma but could be lower as well. So that would mean the ripple voltage would vary depending on the current being drawn at the moment. So how do I ensure that this variation is small enough to be taken care of by the voltage regulator? – Ankit Mar 14 '12 at 14:49

If your load is 500mA you need more than that from the transformer. The 500mA is drawn from the rectified voltage, i.e. the 17V. This means power drawn is 17V $\cdot$ 500mA = 8.5W, which means 0.7A at 12V RMS. A safe value for the transformer's current rating is twice the DC current, so I would pick a 12V AC, 1A transformer.
• @ankit - I added to my answer. If you draw only 200mA only a 560$\mu$F is required, as per your calculations. – stevenvh Mar 14 '12 at 10:01