# Help me with building power supply - materials, calculations

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#### cajun4417

##### Newbie level 3

I am trying to build a power supply that has an input voltage at 120V AC and an output voltage at 3V DC and 3 more at 6V DC, 9V DC, and 12V DC. I know i need a transformer but I'm not sure what size. I think I need a diode and resistor, but once again I not sure what size. If possible could someone help me with the materials and the calculations on how to find the right values. Thank you.

#### dfullmer

##### Full Member level 5

Hi,

best place to start is how much current of each voltage do you need? When you know that multiply the (3V*I)+(6V*I)+(9V*I)+(12V*I)= some VA or watts. Then you need to assume some kind of power supply efficiency. If it is a switching regulator it may be about 80% efficient. So what ever VA you got from the beginning needs to be divided by the efficiency so VA/0.80 = the total VA you should need from the transformer. Now at this point based on your needed voltages I would go with a step down switching regulator design. Say something like the national series LM2576. The also have a decent work bench set up that can design the circuitry for you. Give you a bill of materials and so forth.

h**p://webench.national.com/appinfo/power/webench/index.cgi

hope that helps

dfullmer

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