# A little help needed in understanding DC power supplies

Jan 28, 2017
5
Hi,

Actually, it's a pretty simple question that I wanted to ask. The DC power supplies which we often use in like labs or with some DC operated electronic equipment has its ratings written on it. For example, a DC adapter that I use with my internet modem has a rating of 12V 1A. The question is, that if I am to make a power supply by myself and I need to calculate the parameters, how exactly would I do that. The voltage calculation is pretty simple but what I am confused about is the current. How would I calculate the output current of the power supply that I want to make? If I were to make a power supply with 30 volts 5 Ampere output, how exactly would I figure out that the power supply has an output rating of 5 Amperes? Does it depend on the transformer rating that I am using for stepping down the voltage or load or both or some other thing?

Thank you.

#### Bluejets

Oct 5, 2014
6,922
Does it depend on the transformer rating that I am using for stepping down the voltage

That and any smoothing capacitors and /or chokes used.
In your example match voltage or near and then use max current so 12 * 5 = 60VA.(12V)
If I remember, most quote around 1000uF per amp output for capacitor sizing.

You say you understand the voltage so I won't comment there.

Just don't understand why would you want to build your own when one can buy an already proven design unit for much less than the cost of parts?

Jan 28, 2017
5
Really appreciate the help. I don't really intend to make one but just wanted to understand the concept.

Thank you.

#### davenn

Moderator
Sep 5, 2009
14,260
Really appreciate the help. I don't really intend to make one but just wanted to understand the concept.

Thank you.

OK and the current rating you see on a PSU is the max output for the supply ...
when building a PSU it would generally be designed to be able to supply more current that what will be normally drawn from it
that way it isn't always running at its limit

#### Rayregula

Dec 20, 2016
84
Texas instruments has a handy design tool here for power supplies and other useful stuff.
Just watch out for minimum component quantity's.

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