A homemade power supply

Being poor means that sometimes you have to make your own tools. This is, actually, an amazing exercise and a great way to learn, even if sometimes you end up spending almost as much money as a new tool.

For this project, I built a power supply. It can supply up to 1A from 1V to 20V DC. The main difference from most other DIY power supplies is the fact that this comes with current limitation. This way, I can work with ease in my circuit, knowing that I won’t fry it with a stupid mistake.

Here’s its final form:

The circuit

The circuit for the power supply follows a series of steps, where the energy is transformed from 110/220 VAC to the chosen DC voltage. The steps are shown here:

(if this is too small to see, check the downloadable Kicad source at the end of this article)

(disclaimer: the rectification and voltage/current limiting came from a kit)

The first step is the input, where the power supply receives power from the mains. A switch turns on the supply, and the user can choose the input voltage. Even if the supply has current limiting, we put a fuse here to make sure that nothing goes wrong.

The power is then transformed from 110/220 VAC to 18 VAC.

The next step is full bridge rectification, where the power is converted from AC to DC. The four diodes convert the AC power from negative to positive only, and the capacitor smoothes the wave, transforming in an almost straight line:

Two LM317 power regulators are used to limit voltage and current. Since we might need to reduce a lot of power (20V to 1V, for example), both need to be connected to a heat sink.

I also added an YB27VA voltmeter ammeter to display the voltage used and current being consumed by the circuit.

The final step is, of course, the output. So here is the mounted circuit, in all its might (without the input and the heatsink):

Montage

As usual, the construction of the box and montage was the hardest part of the project.

I started with a standard electric box.

I absolutely underestimated the hardness of the box. A long time was spent sawing and drilling the box. At the end, I learned something useful: instead of trying (and almost drilling myself) with a drill, attach the drill bit into a cordless screwdriver. This way, you can have much more control over the drilling process.

I also ended up using this process for the large hollow areas where I couldn’t fit a saw: drill the outline with the screwdriver, and use a file to give shape and sand.

The final result with the box open, already with the circuit mounted, was this:

You can see that I was very careful to seal all the high voltage areas with heat-shrink tubes (in the power switch, for example).

The detail of the holes drilled with the cordless screwdriver can be seen here:

Files

Get the schematics PDF here.