Bench Linear Power Supply - 1A - 5V, 9V, 12V

Cremona, Apr 2018.
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Hi there,
let's build a simple yet handy power supply. It's a linear power supply, 1A max (fused), with a switch to chose 5V, 9V or 12V.

Here is the baby:


This power supply comes from an old schematic that I found somewhere. Being a linear power supply, it does not generate ripple, but it is also not very efficient (it heats a lot). Yet, the schematic is very simple, so it's worth to build it if you don't have one or just want to practice electronics.
Actually, nowadays you can get a power supply from China for very cheap (around 35 EUR, probably less than buying the components for this one).

A knob allows to chose between 5V, 9V and 12V. Actually, once I built it, I found that the voltage is a bit off. When unloaded the voltages are 5.45 V in spite of 5 V, 9 V sharp and 12.3 in spite of 12 V. When the load is connected they drop by around 0.2 V. Well, unless you need precision, for most purposes like powering some digital circuitry this will be acceptable.

Do you like the casing? The front panel is printed via toner transfer. But before starting:
ATTENTION! This project utilizes hazarduous voltages, which may harm or kill you, your goldfish, your friends and objects. I'm not responsible for any possible damage or inconvenience that you may incur into.


Here is the schematic (click on the image to open it). The transformer must be able to provide 12V * 1A = 12 VA minimum. Go for a 30 W transformer. Remember to put a nice heatsink on the transistor.


Very briefly, the voltage is turned from 230V to 30V by the transformer, and is rectified by the 4 diodes. The large capacitor levels it, and the small capacitor filters the noise away.
In case you wonder why small capacitors are used to filter the noise, the reason is that real capacitors are non-ideal components. Big electrolytic capacitors have a rather large series resistance (called ESR, Equivalent Series Resistance), while ceramic ones have a small ESR.
At this point, the knob selects one of the three Zener diodes to set the voltage. Pay attention to their polarization! They provide a given voltage (pretty much like any diode does) and setup the reference voltage. This voltage will drive the two transistors in darlinghton configuration. Note that two additional silicium diodes are used: they provide a small voltage drop, analogous to the drop, compensating for the voltage drop in the two transistors B-E junctions.

Here's a pic of (a part of) the circuit on a perforated board.

Finally, you may connect a voltage and current meter. Connection will depend on the display you chose.


Ok, now we need to create a casing. I decided to go for a thin steel plate. Yet, this is not thin enough to be precisely bent by hand: for this I suggest you to build a plates bender.

Draw the components on the sheet and have fun drawing a foldable shape. Then cut it, and fold it.

At a certain point, you will need to print the panel symbols, I did it after bending (in this case, be careful not to make ondulations while bending!)

For printing on metal I used the toner transfer technique. First, you need to sand the metal sheet with a veeeery tiny sandpaper. This is needed to make it more opaque (prints will be more visible). Also, this really helps the ink to stay. Then, you need to draw your panel with some software (inkscape! inkscape!) and print it mirrored, on a normal paper sheet. Then, you place the printed face on the metal and you iron it. That's it. It doesn't always come very well. You can use some alcohol and/or sandpaper to remove the print if you wish to retry.

Drill the holes and assemble the components, using some insulation to prevent the circuit from touching the case:
folded-plate-panel-drilled panel-components

And there we go. The top cover is attached by small (3mm diameter) screws placed at the bottom. If you're not very precise with the holes in the panel, use some hot glue to stop the components from moving.
power-supply-open power-supply-closed

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