The regulated power supply is must to have device for each electronics freak.

In this instructable I will show you how to construct such kind of device on the cheap (less than $10)

Step 1: The Core of the Device ...

The most expensive part for a regulated power supply is the input AC/DC converter ( in most cases having quite heavy, expensive and difficult to have transformer, rectifier and filtering electrolytic capacitors). In this design this part is replaced by the recycled from defected printer SMPS (switched mode power supply).

The old and nonfunctional printers are big treasures - they contain a lot of stuff, which can be reused: motors, metallic parts, buttons, LED's, connectors...etc. For our design we need to take away the power supply block of the printer. Usually the whole block is designed as separate module, even in its own case, with contact interface to the other printer electronics. On the picture you can see two SMPS blocks used in "Canon" and "HP" inkjet printers.

I would strongly recommend, before using such block, to check its functionality. Normally on the bottom side of the device can be found a table of all produced by it voltages and the maximum load current for each voltage. The tables of both presented SMPS blocks are shown on the pictures. The existence of these voltages must be checked. You should have in mind, that some of the SMPS devices does not start without load. To measure the voltages, at least one voltage output shall be loaded wit some resistor or light bulb.

The best case, when making the regulated power supply should be if you can assemble the whole device inside the case of the SMPS block. For that reason the block must be open without any damages and it must have enough place for the additional parts. In my case both of the devices was not satisfying these requirements. Additional case was needed.

Step 2: My Choice

Comparing both tables of the produced voltages, I have decided to use the "HP" SMPS (the black one), because it was able to deliver more power, and two voltages (14.4V and 30V) were enough for me on this stage.

Step 3: The Cable Interface

The selected SMPS had to be connected with the regulating circuitry, placed in separate case in the appropriate and easy to use way. Originally it had 3-pin header connector, shown on the picture. I could not find such connector to use it from the other side. I decided to replace it with more comfortable and reliable avionic metallic 3-pin connector ($1.5 at ebay). If you have more voltages to use, you have to buy connectors with more pins.

I have carefully removed the original connector and have mounted the avionic one. To keep the cables collected, I have used a piece of heat shrinkable tube.

Step 4: The DC/DC Converter

For the voltage regulator I have used a LM2596 stepdown DC/DC converter module ($2 on ebay). It is able to produce output voltage from 1.23 V to the value of the applied at its input voltage. I have decided to supply it with the 30V output of the SMPS block, what would give a regulated voltage range from 1.23 V to ~ 30 V. To have easy manual control over the output voltage, I have unsoldered the blue trimmer potentiometer and replaced it with 10 KOhm potentiometer with knob ($1 on ebay). For the shown here module, only two wires are enough for this connection. On the LM2596 module two of the potentiometer terminals are shorted together. I have shorted also these two terminals on the potentiometer (the middle and one of the external ones). The wires connecting the board and the potentiometer shall be kept as short as possible.

Step 5: LED Display

To control autonomously the regulated voltage, I have decided to use a LED Voltmeter (0 V-30 V) ($2 on ebay) .

Step 6: Output Terminals

I have decided to have two output voltages:

1. First : regulated - 1.23 V-~30 V, produced by the LM2596 module

2. Second : fixed 14.5 V - directly from the output of the SMPS block.

For this purpose I needed 3 output connectors. I decided to use banana jacks. I had 3 available ($1.8 on ebay) - unfortunately two black and one red. One black of them became the GND connector. The other two (red and black) I have dismounted and swapped their parts. In this way each of them had a red part, in distinction to the fully black ground one.

Step 7: The Case

All these parts had to be assembled in a small plastic case (local store - $2). Using a "Dremel" tool, I did a openings and holes for the LED voltmeter, the avionic connector, the potentiometer and the banana jacks.

Step 8: The Schematic

On this picture is presented the schematic of the regulated power supply.

In short: The LED voltmeter is supplied by the 14.5 V and senses the regulated voltage at the output of the LM2596 module. In this way the LED voltmeter shows also the presence of both voltages: if 14.5 V does not exist, it will not work, if the regulated is missing, it will show "0". The regulated power supply is connected to the middle output terminal. The 14.V voltage appears at the right output terminal. The 30 V voltage delivered by the SMPS module is applied to the input of the LM2596 module.