Introduction: Pencil Box Triple Output Power Supply

About: I am a retired Electronic Systems Engineer now pursuing my hobbies full time. I share what I do especially with the world wide student community.

Re-purpose your old mobile phone chargers into a triple output power supply for bread-boarding analog and digital circuits.

Mobile phone chargers which connect to the utility wall outlet are efficient switch-mode-power-supplies which convert the utility 110V/220V AC supply to 5V/6V DC with current ratings from 250mA to 1A and provide isolation from the AC mails.

Five chargers when suitably connected can form an excellent triple output power supply for bread-boarding analog and digital circuits.


Input AC 110V-240V 50/60Hz

Output 1 +5V DC 500mA Suitable for Digital IC's

Output 2 +12V DC 250mA Suitable for Analog IC's

Output 3 -12V DC 250mA Suitable for Analog IC's

The images show a demonstration of the power supply when used to power a typical analog/digital circuit being tested at bread-board level.

A LM555 timer IC is operated at +5V and forms the clock for a 74193 4-Bit binary counter. The counter outputs are fed to the 4 msb's of a DAC 0800 whose voltage reference is provided by a 9V regulator chip. The DAC output is fed to a LM741 OP Amp. The DAC and OP Amp operate with +/-12V

The Oscilloscope display shows the 4 counter outputs and the overall DAC output. is used to capture the waveforms.

Caution: Mobile phone chargers operate at the mains voltage of 110V/ 220V and extreme care must be taken while building this unit.

Step 1: What You Need

Five mobile chargers are required. One of these must be a true 5V output unit suitable for digital circuits which have a strict supply voltage range.The other four could have a nominal voltage of 6V. These would be paired and used for the +/-12V output. Analog circuits typically accept a large supply voltage range and the nominal 12V could vary between 10-12V. Any voltage references would need to be separately derived from these voltages.

A plastic pencil box is suitable for this power supply as it would be well insulated from the mains voltage contained within the overall unit.

It is necessary to do a quick check to see that the five smps modules removed from the mobile charges fit comfortably within the box. Space must be catered for the switches connectors and wiring.

A mains patch cord, DPDT switch, output terminals and legs for the box are required.

Electrical tools, soldering iron and wires are essential to complete the wiring and assembly.

Step 2: Building the Unit 1:

Evaluation of the SMPS modules

The mobile charges available were of different types and ratings and it was necessary to evaluate them for input-output performance at 110V/220V AC input and different load conditions.

Based on the test results the module based on the TINY264 circuit was chosen for the +5V 500mA Digital supply.

The other units were paired to get the +/-12V Analog supply with one of the 'Morsing Make' circuits in each pair.

Connection Schematic

AC Mains is connected to the modules through a DPDT switch

The +5V module is independently used for the digital supply.

The other four +6V modules are paired with their outputs connected in series to form the +/-12V analog supply.

Step 3: Building the Unit 2:

1. Cross check that the modules fit comfortably within the pencil box.

2. Fit the output terminals and wire-up the mains cord through the DPDT switch.

3. Raise the cardboard base on which the modules are to be glued using a suitable nonconducting spacer material.

4. Glue the modules in place using a hot glue gun.

5. Connect and wire-up the input and output of the modules to the AC input and output terminals based on the schematic diagram.

6. Print add the terminal voltage labels on the inner side of the plastic box using 3M tape.

7. Fix the legs using a suitable adhesive.

And finally:

Power up and test the triple output power supply re-purposed from five mobile phone chargers.

Step 4: Overall Performance

The image shows the results of the overall performance check at 110V/220V and different load conditions.

The results are within the specifications:

Input AC 110V-240V 50/60Hz

Output 1 +5V DC 500mA

Output 2 +12V DC 250mA

Output 3 -12V DC 250mA

Before and After Contest 2016

Participated in the
Before and After Contest 2016