The purple power supply

The purpose of the purple power supply was for me to give the OSH Park PCB service a try, while still developing a useful tool for myself. (Click here for more information on OSH Park)

Many of my projects require a 3.3V and/or a 5V power supply, so I embarked on the development of a small, yet simple power supply that would supply me with these voltages. Most of my embedded projects require a lot of time working with firmware, so I gave the power supply the ability to be powered via USB.

This basic power supply also includes some basic protection, as to protect the computer if something were to go wrong. (i.e. shorting the outputs of the power supply).

The power supply is only designed to supply about 100mA when supplied power from a USB port. This is due to the default limit imposed by the USB standard. USB hosts can supply larger currents, but this requires negotiation between the connected device and the USB host. To keep the power supply design simple it was decided not to include USB communications between the power supply and the USB host, resulting in a design limited to about 100mA. However, USB is not the only method in which you can supply power to the purple power supply. I included a header on the power supply PCB that allows the power supply to accept a supply voltage between 5V and 12V. This would circumvent the USB current limitation if required. That being said, 100mA is usually more than enough when testing simple digital circuitry. The resetable fuse (polyfuse) limits the total current input of the design to about 200mA, so if your looking for a more powerful power supply you might have to look elsewhere. The LM1117 voltage regulators used in the purple power supply are capable of 800mA, if you need a little more punch out of the power supply the polyfuse can be replaced with a higher current capable replacement.

Anyone wanting their own purple power supply can get theirs by ordering the PCB from Osh Park. The schematic for the design can be found here. The USB connector doesn't need to be included if you only intend to operate the power supply from a battery. I typically use the power supply with a rechargeable 9V NiMH battery.

Designator Component
P1 3 Way 2.54mm Male SIL header (for jumper)
P2 12 Way 2.54mm Male DIL header (outputs connector)
P3 2 Way 2.54mm Male Sil header (for battery power)
U1, U2 LM1117-adj (make sure this is the adjustable version, SOT-223)
C1, C2, C3, C4 10uF, 16V, 1210 ceramic capacitors
R1 120R, 0805
R2 360R, 0805
R3 330R, 0805
R4 150R, 0805
R5 240R, 0805
D1, D2 SS14 or equivalent Schottky diode (I modded a 1n5819 as I forgot to order this)
D3 LED of your choice (0805)
F1 200mA polyfuse (1812) [ERFSD0203020Z]
USB1 Shallin Electronics Y98910




The power supply is simple to use, the output rails are located on the header (P2) with the connections illustrated on the soldermask (legend). The header has two rows of connectors for each output, as such there are two 5V and two 3V3 outputs as well as two unregulated voltage outputs. Each output rail has a corresponding ground connection.

Placing a jumper on P1 allows the user to select an input voltage of 5V or greater than 5V. Powering the power supply from the USB connection requires the 5V jumper position to be selected. This setting bypasses the 5V regulator as the USB already supplies 5V. If the >5V position is selected, while connected through the USB connection, the 5V rail will not be regulated effectively. This limitation is due to the minimum input-output voltage differential required across the LM1117 regulators in order for them to regulate.

The protection diodes protect the connected computer from damage if you accidently connect the power supply to both a battery and the computer. This also protects the power supply from a reverse voltage if a battery is connected with the wrong polarity. The downside to this protection circuit is that the 5V output will be slightly lower than 5V when powered by USB due to the volt-drop across the protection diode. This makes the selection of D1 very important as a diode with a low volt-drop will result in an output voltage closer to the desired 5V.

Warning! Be sure to select >5V when supplying the power supply with an input voltage above 5V, as bypassing the 5V regulator could be catastrophic if you connect the 5V output of the power supply to your projects as the input voltage will be applied instead of 5V.