Electronics

Description:

Fig. 1a PSU1

Fig. 1b PSU2

These power supplies are used to power the tutorials projects. The PSU needs to be small, powered by the computer used to developp, protected from short circuit and efficient. The output requirement is to supply 3.3V with 500mA. It covers most projects.
I could have used a linear regulator like a UA78M33 to convert the 5V from the USB to 3.3V. But for efficiency and experiencing with a buck regulator, I went for a Microchip MCP1602.
It is available in 2 options, one with variable output, the other with a fixed one:

• Board 1 provides 0.8, 1.2, 1.8, 2.5 or 3.3V, 500mA regulated output, selectable with a jumper.
• Board 2 is fixed to 3.3V, 500mA.

They are both powered by a usb port.

Schematic:

The schematics are based around the datasheet of the MCP1602. The boards are powered by a USB bus via a USB B connector. A ferrite bead and a capacitor filter the input voltage (L1 and C1).

On board 1, the output voltage is determined by R5 and the resistor selected with jumper J1. The output voltage is determined by the following formula: Vout = Vref x (R5/Rx + 1).

R5 is the recommended 200K and Vref is 0.8V typically. The required resistor Rx is calculated with:
Rx = R5 x Vref / (Vout - Vref)
The jumper J1 determine which Rx resistor is selected. With the selected values, the output voltage can be set to 3.3, 2.5, 1.8, 1.2 V, or 0.8V if left open.
R10/C4 is a compensation network to stabilize the control loop.

Fig. 2a PSU1 schematic

On board 2, the output is fixed and determined by the version of MCP1602 used. The voltage divider is not required anymore, saving space.

Fig. 2b PSU2 schematic

R4 is here to have an easy Gnd point to connect a test instrument.

Construction:

All the PCBs fit on a small single sided board. U1 (MSOP package) and L2 are on the bottom (copper) side of the boards.

Fig. 3a PSU1 layout

Fig. 3b PSU2 layout

The USB connector is on one edge, and the output connector on the other. Start with the lowest components on the top of the board, and continue with the higher ones.
R4 is made up of a piece of resistor's lead. It's here to easily attach a clip from a test instrument or have a Gnd test point.

Fig. 3c Bottom side of PSU2

Resistors R6 to R9 on PSU1 can be made up of 2 resitors connected together if you can't get the proper value. I used resistors I had in my lab.

Finish with U1 and L2 on the bottom (copper) side.
U1 can be a bit tricky because it is very small. First tin plate the pads of U1: put solder on the copper pads of U1 and clean them with some desoldering wick. Position U1 and solder one pin of an angle to secure it in place. Solder each side of U1, starting with the opposite side of that angle. Don't worry about soldering the pins together. Once both sides are done, use some desoldering wick to remove the excess solder. The main thing is to have clean wick, so the solder is remove properly and neatly. 

How to use / Testing:

To check the power supply, plug a USB cable in a computer and the other side in the power supply.
For board 2, just check that the output voltage on K2 matches the selected MCP1602 (3.3V typically)
For board 1, the output voltage is selected by jumper J1. The output should be 0.8V if no jumper is set. Check each output voltage in the various position for J1. It should be 1.2, 1.8, 2.5 and 3.3V.

To use, just plug in the usb cable, and your project board with a cable.

Files and links: