I recently submitted an idea for beta-testing the new Cortex-M0 mBed and, lucky me, I was selected as one of the lucky few to test out the light-weight board.

Idea

The idea is rather simple, I want to make a workbench power supply. Since I started electronics I looked for development boards that were powered by either the USB port of my computer, or came with their own wall-wart.

Since I am thinking on starting to make my own custom made circuits for specific tasks, a workbench power supply is a must. I toyed with the idea of using a old PC power supply for this because their readily available, cheap and give a variety of voltages to play with: -12V, 3V, 5V and 12V. This is OK for most projects, but there may rise a need for other voltages.

So I proposed the idea for a workbench power supply with the following features:

• PC controlled, over a USB-Serial link
• Continuous status reporting from the power supply
• Send requested settings to the power supply (voltage, output enable etc)
• Two independent channels with voltages from 1.2 to 12 V
• Internal temperature control (make it as cool and as silent as possible)
• Display status of power supply by means of two 0.96" OLED displays and LEDs
• Short-circuit, over-current and over-temperature protection

Description

The power-supply will be based on a PC power supply and the Cortex-M0 mbed for controlling it. The two adjustable power channels will be controlled by an LM350T voltage regulator with their adjustment voltage running through two I2C digital potentiometers (although I am reconsidering this) to achieve a variable output voltage. The mbed will check the required voltage and switch from between the fixed voltages supplied by the PC power supply (3.3V, 5V, 12V, -12V(if available)) and the LM305Ts when the voltage required is not in the voltages supplied by the power supply.

The mbed will use the analog inputs to check and display the voltages supplied. The status of the power supply will be displayed on the mbed LEDs, output voltage and output current will be displayed on one or two (one for each channel). The status of the power supply will also be sent to the PC by a USB serial connection.

The power supply fan will be also controlled by the mbed as to be as silent as possible. The mbed will vary the rpm of the fan by means of Software PWM.

The control of the power supply will be handled by means of an rotary encoder for each channel and from the PC using the USB serial connection.

The mBed can be powered at all times using the 5V stand-by voltage supplied by the PC power supply.

Other attempts

Dave Jones, from the EEVblog also started a multi-part tutorial on the subject and I will be looking at it closely as what he presents will most definetly affect my design (That guy knows his stuff, always learn from the best!).