Experiment #2: Power supply

Introduction

Following the decision to power the diorama using 9V DC, the next step was to design a power supply with the following features:

1. To enable either centre positive or centre negative power supplies to be used - implying the use of a bridge rectifier.
2. To have an output for a “power-on” LED.
3. To have a tap making available the full voltage after rectification.
4. To have a tap for a 5V supply to power the microcontroller(s) and (probably) the LEDs.

Testing was done with both centre-positive and centre-negative 9V DC supplies. In fact the same centre negative supply was used for both tests using an adapter cable to swap polarity as required.

Circuit

The following circuit was drawn up:

It was tested on a breadboard:

Since the voltages obtained were as expected, a trial circuit was constructed on perf board that replicated the breadboard design. A bank of 8 pins were provided for external connection, as follows:

• 2 pins for 9V input - polarity agnostic.
• 7.8V and GND pins for the power-on indicator LED (the LED is mounted above the 9V socket, not on the perf board).
• 7.8V and GND pins made available for connection to other circuit boards.
• 5V and GND pins made available for connection to other circuit boards.

Given that this was my first ever soldered circuit there were some problems and the circuit wasn't as tidy as it could be!

Conclusion

Evaluating the soldered circuit led to a redesign of the layout of the board along with some changes in functionality. These changes were incorporated in a second version of the board. The functional changes were:

• The LED current limiting resistor was omitted from the circuit board – it was decided to solder the resistor directly to the LED, making it easier to change the value if necessary at a later date.
• The number and layout of the output pins were changed. From left to right the pins were arranged as follows:
  1. A pair of 9V input pins to be used to connected to the power input socket.
  2. A pair of pins for connecting to the power-on LED – these were 7.8V and GND respectively.
  3. A pair of 7.8V pins for connection to other circuit boards.
  4. A rank of five GND pins for connection with other circuit boards.
  5. A rank of three 5V pins for connection with other circuit boards.

This final version of the circuit board is pictured on the main project page.

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