This project was inspired after building a linear DC power supply at BCIT
(and after finding out how much DC power supplies cost!). Most of the parts
are from old equipment I had collected. Since I was only making one, I
used whatever I had around. (The case is an old external hard drive case,
the dual voltmeter is from a radio VU meter, the heat sinks and connector
is from a Macintosh SE power supply, etc.)
Specifications
Specification:
Value:
Input Voltage
120V, 60Hz
Input Current
0.5A max (except for a split second at power-up when the capacitors
charge)
Output Voltage
1.2-30V x2, +5V, +12V
Ouput Current
~1A per output
Documentation
The power supply is based on one of the simplest linear power supply designs.
It consists of a transformer, capacitors and 4 voltage regulators. Here
is the schematic. There is also a 2A fuse that
is not included in the schematic. Both supplies have a 1.2-30V ouput and
the left supply also has a +5V tap and a +12V tap. Since the two sides
operate independently, they can be bridged to get up to +60V or to make
positive and negative rails (eg. for op-amps).
Design Notes
Here are some notes on the different aspects of the design:
Transformer - I was able to get away with one transformer because
I happened to have one with multiple secondary windings. I divided the
windings and used them for the different halves of the dual power supply.
Power Board - The 1.2-30V outputs use LM317 voltage regulators,
the +5V uses an LM7805, the +12V uses an LM7812. The capacitors were
quite large (overkill), but my reasoning at the time was the bigger
the better. The wire size was also overkill but it came with the nice
connector that I used to separate the board from the rest of the box. The
heat sinks were to ensure I could run at fairly high current without having
to worry about the thermal protection kicking in on the regulators. The
components are attached to vector board because I built this before I started
etching my own circuit boards.
Front Panel - The voltmeters were VU meters in a past life and now
they run in parallel with output posts. On the left power supply, there
is a switch to select whether you want to monitor the 1.2-30V, +5V or +12V
ouput. Behind the panel, there are potentiometers to fine tune the meters.
