12V Power Supply

[Duane Attaway]

On Thu, 25 Sep 2003, james McAllaster wrote:

> if anyone knows how to step to different voltages i'd appreciate the
> feedback. i'd like to replace the half dozen transformers i've got
> running at 6, 7.5, and 9 volts with a single "at" power supply.

Its easy.  Or fun.  Or a science experiment.  If you love pyrotechnics,
you will love modifying switching power supplies.  Changing parts or even
attempting to repair one of these can make a slow day filled with the
sounds, smells, and visual effects usually only seen in freshman
electronic labs:

If you don't mind changing the resistor voltage divider feedback circuit
for the pulse width modulator regulator, you can vary the 5 volt output a
little bit.  Changing the winding ratio is required to get beyond much
from that.  The high amperage 12 volt output may be another winding from
the same transformer with not much voltage tolerance.  There is likely to
be a pair of resistors to measure voltages out of tolerance for a safety
shutoff.

The other smaller amperage outputs are usually drawn from extra windings
and regulated from popular 3 pin linear regulator chips.  The part numbers
for these chips are usually 7905 for -5 volts and 7912 for -12 volts.

The main pulse width modulation switching circuit of these supplies are
funny.  Be careful with adjusting the output ratio.  To large of a duty
cycle and the transformer can become quickly saturated.  By altering this,
you can make its dozen or so primary turns of wire into an effective short
circuit to your wall outlet.  Back in the old days, they used to be known
for producing spectacular pyrotechinic displays.  You may have to use a
lightbulb in series with the line to limit maximum current for your
project.  A scope is handy to make sure the switcher is not entering the
linear region, allowing heat to be generated inside the supply.

They put a fuse in these for a good reason.  You may also want to wear
safety glasses.  Or a full face shield if you hate the high velocity of
flying parts.  High voltages often deliver quite a punch of energy.