12V Power Supply

[Brian Densmore]

You make it way too complicated.

Simply take the 12 volt output and hook up a series of
resistors to get the required voltage drops. Adding an
optional toggle switch to open the circuit. And do not
forget to add som nice little banana plugs to access your
new variable voltage power supply. It is also a 
simple matter to add a second circuit that provides
variable current.
For more sophisticated tastes you could simply add
a power meter and a variable resistor in series with
a fixed resistor or two variable resistors. 
There is no need to go hacking inside the box for this.

enjoy,
Brian

> -----Original Message-----
> From: Duane Attaway [mailto:dattaway at dattaway.org]
> Sent: Thursday, September 25, 2003 12:03 PM
> To: james McAllaster
> Cc: Jonathan Hutchins; kclug at kclug.org
> Subject: Re: 12V Power Supply
> 
> 
> 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.
> 
> 
> majordomo at kclug.org
>