RadioBanter - View Single Post - What's in a "wall wart" so-called "transformer"?

Al · September 24th 06, 03:46 PM posted to rec.radio.amateur.homebrew

In article , wrote:

I have a collection of "wall wart" (others may call them "transformers")
and I'd like to use one to cobble a little power supply. I've never
opened any to see what's really in them (and suspect it'll be quite a
chore), but I'm quite sure they are not ALL "transformers", because some
claim AC and some claim DC output, yet the first I grabbed out of the
junkbox claimed DC AND ALSO provided a HEALTHY AC output on the same
two wires! Furthermore, NONE of them seem to be voltage regulated!
(But if one says "X volts at Y milliamps", then loading it to Y mils
usually gives an output close to X volts.)

So what IS in them? Do the really light-weight ones, for example,
use a capacitor for relatively low-loss voltage dropping? And
why would one give *BOTH* AC and DC? Bad diode(s)?

There are four types I have come across:

1: transformer only for AC
2: transformer and diode bridge for DC
3: transformer, diode bridge and linear regulator
4: switching regulator

The voltage output and the current output of the first two type depend
upon the load. So for a type 2, for example, if the nameplate says 6
volts and 500 mA. That's exactly what it means. With a laod of 500 mA,
the DC voltage will be 6 volts. a lighter load will result in a higher
voltage, and a higher load will result in a lower voltage.

The last two will provide you with a fixed voltage for which they are
rated. So, for 6 volts and 500 mA, the voltage will always be close to 6
volts as long as the current does not exceed 500 mA. Switchers may
require a minimum current draw of about 10% of their maximum current
rating. Thus one rated ate 500 mA needs a minmum load of 50 mA to
provide the rated voltage output.

I have disassembled wall warts and used their guts for power supplies in
projects. It works quite well. As our town's recycling center is full of
old electronics, I have assemble quite a nice selection of wall warts
that I use as PSs in projects. And the price is right, zero, except for
your time.

Al