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Auto-shut-off step-down transformer
written by Richard D. Gregg
Wasted Electrical Energy
Most people don’t realize it,
but a significant electrical energy drain exist within their
homes---leaky appliances. An estimated 5 billion watts of
electrical energy, equivalent to four average sized power plants,
are lost to appliances that the consumer assumes are off. It
is calculated that the average home leaks about 50 watts or $4.20
a month in electricity. There are many sources for these
losses, including television memory and remote control circuits
that need to be powered in a ready state for the convenience of
the user. The ow voltage step-down convertor (wall adaptor), or
little “black cube” is one of the worst offenders. They
on average consume one watt of electricity when not in use. Even
more numerous are less expensive versions that have been coined
“vampires” that use about three watts of
electrical energy when not in use. You can feel how warm
they are even when they aren’t powering anything. Another
huge loss are the increasingly popular compact audio devices.
These are stereos that are bigger than a boom box but somewhat
downsized from a typical component system. They consume
approximately nine watts of electrical energy after they are shut
off. It has been estimated that the consumers of these
stereos and the like only listen on average of one
hour per day. It has been calculated that over ninety
percent of the compact audio energy use in the United States
alone is worth approximately 370 million dollars per year and
that cost is incurred when the stereos are thought to be off.
It cost more to have them plugged in and ready to go than to
listen to them. That’s down right wasteful!
Transformers under load exhibit
many kinds of energy losses including magnetic
field leakage (leakage flux), iron loss which includes hysteresis
loss and eddy current loss and reflected impedance.
Most reference books indicate ideal calculations for a perfect transformer
with 100 percent efficiency which doesn’t exist in
practice. Another kind of energy leakage (the one this
project solves), comes from dc resistance within a coil of
copper wire (copper loss), such as the primary of a
typical step-down transformer. The resistance of a conductor
increases as the length of conductor increases. The dc
resistance of a primary coil on a typical step-down transformer
usually is no more than 200 ohms. Here lies a major loss of
energy in the United States----the constant electrical drain
within the primary coil of millions of “black cubes”
intermittently powering all sorts of electronic devices and
gadgets from appliances to Xenon flash lamps. In the
majority of cases these vampire “black cubes” are
plugged into the AC quietly wasting energy even when they are not
supplying current to a low voltage gadget.
The inexpensive “Auto-Shut-Off Transformer” project you can easily build in a few hours is a method of accomplishing power savings in the common wall adapter, AC powered radios and tape recorders, and any appliances which use step-down transformers to supply power. The alternative method of saving this wasted power would be to pull the plug or use a switch between the AC outlet and the transformer. In many cases you can add this small amount of circuitry to your radios, cassette players, etc. If you are unafraid of opening the little black cubes, sometimes there is enough room within the cube to add the Auto-Shut-Off transformer components and glue the cube back together. Very tiny components can be purchased (maybe even surface mount components,SMC), in the through-hole variety. The largest components beyond the transformer will be the relay and 1000 uF. capacitor. Many black vampire cubes will already contain the capacitor (smooths out the ripples after dc conversion). DPDT relays can be had for a buck or less and many are made very small. Using the schematic / plans circuit I provide in this project, you can build an Auto-Shut-Off transformer that will control a transformer that outputs between 6 and 12 volts at up to 1.5 amps. Any voltage / current ratio below or beyond those just stated, you would have to use different component values than is listed in the schematic. The auto-shut-off transformer plans project #0007 contains schematics, parts list and how-it-works descriptions.
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