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 low 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 down-sized 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-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.
The auto-shut-off transformer
Project 0007, (the auto-shut-off
transformer), I built, cost me $2.31 in extra parts not including
the transformer and four rectifier diodes and filter capacitor
already contained in most power wall adaptors. The relay is
the most pricey component. You probably have some of these
lying around in your junk box. It is cheaper to buy surplus
wall adaptors and carefully cut and open them with a hack saw,
than to purchase outright a transformer alone. There are
many on the surplus market. In addition most have a
rectifier circuit including a filter capacitor. See the parts
list for a list of suppliers.
Many portable audio units have built in step-down transformer /
rectified / filtered power supplies. You can open up
these units, and if there is enough room to accommodate the added
circuit board of figure 1, then you can add the auto-shut-off
transformer feature to your audio unit.I say this with caution
because you might void your warranty if you tamper with the guts
of your audio unit. And if your audio unit is remote
controlled, you also might not want to cut power to the unit’s
circuitry. The audio unit’s infrared receiver is not likely
to function without the standby power it receives from the step-down
transformer.
How the auto-shut-off transformer works
In reference to figure 1, connecting a device
to the power output causes a brief positive going current to flow
through the device and to the relay common (pin 7), and to the
closed relay contact (pin 3), then to R1 and the base of
the NPN transistor, Q1. Q1 is forward biased to conduction. A
negative going current flows from the collector Q1, through D6
and the relay coil of relay 1. LED, D6 and signal diode, D7
prevent voltage spikes that could destroy Q1. In addition D6 acts
as a visual indicator showing that relay 1 is activated. Both
sides of the relay are pulled in making dual electrical isolated
contact with the negative side of the rectified and filtered
secondary voltage from the transformer to the loading device
through the common, pin 7 and the normally open, pin 4
and full AC contact with the primary coil shunting or by-passing
C1, through common, pin 1 and normally open, pin 6 of relay
1.
You now have full step-downed power to the
device. In addition to the conduction of transistor, Q1 as
previously mentioned, a positive going current also flows through
diode D8, neutralizing any charge on C4. When relay 1 closed,
this action disconnected the base of the transistor Q1 from the
positive going current through the powered device and
connects that side of the device’s power input to the
negative as previously mentioned. The positive going
current also no longer flows through D8 and Q2. Now C4 is
allowed to charge via the base / emitter junction of NPN
transistor Q2. While C4 is charging the base of Q2 is more
negative, and therefore Q2 does not conduct. When C4 charges to
the point that the base becomes more positive, Q2 conducts a
positive going current through R3, R4, and NPN transistor Q3,
causing Q3 to conduct. Q1 in effect stops conducting. Only
briefly though, because as soon as the relay coil magnetic
field collapses, the common contacts in the relay go to the
normally closed position and once again a positive going current
flows to the gate of Q1 and to C4 neutralizing its charge. The
SCR, Q1 is quickly triggered into conduction and the relay
rapidly snaps back to the full power “on” condition.
The surplus charge on C2 holds enough current
to keep power flowing to the load during the very brief cut-off
incurred when the relay magnetic field collapses. In short
the circuit is constantly trying to shut off the main power to
the transformer. As long as a load is ‘sensed’,
the relay will stay on. As soon as the load (device),
is disconnected or turned off, the SCR, Q1 and relay
1 stay off when the next shunt pulse from Q3 is
administered to the anode of Q1. In addition, if the load
you connect to the project is more than the transformer can
handle, it will not switch on. This feature could act as a safety
feature. Wall adaptors and the like, do sometimes cause
fires.
Further Explanation and set-up
Essentially AC current / voltage are reduced
substantially within the the primary coil of a typical step-down
transformer by placing a capacitor in series with the primary
coil of the AC line current / voltage. (The
transformer stays cold---you can feel the heat from a typical
wall adaptor even when not under load.) The project’s transformer,
T1 is rated 117 volts primary input and 12 volt secondary @
500mA output. It wouldn’t be difficult to adapt
my circuit to the multitude of different voltage / current
ratings of wall adaptor transformers currently manufactured. With
a .047 uF 250 volt capacitor in series with the primary
coil, the primary coil voltage measured approximately 15
volts AC. Calculation of the capacitor reactance across the
capacitor C1, (0.047 uF), should be a total of 56,437.923 ohms between
the AC line and the primary coil. As you can see from the
formula for calculating capacitance reactance, the higher the
input frequency (F), the lower the resistance (Xc).
Xc= 1 ÷ 2?FC
Xc= 1 ÷ 6.28 x F x C
Xc= 1 ÷ 6.28 x 60 hz (cycles per second) x .047
x .000001
Xc= 56,437.923
The dc resistance of a primary coil on a
typical step-down transformer usually is no more than 200
ohms. This would figure also in your calculations, but for
the purpose of this project is not necessary. A capacitor in
series with a transformer primary will substantially reduce the
amount of current flowing across the coil and therefore reduce
the amount of electrical loss.
Referring to the schematic figure 1, there is a momentary push button switch S1, electrically connected in parallel with the capacitor C1. When you first plug in the project, there will be little current flowing within the primary coil as mentioned previously. The next step is to actuate the wall adaptor by pressing briefly the momentary switch. This action will by-pass C1 and allow full AC power to the primary coil. The secondary coil will conduct until C2 is fully charged as long as the momentary switch is pressed on. The relay will briefly (10 to 15 seconds), pull in closing contacts within relay 1, and again shunting C1. The relay then shuts off cutting full AC to the primary coil reducing voltage drop to 15 volts AC. C2 is fully charged and holds that charge indefinitely with very little leakage. The 15 volt AC drop across the primary coil supplies just enough power to keep C2 filter capacitor charged. When I used a 0.1 uF 250 volt capacitor in place of the 0.047 uF capacitor----I could actually measure the slow charge to C2. If a 0.1 uF capacitor is used, the momentary switch could possibly be eliminated. But it does take a number of minutes for C2 to charge up. In addition increasing the value of C1 to 0.1 uf lowers the capacitor reactance to 26,525.824, and therefore the current / voltage drop increases proportionately. Now that C2 is fully charged and the relay has opened, you can connect a 12 volt, 500 milliamperes or less load (radio, tape recorder, etc.), to the output of the power saving auto-shut-off transformer project.
Summary
This circuit is very simple and
effective. I hope it has potential as an energy saving
circuit. I am experimenting on further circuitry that would
incorporate this circuit and a simple voltage level detecting
circuit for use with charging nickel cadmium batteries, utilized
extensively in many appliances. Much power is consumed and
wasted after the batteries have been fully charged. After they
are fully charged the primary coil should be powered down. When rechargeable
batteries discharge to a predetermined point, only then should
the primary coil automatically power-up.
My goal is to help stop the extravagant waste of electrical
energy and further destruction of the earth’s present environment through
increased alteration of earth’s weather patterns
due to increased green house gases. We must not exchange
long term environmental balance for short term material
gains. We must reduce humankind’s influence on the natural
systems to a minimum and still maintain our profitable economy. By
planning and inventing, this can be accomplished over time.
Several people have said to me, “who cares if you spend an
extra $20 to $50 a year in electricity---its such a small amount
to worry about”. But I say multiply that by millions.
Their eye-brows raise like Mr. Spock!
figure 1
Parts List
RESISTOR
R1,4-------1K ohm
R2--------100 ohm
R3------100 K ohm
CAPACITORS
C1---------0.047 uF to 0.1 uF, 250 volt
C2---------1000 uF 25 to 35 volt
C3---------100 uF 25 volt
C4---------10 uF 25 volt
DIODES
D1,2,3,4---------IN4002 rectifier diode
D5,7,8---------1N914 signal diode
D6----------------Bright red or green LED 20mA
SEMICONDUCTORS
Q1,2,3-----------------2N3904 NPN transistor
MISC.
T1----------------Step Down transformer, (ie. 12 volt, 500
mA)
RLY 1------------Double pole, Double throw (DPDT), relay, (ie. 12
volt)
S1------------------momentary push button switch
List of Suppliers
Various Electronic Components Mail order companies
Electronic Goldmine, P.O. Box 5408, Scottsdale AZ. 85261, Call (602) 451-7454----(very good)
All Electronics Corp., P.O. Box 567, Van Nuys, CA. 91408-0567, Call 1-800-826-5432---(good)
American Science & Surplus, 3605 Howard Street, Skokie, Il 60076, Call (847) 982-0870 ----(fun)
Gateway Electronics, Inc., 8123 Page Blvd. St. Louis, MO. 63130----(good & unusal)
American Design Components, 400 County Ave., Secaucus, NJ 07094, Call 1-800 776-3700---(good)
Surplus Center, P.O. Box 82209, Lincoln, NE 68501-9973 Call 1-800- 488-3407------(motors +)
Edmund Scientific Co., 101 E. Gloucester Pike, Barrington, NJ 08007-1380, U.S.A.-------(fun)
Herbach & Rademan Company, 18 Canal St., P.O. Box 122,Bristol,PA 19007-0122, Call 1-800-848-8001----------(good)
Marlin P. Jones & Assoc. Inc., P.O. Box 12685, Lake Park, FL 33403-0685---(good & interesting)
Fort Apache Electronics and Trading Company, Inc. ,31902 hayman St., Hayward, CA. 94544-----(fair)
SouthPaw Electronics, P.O. Box 886, New Hyde Park, NY 11040-0311----------(fair)
Unicorn Electronics, 10000 Canoga Ave., Unit C-2, Chatsworth, CA. 91311----------(fair)
Premium Parts, P.O. Box 28, Whitewater, Wisconsin 53190-0028-----------(fair)
International Microelectronics, P.O. Box 17041, Arlington, TX 76003---------(fair)
Hosfelt Electronics, Inc. , 2700 Sunset Blvd., Steubenville, OH, 43952-1158--------------(good)
Abbott Electronics, 155 New Boston Street, Woburn, MA, 01801 --------(fair)
Bigelow Electronics, PlO. Box 125, Bluffton, OH 45817-------------(fair)
Fair Radio Sales Co., P.O. Box 1105, 1016 E. Eureka St., Lima, Ohio 45802---(cool antiques)
Edlie Electronics, 2700 Hempstead Turnpike, Levittown, L.I., NY 11756-1443---------(good)
Ocean State Electronics, P.O. Box 1458, 6 Industrial Drive, Westerly, RI 02891-------(fair)
Digi-Key Electronics Corp., 701 Brooks Ave. South , P.O. Box 677, Thief River Falls, MN 56701-0677 (most any electronic part you could want to buy is available)------(excellent)
Mouser Electronics, 958 North Main Street, Mansfield, TX 76063-4827, Call 1-800-346-6873 (most any electronic part you could want to buy is available)------------(excellent)
MCM Electronics, 650 Congress Park Dr., Centerville, OH 45459-4072-----(excellent)
DC Electronics, P.O. Box 3203, Scottsdale, AZ 85257-3293------------------(good)
Circuit Specialists, Inc., P.O. Box 3047, Scottsdale, AZ 85271-3047---------(interesting / good)
Jameco Electronic Components,1355 Shoreway Road, Belmont, CA. 94002-4100------(good)