THE ELECTRIC WAVE
If there is
a need to feed very low power devices you may resort to infrared optocouplers,
solar cells, batteries or low power transformers although the latter might be
rather oversized for the intended purpose. Eventually, with the exception of
solar cells, all of them draw power from the mains so it might be convenient to
use a piezoelectric transformer if the power required is in the range 0.1 to 0.3
mW. The schematic shows an easy implementation of such a transformer.
piezoceramic sounders are glued back-to-back so that the mechanical movement of
the first, the primary, is transferred to the second, the secondary. The ac
output voltage can be used as it is or rectified in order to feed micropower
electronic equipment or trickle charge small backup batteries.
actual implementation requires two ceramic sounders with high intrinsic
capacitance: sounders with 80 to 110nF are readily available and usually come as
50mm discs. Two of these discs are cut down to 35mm in order to have a more
compact unit and a lower stray capacitance between primary and secondary. A
layer of double-sided adhesive tape is laid on the larger plate of each sounder
in order to assure proper electrical insulation between primary and secondary.
The sides of the sounders are then pressed against each other and the
transformer is ready to operate.
table shows the measured output under several loading conditions: the ac output
was measured with the load directly across the output terminals while the dc
output was measured with a full wave rectifier in place.
measured dc voltage refers to a schottky bridge rectifier but the use of
standard 1N4004 diodes will only show a modest 6-8% voltage decrease.
were taken with the transformer operating in free air, without any holder, but a
proper mechanical layout would require the transformer to be firmly held by the
edge of the disc. The use of a plastic box is mandatory for safety reason and improves the transfer of mechanical energy to the
secondary thus obtaining a 15-20% voltage increase.
principle of operation of a car hooter has been applied to both a ceramic
sounder and a loudspeaker. A break in the supply current is caused by the
vibration of the ceramic sounder plate or the speaker membrane. You could
implement similar circuits even without a transformer but the voltage range will
be limited, there will be too much sparking at the contact point and pressure
and position of the contact become critical. The transformer introduces a
feedback mechanism thus eliminating or drastically reducing all mentioned
negative effects. An output transformer is used in both circuits: one of the
winding is normally 4 or 8Ω
while the other is at a higher impedance. The larger plate of the
piezomechanic oscillator goes to positive through the contact, typically an
adjustable screw, and the transformer low impedance winding. To get the correct
phase relationship you may need to reverse one of the windings.
similar transformer is used for the electromechanic oscillator with the low
impedance winding connected to the speaker. Also in this case you may need to
reverse one of the windings but first you must make sure that the speaker cone
goes forward when the voltage is applied: reverse the speaker connections if
necessary. A small copper strip is glued on the back of the speaker membrane
with a screw placed in the speaker casing so that it just touches the copper
of operation is from 1 to 1.5 KHz for both oscillators. The frequency for the
electromechanic oscillator depends mainly on the speaker damping factor: best
results are obtained with the speaker laid against a flat surface or sealing the
front side with a wooden panel.
below 0.4-0.6V depends on the careful adjustment of the screw and mechanical
precision of the assembly.
charger is suitable for lead-acid car batteries and it is assembled in two
units: a metal box with the toroidal transformer, instrument, lights, etc, and a
small plastic box housing the voltage and temperature circuit. Connection
between main box and sensor is realized with a standard 3 core x 1mm, electric
cable, 4m long. Its resistance is factored in the circuit calculations and it is
the limiting resistor against overcurrent. Do not change type or length as it
may alter the overall performance and safety of the charger. The sensor box is
typically positioned close to the battery to be charged and two short flexible
leads, 2mm section, 30cm long, one red and black the other, terminated with good
quality clamps make up the connection from the sensor to the battery.
This solution assures that
the battery is charged up to the correct voltage which depends, in turn, upon
the ambient temperature. The final voltage should be set, with the 200Ω
multiturn pot, at 14.8V at 20°C-68°F and derated +/-30mV/°C (17mV/°F) at any
other temperature. For example, if the prevailing ambient temperature is 10°C
then the final voltage should be set at 15.1V, if the prevailing ambient
temperature is 30°C the final voltage is 14.5V and so on. Once set, the circuit
will automatically adjust the voltage to within 1°C. You have to connect a
battery in order to carry out this setting.
A thermistor would have
simplified the circuit but its correct implementation is not easy and it was
preferred to employ a number of diodes. A red led in the sensor box gives an
indication of the correct connection to the battery. However the circuit is
quite tolerant to mistakes: shorting the output will do no arm as there is no
voltage at the output terminals, not until you connect it to the battery. It is
the battery voltage that triggers the circuit into operation and once it is
disconnected from the battery the voltage too disappears from the output. Only
if the battery voltage is above 7-8V then the circuit will operate. A reverse
connection of any battery will do no arm either as the circuit will simply not
operate. It will withstand a temporary connection of a 24V battery; above this
voltage the input circuit is overloaded and could be damaged.
Current control is
achieved by switching the SCRs at the appropriate time through the BF761
collector current. The blue led, but any other colour will do, gives an
indication that the unit is charging the battery. The led will start flickering
at the end of the charging cycle so you know at a glance that the charge is
coming to an end. You may leave the battery connected after it has fully charged
as there will be a trickle charge which will keep the voltage at its optimum
level. Switching noise is eliminated by the 85μH
choke made up by winding 27 turns of 1mm enamelled wire on a ferrite ring
27x11mm. Due to the way SCRs operate, the common line is positive and not
negative as one would expect. Care must be exercised when connecting all the
polarity sensitive devices.
A toroidal transformer has
many advantages: it is small, highly efficient, will tolerate a moderate
overload and will consume little power, only 3.5VA when switched on and no
battery connected. Cost, at this power range, is surprisingly close to a
traditional transformer, yet, the inrush current when switched on can be so
high, depending on the exact time with respect to the mains sinewave, that the
collapse of the ensuing strong magnetic field will produce mighty spikes up to
500V at the secondary, destroying whatever they find in their path. A few
capacitors, the use of fast diodes UF4006 and the high voltage transistor BF761
take care of the problem. The main switch should be rated at 10A.
SCRs can get rather hot;
the best solution is to mount them on the metal case itself using appropriate
insulating kits. As a consequence the box will warm up especially at the
beginning of the charging cycle when the unit may be temporarily overloaded. A
thermal switch is provided to cut out the mains supply under extreme temperature
and overload conditions. This switch is mounted at about 6-8cm away from the
SCRs so that it will take care of the heat coming from other sources as well,
such as the transformer and the choke.
The unit has been tested
with batteries from 44 to 100Ah for over a year, from 0 to 38°C (32 to 100°F);
the upper temperature limit caused the thermal switch to operate. I should
relocate the thermal switch in a cooler place if the designed max operating
temperature of 40°C-104°F is to be met. You may have different temperature
limits depending on the mechanical configuration of the box and internal
components layout. Pay attention to the fact that this charger behaves like a
fast charger for the smaller batteries and precautions should be taken
concerning gas production and it is good practice to disconnect the battery from
the car before charging it.
SIMPLE WARNING SIGNAL
power audio oscillator could be used as a warning signal for alarm systems or to
attract attention if something is wrong with an equipment. The oscillator, about
750Hz, exploits the characteristic of certain NPN transistors, in this case a
BC337, to oscillate if reverse biased and with the base open. Other equivalent
transistors might not work. Despite its simplicity the circuit is quite
flexible: the 390Ω resistor, normally connected to
negative could be switched in through a logic circuit, so it can be driven
directly by the circuit to be monitored; the base is normally not used but
frequency modulation of the circuit is possible by connecting a modulating
signal to the base via a high value resistor, typically 2.2MΩ.
A 3W loudspeaker is
adequate for the circuit and it can be either an 8 or 4Ω
loudspeaker, although in the latter case a small heatsink is necessary for the
BD436. Peak current for an 8Ω loudspeaker can be as
high as 1.2A but because the duty cycle is relatively small, the average current
was measured at 0.2A hence the overall power requirement is only 2.4W despite
the high volume the circuit is capable of. The feed line must be well filtered
and can be anything between 9 and 15V although adjustment of the resistor might
be required as the oscillation frequency is sensitive to the supply voltage.
DOMESTIC POWER LIMIT WARNING
a new electricity meter, the electronic variety, was installed at my place, I
get cut off if I exceed the set power level, 3.3KWh in my case.
meter is unforgiving and although there is a little tolerance built in, you really never
know when it has gone over the cut off limit, given the number of electric appliances
which are continuously switched on and off.
The circuit was designed to give an audible
warning when the 3.3KWh limit is exceeded. The transformer is a disused
transformer from a soldering gun. It is relatively easy to remove the few turns
of the secondary winding and rewind two turns of thick wire, as thick as the
wire coming from the meter at least. One turn should be enough if you have a
limit of 6.6KWh, but operation at this power level was not tested. As an
alternative you may try a small toroidal mains transformer: it
is easy to add a few turns of thick wire. Ignore all other windings, if any,
except the primary winding, which in our circuit becomes the secondary winding.
The circuit is to be installed between the electricity meter with its breaker
and the house wiring. With the given components, the circuit will oscillate at 1
sec. on and 1 sec. off, depending on the load. Adjust the potentiometer so that
there is no sound below the power limit. The varistor is necessary in case there
is a short in the house wiring: the extra voltage at the secondary may damage
the circuit. The piezo buzzer can even be placed away from the circuit in any
place where it can be easily heard.
It goes without saying that you must know what
you are doing as working with the household mains can be dangerous and remember
to switch off the mains breaker before doing any work on the electric wiring. Do
not attempt to install this circuit if you have doubts on its operation, connections
and relevant safety measures.
LIGHT BULB TIMER
you might have a need to keep a light on for a certain time, usually a few
minutes, and be sure that it switches off even if you forget to turn off the
switch. This could be useful in a cellar or in a closet. The circuit will switch
on a light bulb simply by pressing the push button. After a time of 3-7 minutes
it will switch off automatically. The long delay is achieved by partly using the
leakage current between anode and gate of the scr. This current is dependent
almost on anything: voltage, temperature, lamp power, scr device, etc., this is
the reason why the timing is not constant but for the intended application it is
not important. If the delay is too short you may increase the 220nF capacitor up
to 470nF. Too high a value will keep the light always on. It will work with
incandescent light bulbs only. its operation with electronic lamps is erratic
and the delay is only 1 or 2 minutes. The scr must be the sensitive gate type
and no other type was tested except the TIC106N.
The circuit is rather small and could be housed in the same case
as the push button, if there is enough room. Of course you have to substitute
the standard switch with a push button. Operation with a 110Vac mains has not
been tested although I expect that a 100μF
100V capacitor instead of the 47μF capacitor
should do the trick.
SCR AUDIO OSCILLATORS
follow a similar one shown in a previous page but operate at 5V,
and down to 2V (1.5V for the circuit with the transformer), and it is also louder. To set them properly
start with the trimmer at its highest resistance, connect the 5V supply
and slowly adjust the trimmer until oscillation sets in. Oscillation
frequency can be adjusted, within limits, using the same trimmer. The
loudspeaker must not have a DC resistance bigger than 5.5 ohm or else
the higher damping factor will prevent the scr from switching off and
it will latch on forever. This means that you cannot use a speaker too small,
less than 2 inch diameter. For the same reason it is proper not to use
any electrolytic capacitor. If, while setting it up, you find that the
scr has latched in the on state, temporarely remove the supply voltage
or temporarely make a short between anode and catode. In order to
operate it at 12V you must change the resistor from 82 to 120 ohm and
the trimmer from 1k to 2.2k with an additional 1Kohm resistor in
series. The circuit on the right uses an output
transformer, admittedly not readily available nowadays but was common
on old transistor radios and amplifiers some years ago. Operation and
setting is similar but you may need to invert the secondary winding in
order to have a better stability corresponding to a lower oscillation
frequency. In this last circuit it might be possible to use a low esr
electrolytic capacitor instead of the standard capacitor. If the
voltage is increased beyond 5V pay attention to the maximum current
flowing through the components, would the scr latch on. Other scr types should work well even if you might be required to adjust the trimmer value.
Here we offer some circuit configurations for night lights, other similar schematics are easily available from the web.
In these circuits we have used a plastic case from some old night
lights: the neon bulb and relative resistor have been removed to make
room for the circuits shown on the left. The first circuit has 4 leds
of any color, although the white ones seem to be the best option. Power
consumption is about 270mW. Please pay close attention to the leds
if you make a mistake in connecting just one of them, then all of them
will blow up on the spot. The only critical component is the line
capacitor, 220nF, which must be properly rated for the mains voltage, at least 400V.
The resistor in parallel with the capacitor has the purpose to
discharge it in a relatively short time to avoid any electric shock if the plug pins are touched soon after it is removed from the relevant socket.
This resistor is not required if the circuit is permanently wired to
the mains and there is no chance to touch the connecting pins. The 47nF
capacitor, together with the 560 ohm resistor is used to protect the
leds against voltage spikes due to lightning, switching operations of highly inductive loads on the same supply line or simply due to the connection of the night light in the relevant socket. All resistors are 1/4 or 1/2 W and the voltage rating for the 47nF capacitor is 50-100V. Perhaps the only drawback of this circuit is that there is too much light emanating from it; During
the night our eyes are accustomed to darkness and this circuit might be
too bright for our taste and it is therefore most suited to light up a
large area rather than a single room. 5mm, clear type leds were used in all these circuit.
second circuit is a less powerful version with two leds only and really
meager power consumption, just 11mW. This version is the one which more closely matches the concept of night light: low power, not a blinding light but enough for our purpose and a limited cost to build.
third circuit is a combination of the others and it is housed in a case
that comes complete with a switch; in fact, some commercially available
night lights have a switch to activate or deactivate them and here we
use the switch to choose between two power levels: a weak one and a
more powerful one. Power consumption is 15mW with the low power
setting, open switch, and 310mW with the high power setting, closed
switch. The power rating of commercially available night lights is
between 0.6 and 1W while the old fashioned type with the little neon
bulb is between 0.21 and 0.33W.
In this third circuit we used colored
leds: the resulting effect is pleasant because the room is bathed in a
light close to a white light but the source is colored.
the fourth circuit, a very small 80mAh NiMh battery was
added. Several tests were carried out to find the best matching among
the charging current, led color, number of battery elements, two in our
circuit, and the time the green led is shinining when there is no
mains. Normally, closed switch, the NiMH battery is charging while the
green led will avoid any overcharging effect. The switch comes handy
when you do not use the
night light thus avoiding the battery discharge. If you
wish, the green led can be exchanged with the blue led or you may use
white leds for all three of them with the advantage of a longer
illumination time when there is no mains. A very nice color
combination is to replace the red led with a yellow one and the green
led with a white one. Only three leds
were used in this circuit. This was necessary to reduce the excessive
reverse voltage across the red led. Power consumption is around 140mW.
Please note that the forward voltage of the green led will ultimately
set the battery voltage. This means that other green leds might give a
slight different battery voltage. The one used in this circuit would
battery voltage at 2.78V. This voltage becomes 2.8V with a blue led and
2.82V when white leds are used. With no mains, we have a range of three
days for the circuit with the white led connected to the battery and
about two days with the green led and one day with the blue led. The
will go on for twice or three times longer with ever decreasing
output.The light intensity is minimal but it is sufficient in
complete darkness. Remember that NiMh batteries have to go through 3 or
4 charge-discharge cycles in order to get the best efficiency.
The last circuit is a classic diagram with bridge rectifier (800V, 1A) and a higher capacity battery.
circuits operate from a 230Vac mains but they can be scaled to work
from a 110-115Vac mains by doubling the capacitors value and halving
the resistors value and voltage rating.
Full astern to main