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.
|
R Load |
VAC |
VDC |
|
100 KΩ |
5.1 |
4.67 |
|
47 KΩ |
4.22 |
3.29 |
|
22 KΩ |
2.77 |
2.06 |
|
10 KΩ |
1.41 |
1.1 |
|
4.7 KΩ |
0.68 |
0.56 |
The
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.
The
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.
Frequency
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.
The 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.
This 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.
Since
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.
The new 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.
Occasionally
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.
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.
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