THE ELECTRIC WAVE
This circuit was specifically designed to recharge
alkaline cells. The unusual connection of the transistor in each charging unit
will cause it to oscillate, on and off, thus transferring the charge accumulated
in the capacitor to the cell. The orange LED will blink for around 5 times a second for a
1.37V cell. For a totally discharged cell the
blinking is faster but it will decrease until it will
come to a stop when the cell is charged. You may leave the cell in the charger
as it will trickle charge and keep it at around 1.6V. To set the correct voltage
you have to connect a fresh, unused cell and adjust the trimmer until
oscillations set in, then go back a little until no oscillation is present and
the circuit is ready to operate. You should use only the specified transistors,
LED colors, zener voltage and power rating because they will set the final
voltage across the cell. A simple 9V charging circuit was also included: it will
charge up to around 9.3V and then keep it on a trickle charge: the green LED
will be off while charging and will be fully on when the battery is close to its
final voltage.
A 2.5VA transformer will easily charge up to 4 cells at
the same time although 2 only are shown in the schematic. In order
to minimize interference from one circuit to the other they have nothing in
common except the transformer and, in order to show a balanced load to the
transformer, half of the charging units will use the positive sinewave and the
other half the negative sinewave.
All types of alkaline cells can be recharged: it will
take 1 day for a discharged AA cell or 9V battery and up to several days
for a large D type cell. The best practice is not to discharge completely the
cell or battery but rather to give a short charge every so often although
admittedly this is not easy to achieve.
I tried successfully to recharge NiMH cells as well. Although the charging profile for these cells is quite different from alkaline cells, the circuit seems to work fine provided you do not leave them in the charger forever, because of the possibility of overcharging especially for the smaller batteries.
The mains transformer must be suited for the voltage available in each country: usually 230Vac or 115Vac.
A single
transistor is all you need for this simple inverter. The main aim of this
circuit is to provide a suitable supply for all kind of low power battery
chargers that normally connect to the mains such as mobile phones, electric
shavers, etc, even an electronic neon light rated at 5W was successfully connected. Only easily obtainable components are used. The transformer is a
standard 10VA mains transformer with two 6V windings connected as shown in the
schematic. Frequency of operation is between 70 and 190Hz depending on the
nature of the load. This frequency is acceptable by most devices but obviously
it is not suitable to drive frequency dependent appliances such as clocks or
small motors that depend on the mains frequency in order to operate reliably.
The transistor will not require any additional heatsink if it is assembled on
the metallic case provided for the inverter. The neon glow light will give a
useful indication, and warning, on the presence of a dangerous voltage at the
output. A 2.5A fuse on the input supply line would be a useful addition.
Operation is simple: switch on the unit and connect the load keeping an eye on
the neon glow light which should be always on: certain switching chargers demand
an initial peak current effectively shorting the output and switching off the
neon: in this case you have to try repeatedly to connect the load until it
works. A temporary short at the output and a temporary voltage reversal at the
input will not damage the unit. Efficiency was not a design parameter however it
was measured to be between 50 and 60%. If you have a 110V mains
transformer and consequently a 110VAC output you should change the 0.1μF
capacitor to 0.22μF, 400V. The waveform is only
vaguely sinusoidal. Invert the connection of
one of the 6V windings if oscillations do not set in.
Checking the
status of your car battery (accumulator) should be easier with this circuit
which measures the internal resistance of the battery. Pulses generated by the
555 are used to drive a dummy load and the AC voltage which develops across the
battery gives an indication of its internal resistance: the lower the voltage
the healthier the battery. The AC voltage is read out by means of a digital
meter connected at the output. Separate leads are used for the dummy load and
for the metering circuit. They should be connected to their respective battery
lugs but they should not touch each other. This avoids erroneous readings due to
less than perfect contacts of the dummy load. The internal resistance depends on
the battery temperature as well; this is the reason for the switch: hot
means a battery (not ambient) temperature between 35 and 52 degrees Centigrade, normal
is for a temperature between 16 and 34 degrees and cold is good for a
temperature from -4 to 15. Beyond these ranges the reading is unreliable. The
internal resistance depends also on the rated capacity of the battery. The 100
ohm potentiometer sets the battery capacity: it is rotated totally to positive for a 100Ah battery and totally to
negative for a 32Ah battery. A
dial with uniform markings from 32 to 100 was used in the prototype. This means we
can measure internal resistance of batteries rated from 32 to 100Ah. As there
are a number of smaller 12V batteries around, specially for alarm systems, a
switch was introduced that, in the X1 position, will change the capacity range
to 3.2 - 10Ah. The unit has six leads going out of the box: two for the dummy
load, two for the metering section and two going to the digital meter. Operation
is simple: set the range, temperature and battery rating, then connect the dummy
load and the metering leads to the battery lugs and read the ac voltage: you should
be safe if it reads below 10-12mV otherwise it is better to give the battery a good
recharge and if it is still beyond 10-12mV then probably you need a new
battery.
A bright orange LED shows that the unit is connected and in operation.
Full
short-circuit and overcurrent protection is given by this circuit suitable for
workbench applications in technical schools and laboratories where there is a
need to work directly with the mains. Additional features are a clearly visible red lamp
indicating that the voltage is present, good isolation of the output circuit
when the unit is off, only a few millivolts were measured with no load, current
threshold adjustable over a limited range and the possibility of remote cutout:
the 6V from the secondary can be taken anywhere, normally where you are working,
even far away from the protection circuit. Pressing the push button will
short-circuit the winding and the circuit will switch off thus removing the
mains voltage. A suitable led is placed together with the push button to show
whether the circuit is in operation or not. Additional remote cutout circuits
can be wired in parallel if so required. The circuit will switch off if a short
is applied at the output without blowing the fuse but it will blow if you try to
activate the circuit if a short is already present.
This
circuit will convert a standard relay to a pulse relay; pressing the button
will switch it on and pressing it again will switch it off. For this purpose you
need a relay with 2 sets of contacts: one is used for the circuit and the other
is available for an outside circuit. Sometimes it is difficult or impossible to
find a stepping relay, normally used in electrical wiring, and this is a viable
solution. The relay used in this circuit was a power relay with 10A contacts and
a coil resistance of 28Ω. The circuit will draw no
power when idle and it is possible to scale up the circuit to operate at a
higher voltage. The relay must be always rated at half the supply voltage, in
our case it is a 6V relay for a 12V supply. The resistor in series with the coil
must have a similar resistance as the coil or slightly higher and the other
resistor should be twice the coil resistance. All capacitors are 25V. The
capacitors value depends on the coil resistance: the higher the resistance the
lower the value. As it takes a certain time to charge the capacitors it is
necessary to wait about 0.5-1sec between one operation of the push button and
the next. An unregulated 12V power supply is adequate for this circuit.
If
you wish to have some really nice looking LED's shining out of your equipment
panel, you may try the following trick: pass repeatedly a fine sandpaper on the
surface of any transparent and clear LED until the same
surface is all worked
out to a
whitish look. 
There
is nothing else to do but to switch it on and enjoy the pleasant look of it. Do
not use the extra fine sandpaper as it will not cut deep enough in the LED
plastic material, in other words the sandpaper normally
used for metals is
not suitable.
As the difference
with a standard LED was remarkable I did some tests
in order to compare 
them:
picture 2 and 5 refer to the normal clear LED, red in
these tests, shining right in front of a screen and tilted at about 60°
respectively. The results were as expected: very bright when viewed on axis and
dimmer when off axis. The same LED (picture 1 and 4) after the
"treatment": it is slightly dimmer when viewed right in front but it
is much brighter when it is off axis and it gives a much better overall
appearance. Picture 3 and 6 refer to a standard diffused LED and as one can clearly
see, it is just too dim. The white part of the picture is where the
light is most intense and full of infrared light. As most digital cameras are quite
sensitive to infrared light, it is recorded as a white area. This is not really
a circuit but I thought to share it with you and unless you need the extra brightness
of a front shining LED you may use this trick with any clear LED, blue LED's
being especially attractive.
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