This is only a preview of the November 1994 issue of Silicon Chip. You can view 29 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "A Novel Alphanumeric Clock":
Items relevant to "80-Metre DSB Amateur Transmitter":
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Build a dry-cell
battery rejuvenator
Are you sick of throwing away those AAsize dry batteries? Well, don’t – rejuvenate
them instead with this Dry-Cell Rejuvenator.
Depending on the state of the cells, you could get
up to 10 times their rated life & save big money!
By DARREN YATES
That’s right – this circuit allows you
to rejuvenate dry-batteries. Of course,
you’ve read the warnings printed on
dry battery cases quite a few times.
While the exact wording may differ
from brand to brand, they all say
much the same thing: “do not charge
this cell”.
It’s true that placing a dry cell into
14 Silicon Chip
an ordinary nicad charger will create
serious problems. These constant
current chargers cause heat which
produces steam and pressure, and this
can easily burst the battery casing.
This Dry-Cell Rejuvenator overcomes that problem by using a
charging technique that results in
very little heat production, thereby
greatly reducing internal stress. It
can recharge dry cells up to 10 times
and save your hard-earned dollars in
the process.
In addition, this circuit helps our environment. We currently throw away
millions of dry cells each year –cells
that eventually rust out and release
their chemical cocktail of electrolytes.
So reducing the number of cells we
throw away provides definite environmental benefits.
Both alkaline and conventional
carbon-zinc batteries can be recharged
by the Dry-Cell Rejuvenator. And
because it employs two independent
(and identical) charging circuits, it
can charge either a single cell or two
cells at the same time. The circuit runs
off a standard 12VDC 300mA plug-
+9V
4.7k
D2
1N914
1M
100k
10k
4
IC1a
2 LM324
15k
100k
1
1k
11
100k
6
10k
7
IC1b
A
LED1
CHARGE
10k
K
E
10k
5
10k
Q3
BC547
B
2x1N914
D3
D4
C
1.5V
CELL
100
Q4
BC547
C
B
E
10k
E
10k
.01
Q2
BC327
C
4.7k
Q1
100
BC547
C
1W
B
68k
3
E
B
100k
2.2
25VW
+9V
4.7k
D5
1N914
1M
100k
15k
13
IC1c
100k
14
1k
10k
100k
9
10k
8
IC1d
A
LED2
CHARGE
10k
K
E
10k
10
10k
10k
.01
Q6
BC327
C
4.7k
Q5
100
BC547
C
1W
B
68k
12
E
B
Q7
BC547
B
2x1N914
D6
D7
C
100
Q8
BC547
C
B
1.5V
CELL
E
10k
E
100k
2.2
25VW
D1
1N4004
B
E
C
E
B
C
VIEWED FROM BELOW
I GO
IN
12VDC
300mA
PLUG-PACK
100
16VW
7809
GND
OUT
+9V
100
16VW
DRY-CELL REJUVENATOR
Fig.1: the circuit employs two identical sections to individually charge two 1.5V cells. IC1a is a Schmitt
trigger – when the battery voltage is low, its output is high & Schmitt oscillator IC1b drives Q1 & Q3.
These transistors in turn switch complementary pair Q2 & Q4 to provide the charge/discharge cycles.
pack and will recharge an alkaline
cell in about 18 hours. Conventional
zinc-based cells are recharged in
around 12 hours.
Note however, that a recharged cell
will not have “as-new” capacity. Provided the cell is in good condition, it
will typically recharge to about 60%
of the new capacity, at least for the
first 7-8 cycles for an alkaline cell
and 3-5 cycles for a zinc-carbon cell.
After that, its performance will begin
to deteriorate quite markedly.
As a point of interest, a recharged
alkaline cell will have greater capacity
than an equivalent-size nicad cell,
with the added benefit that it charges
up to 1.6V. This figure is equivalent to
new cell voltage and is much higher
than a nicad’s 1.2V rating.
A feature of the unit is that it is very
easy to use – you simply switch it on,
slip the battery into its holder and,
after about a second, the circuit will
decide if that battery can be charged.
If so, an indicator LED on the front
panel will light up and the battery
will be charged until its voltage rises
above 1.65V. At this point, the circuit
automatically switches into trickle
mode and the indicator LED goes out
to signal the end of the charging cycle.
Faulty cells
What happens if you attempt to
charge a cell that has gone open circuit
November 1994 15
PARTS LIST
1 PC board, code RAT002, 102
x 57mm
1 zippy box, 130 x 68 x 41mm
1 front panel artwork
1 12VDC 300mA plugpack
2 “AA” size cell holders
1 2.5mm DC panel mount socket
1 Mini-U heatsink
Semiconductors
1 LM324 quad op amp (IC1)
6 BC547 NPN transistors
(Q1,Q3,Q4,Q5,Q7,Q8)
2 BC327 PNP transistors
(Q2,Q6)
1 7809 9VDC regulator (REG1)
1 1N4004 rectifier diode (D1)
6 1N914 signal diodes (D2-D7)
2 red 5mm LEDs (LED1,LED2)
Capacitors
2 100µF 16VW electrolytics
2 2.2µF 25VW electrolytics
2 .01µF 63VW MKT polyesters
Resistors (0.25W, 1%)
2 1MΩ
14 10kΩ
8 100kΩ
4 4.7kΩ
2 68kΩ
2 1kΩ
2 15kΩ
4 100Ω 1W
Miscellaneous
Light-duty hookup wire, machine
screws & nuts, washers, solder.
16 Silicon Chip
5
1
VOLTAGE
or high impedance? In the first case,
the circuit will remain in trickle mode
and the indicator LED will stay out.
The same goes for a cell that’s already
fully charged.
So if the circuit refuses to start when
you install a cell, check its output
voltage. If the voltage is close to 0V,
that cell has passed the point on no
return and should be discarded.
On the other hand, the circuit will
attempt to charge cells that have discharged to a low voltage (ie, below 1V)
and, as a result, have a high internal
impedance. Cells in this condition will
charge to 1.6V very quickly however,
typically in less than five minutes,
after which the circuit switches to
trickle mode. The cell then quickly
loses its charge so that, after a few
minutes more, the circuit reverts to
the full charging mode again.
Any cell which causes the circuit
to exhibit this behaviour should also
be discarded, since it is obviously
Fig.2: the charge/discharge waveform
used by Hollows in 1955. The charge/
discharge ratio was about 5:1.
incapable of holding any worthwhile
charge.
General guidelines
In order to get the most out of the
Dry-Cell Rejuvenator, there are several important guidelines that must be
followed. Let’s take a look at these.
First, never let the cell voltage fall
below 1.0V. This is basically a cell’s
“point of no return”. If its output
voltage falls below this figure, it will
generally not hold a sufficient charge
to make recharging worthwhile.
Second, recharge the cells as soon
as possible when they go “flat”. The
longer they are left lying around, the
harder it is for the Rejuvenator to recharge them. Similarly, use them again
as soon as possible after recharging,
otherwise they will begin to deteriorate. This fast recycling technique
will allow you to get the most out of
your batteries.
Third, don’t leave a battery on
charge for more than two days (48
hours). If a battery hasn’t charged up
in this time, it can be considered a lost
cause and should be discarded. If you
persist for longer than this, heat will
slowly build up and some lesser-quality batteries may begin to leak.
Finally, if a cell does begin to leak
as a result of charging or was already
leaky, it should be discarded at once.
The fluid discharge from a leaky cell
is highly corrosive and can damage
valuable equipment.
Note that the Dry-Cell Rejuvenator
works best on alkaline and heavy-duty (or super heavy-duty) zinc-carbon
cells, so you are definitely better off
spending a little extra for these types.
Warning: under no circumstances
should you try to recharge lithium
batteries.
Charging principle
The charging principle relies on
the chemistry inside the cell. If a carbon-zinc cell is charged with plain
DC, the zinc is returned to the negative
electrode in spongy blobs. Although
this results in a cell with reasonable
output voltage, it will also have a high
internal impedance. Hence, it will be
unable to deliver the expected power
to the load.
Much of the initial study into dry
cell recharging was done nearly 40
years ago by R. Hollows and the results
published in a 1955 edition of “Wireless World”. Hollows found that if the
cell was charged using “dirty DC”, the
zinc was distributed more evenly and
compacted on the casing. The result
was a cell which resembled its original
charged state. A similar process occurs
in alkaline cells.
In this case, the term “dirty DC”,
refers to a half-wave rectified DC voltage with a small negative offset. Fig.2
shows the details. When applied to a
battery, this resulted in a 5:1 charge/
discharge ratio; ie, the battery was
charged during the positive half cycle of the waveform and discharged
during the much shallower negative
half cycle.
In effect, the principle could be
called “five steps forwards and one
step back”.
Hollow’s work was based on a circuit which used a 3VAC transformer,
an item not commonly found these
days. In addition, Hollow’s circuit
would not have been the most efficient
way of recharging a dry cell, due to the
low frequency of the charging waveform (50Hz). This circuit overcomes
those problems by using a square-wave
oscillator to generate the charging
waveform and by operating at a much
higher frequency (4.5kHz).
Circuit details
Fig.1 shows the circuit details for
the Dry Cell Rejuvenator. As already
mentioned, it consists mainly of two
identical charging circuits, one for
each cell. These two circuit sections
are powered from the plugpack via
reverse polarity protection diode D1
and a 3-terminal regulator which delivers a 9V rail.
IC1a is one-quarter of an LM324
quad op amp and is connected as a
Schmitt trigger. The 68kΩ, 15kΩ and
1MΩ resistors set the reference voltage
on pin 3 to approximately 1.6V, while
the inverting input (pin 2) monitors the
battery voltage via a 100kΩ resistor and
a 2.2µF filter capacitor.
If the cell voltage is less than the
LED1
CELL 1
LED2
CELL 2
Fig.3: install the parts on
the PC board & complete
the wiring as shown here.
Be sure to use the correct
transistor at each location
& note that a small finned
heatsink is bolted to the
metal tab of the 7809
3-terminal regulator.
Q5
reference voltage, pin 1 of IC1a switches high and lights LED 1 to show that
charging has begun. At the same time,
pin 5 of op amp IC5b is biased to about
half supply via a voltage divider consisting of two 100kΩ resistors. This op
amp is connected as a Schmitt trigger
oscillator. When pin 1 of IC1a switches
high, IC1b oscillates at a frequency of
about 4.5kHz and with a 50% duty
cycle.
The square-wave output from IC1b
appears at pin 7 and drives transistor
inverter stages Q1 and Q3. These
transistors, in turn, switch the main
output devices (Q2 and Q4) on and off.
In effect, Q2 and Q4 function as a
complementary output pair. When
pin 7 of IC1b goes high, Q1 and Q3
turn on, Q4 turns off and Q2 turns
100
100
10k
.01
.01
D3
Q3
4.7k
4.7k
10k
Q4
10k
100k
68k
D2
100k
100k
10k
10k
Q2
15k
1M
10k
100uF
1k
2.2uF
100k
D7
10k
100k
100k
100k
100
Q7
10k
1
1M
68k
7809
100k
10k
10k
D6
4.7k
1k
15k
D5
Q8
12VDC
PLUG-PACK
100uF 2.2uF
IC1
LM324
4.7k
Q6
100
D1
D4 Q1
10k
10k
10k
10k
on and supplies charging current to
the cell. Subsequently, when pin 7
of IC1b goes low, Q1 and Q3 turn off
and so Q2 also turns off to end the
charging pulse.
At the same time Q4 turns on, since
diodes D3 and D4 are now forward biased via a 10kΩ pullup resistor (more
on these in a moment). The cell now
discharges through Q4 and its associated 100Ω collector resistor.
Because oscillator IC1b has a 50%
duty cycle, Q2 and Q4 also operate
with a 50% duty cycle. This means
that the cell is charged for half the
time and is discharged for the other
half of the time. However, when Q2
turns on, its 100Ω collector resistor
has about 7.5V across it, whereas when
Q4 turns on its 100Ω collector
resistor only has about 1.5V
(ie, the cell voltage) across it.
As a result, about 75mA flows
through Q2 to charge the cell,
while only about 15mA flows
through Q4 to discharge it.
This means that the
charge-discharge ratio works
out to be about 5:1, although
this will vary somewhat according to the cell voltage.
Trickle mode
As the battery charges, its voltage is
monitored by pin 2 of IC1a. When it
exceeds 1.6V (the reference set on pin
3), pin 1 of IC1a switches low to about
0.7V and this set the bias applied to pin
5 of IC1b to about 0.35V. As a result,
IC1b changes its output to a low-duty
(1:10) square-wave with a frequency
to about 2.2kHz.
This change in frequency (from
4.5kHz to 2.2kHz) is due to the different bias, while the lower duty cycle
is partly due to the Schmitt trigger
action and partly due to asymmetry
in the output of IC1b. In operation,
IC1b’s output (pin 7) swings closer to
RESISTOR COLOUR CODES
❏
No.
❏ 2
❏ 8
❏ 2
❏ 2
❏
14
❏ 4
❏ 2
❏ 4
Value
1MΩ
100kΩ
68kΩ
15kΩ
10kΩ
4.7kΩ
1kΩ
100Ω (5%)
4-Band Code (1%)
brown black green brown
brown black yellow brown
blue grey orange brown
brown green orange brown
brown black orange brown
yellow violet red brown
brown black red brown
brown black brown gold
5-Band Code (1%)
brown black black yellow brown
brown black black orange brown
blue grey black red brown
brown green black red brown
brown black black red brown
yellow violet black brown brown
brown black black brown brown
not applicable
November 1994 17
Take care to ensure that the DC socket is wired to suit the plugpack, so that the
correct supply polarity is applied to the board. The external wiring connections
to the board can be made via PC stakes.
ground than to the positive supply rail
and this situation is exaggerated when
the input threshold is pulled low.
What happens now is that IC1b delivers a train of narrow positive-going
pulses and these briefly pulse Q1 and
Q2 on to trickle charge the battery.
Q4 remains off in this mode, however. That’s because D2 now clamps
Q3’s collector to a maximum of 1.4V
(remember that pin 1 of IC1a in now
at 0.7V) and this, coupled with the
voltage across D3 and D4, means that
there will be insufficient bias to turn
Q4 on.
This means that the battery is not
discharged for part of the time when
the circuit in trickle mode.
If the battery voltage now subsequently falls below the reference
voltage, pin 1 of IC1a switches high
again and the circuit reverts to its full
18 Silicon Chip
charge/discharge mode of operation.
In this mode, D2 is reverse biased and
so Q3 is now able to turn Q4 on and
off to provide the discharge cycle, as
described previously.
Note that because IC1a is connected
as a Schmitt trigger with about 100mV
of hysteresis, the circuit is effectively
prevented from oscillating when the
cell voltage reaches the reference voltage on pin 3. Instead, the cell voltage
must fall from 1.6V to 1.5V before
the circuit will revert to full charging
mode and must then reach 1.6V again
before reverting to trickle mode.
If the cell is removed, the circuit
behaves as if a fully charged cell is in
position; ie, it switches to trickle mode
and the LED goes out. That’s because
the 2.2µF capacitor on pin 2 of IC1a is
charged almost to +9V (via the 100kΩ
feedback resistor) by the current pulses
generated each time Q2 turns on.
The second charging circuit, based
on op amps IC1c and IC1d and transistors Q5-Q8, functions in exactly the
same manner.
Construction
Most of the parts are installed on a
PC board measuring 102 x 57mm and
coded RAT002. Begin construction by
fitting PC stakes to the external wiring
points, then install the various parts
as shown on Fig.3. The resistor colour
codes are shown in the accompanying
table but we also recommend that
you check each value using a DMM,
as some colours can be difficult to
decipher. Take care to ensure that all
semiconductors are correctly oriented
and don’t get the transistors mixed
up – Q2 and Q6 are BC327 PNP types,
while the rest are BC547 NPN types.
The 7809 regulator must be installed
with its metal tab adjacent to the edge
of the board – see photo. It is fitted with
a TO-220 Mini-U heatsink to aid heat
dissipation.
Once the board has been completed,
it can be installed in a plastic zippy
case measuring 130 x 68 x 41mm. Use
the board as a template to mark out its
mounting holes, then drill the holes to
3mm along with a hole in one end of
the case to accept the power socket.
This done, attach the front panel label to the lid and drill out the
mounting holes for the battery holder
and the two LEDs. The latter should
be made just large enough so that the
LEDs are a push fit. Finally, mount the
various items in position and complete
the wiring as shown in Fig.3. The PC
board is secured using machine screws
and nuts, with additional nuts used
as spacers.
Take care to ensure that the LEDs are
wired with the correct polarity. The
anode lead of each LED is the longer
of the two.
Testing
Before switching on, temporarily
disconnect one of the leads to the DC
power socket and connect a multimeter set to milliamps across the break.
This done, apply power and check
Where to buy a kit of parts
The Dry-Cell Rejuvenator is only available from RAT Electronics. Complete
kits, including all specified components, instructions, case, front panel and
12V DC plugpack, are available for $44.95 ($39.95 without plugpack). Please
add $5.05 for postage and packaging for delivery within two weeks.
To place your order, phone or fax RAT Electronics on (047) 77 4745 or
send your cheque/money order to: RAT Electronics, PO Box 641, Penrith,
NSW 2750.
Note: copyright (c) 1994 RAT Electronics. Copyright of the circuit and PC
board art associated with this project is owned by RAT Electronics.
that the current drawn by the circuit
is about 10mA with no cells in place.
Note that both LEDs should flash briefly when power is applied.
If you now install a single “flat” cell,
the circuit should switch to charge
mode – the appropriate LED should
light to indicate that charging it taking
place and the current drain should rise
to about 50mA. This should increase
to about 90mA if a second “flat” cell is
installed. Check also that the second
LED is now lit.
If you don’t get the correct current
readings, switch off immediately and
check the board carefully for incorrect
parts placement or orientation. Check
also that the 7809 3-terminal regulator
is delivering +9V and that this voltage
appears at pin 4 of IC1 and on the
emitters of Q2 and Q6.
Finally, remember that a dry cell
should not be discharged below 1V if
it is to be successfully recharged and
don’t leave any cell on full charge
for more than 48 hours – if it hasn’t
charged up in this time, it can be considered defunct.
That’s it – you are now ready to
start recharging those expensive dry
batteries and do your bit for the enviSC
ronment as well.
AC/DC digital clamp meter
with 4000 count display
and bargraph!
● High speed auto-or manual ranging
● High speed sampling for 40 segment
bargraph display
● Average, Temperature test, Max hold,
Peak hold functions
● Sleep mode to reduce battery con-
sumption
● Continuity beeper, Data hold, Diode
test and analog signal output
● Battery or AC adaptor operation
Brief Specifications
Functions : AC/DC current, AC/DC voltage, Ohms,
Continuity, Diode test, Frequency, Temp, Data/
Peak/Max hold, Average., Analog signal output
Display :
LCD 3.5 digits, 4000 (Hz: 9999) count
Bar Graph Display : 40 segments
Ranges :
Auto or manual ranging
Aac, Adc : 400, 1000A
Vac, Vdc :
40, 400, 650V
Frequency : 10.0-999.9Hz
Temperature : -50.0 to +150°C
Jaw Opening : 55 mm ø or 65 x 18mm busbar
Withstand Voltage: 2.5kVac, 1 minute
Lloyd’s Register
Quality Assurance
to ISO-9001
2343 – one of the NEW Generation of Multimeters from
Centrecourt D3, 25-27 Paul Street North, North Ryde
Call Robyn for more information on (02) 805 0699
or fax : (02) 888 1844
November 1994 19
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