This is only a preview of the October 1995 issue of Silicon Chip. You can view 27 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. Articles in this series:
Items relevant to "Build A Compact Geiger Counter":
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Items relevant to "A Fast Charger For Nicad Batteries":
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A fast charger for
nicad batteries
This nicad charger lets you fast charge nicad
battery packs from a 12V car battery. It can
charge packs of 5 to 10 cells at once and
automatically reverts to trickle mode at the
end of the charging cycle.
By JOHN CLARKE
Nicad battery packs seem to have
a habit of going flat just when you
want to use them, particularly when
there is no readily available source of
mains power to operate a recharger.
Of course you can always call upon a
spare battery but what happens when
it also goes flat after some use?
This Extra Fast Nicad Charger is the
answer to your nicad battery problems.
It operates from a car battery so you
don’t need mains power and it can
recharge a nicad battery pack in far less
54 Silicon Chip
time than it takes using a conventional
charger.
At the maximum charge current of
4A, you can charge a 1.4Ah battery
in less than 30 minutes. For higher
capacity batteries, the charge time will
be longer but most batteries with less
than a 2Ah capacity can be charged in
under 45 minutes.
Sensing techniques
With high charge rates, nicad batteries can be damaged if they are not
charged correctly. As a result, the
Extra Fast Nicad Charger employs
four sensing methods to ensure that
charging ceases before any damage is
done to the cells. These are as follows:
(1). Over-temperature sensing: If
nicad cells are overcharged, they become hot and this causes cell damage.
To prevent this from happening, the
circuit monitors the temperature of
the battery pack using a thermistor
and switches the circuit to the trickle
charge mode if the temperature rises
above a preset level (45°C).
(2). Low voltage sensing: Nicad cells
that have been discharged to a very
low voltage can be damaged if initially
fast charged. The circuit prevents this
by monitoring the battery voltage and
initially trickle charging the battery
until it reaches a preset value. It then
automatically switches over to fast
charging.
(3). Voltage sensing: When a nicad
VP
12
VS
6
Vref
10
Rn
11
IB
5
NTC
3
CP
9
Vr1
In
SUPPLY
GND
16
Vhigh
PROTECTION
Vr3
MAINS
ON
RESET
V
LSP
Iref
>
t
AO
2
A2
Vr2
Vlow PROTECTION
Vr4
LS
4
A1
>
>
OSC
DISABLE TIME OUT
>
R
s+h
BATTERY
FULL
DETECTION
VAC
7
PWM
1
PWM
&
R
TIME
OUT
PROTECT
LED
15
>
R
1/10
OSC
TO
PWM
:1:2:4
PRESCALER
COUNTER
CONTROL
CURRENTLESS SENSING
AUX PULSES
13
OSC
8
PR
14
SYNC
Fig.1: block diagram of the TEA1100 which is designed specifically for nicad
battery charging. It includes automatic timeout and -dV detection circuitry and
features both linear and PWM outputs.
the LED flashes, the charger is in trickle charge mode.
Battery charger IC
battery pack is fully charged, further
charging causes its output voltage to
fall slightly. This slight voltage drop
is sensed using a method known as
-dV detection, at which point the circuit is switched to the trickle charge
rate.
(4). Automatic timeout: As a final
precaution, the circuit em
ploys a
timer which can be set to one of six
intervals ranging from 30-180 minutes.
If, for some reason, the battery voltage
does not drop within a certain time,
this timer automatically switches the
circuit to trickle charge mode.
Note that -dV detection can be
unreliable if it takes longer than one
hour to fully recharge a battery. This
is because the output voltage drops
very slowly after full charge at the
lower charging currents and may not
be detected.
The timer is a “belts-’n-braces”
feature – it’s there as a backup if the
-dV sensing circuit fails to detect full
charge.
compact plastic case. There are just
three switches on the front panel:
(1) an on/off switch; (2) a 6-position
rotary switch to set the timer (30-180
minutes); and (3) a 5-position rotary
switch to set the charging current
(1-4A).
A table on the front panel shows
the required switch settings for the
various battery capacities available.
These settings must be used in order
to prevent battery damage.
Also on the front panel is a LED
indicator which shows the charging
mode. When the LED is continuously
lit, the charger is fast charging. When
Main Features
•
•
•
Indicators & controls
As shown in the photos, the Extra
Fast Nicad Charger is housed in a
The circuit is based on a Philips
TEA1100 charger IC which is specifically designed for nicad cells. Its
schematic is shown in Fig.1.
Most of the IC circuitry is controlled
by a single oscillator which is used
for timeout counting, driving a pulse
width modulator (PWM) for switch
mode operation, and for various timing
processes.
These timing processes include a
periodic “quiet time”, during which
battery charging ceases so that its voltage can be measured without switchmode noise. In the trickle charge mode,
the PWM output is applied in short
•
Fast charging
Powered from a car battery
Charging stopped using three
detection methods: by monitoring battery temperature, drop in
battery voltage at full charge and
charging time
Five charging currents from 1-4A
•
Suits most battery packs with
5-10 cells
•
•
•
Charging indicator LED
•
Trickle charging after fast charge
Fuse protection for reverse polarity
and shorts
Short-circuit proof
October 1995 55
F1
10A
S1
12V
BATTERY
10
4700
50VW
4700
50VW
4700
50VW
0.47
+8V
4
1.8k
6
8
IC2
7555
2
6.8k
3
10
2.2k
0.1
C
A
Q3
BC328
D1
MBR735
K
A
56k
NICAD
BATTERY
N2
Q1
IRF540
D
0.1
5W
2.2k
3.9k
1
S2 : 1 : 4A
2 : 3.5A
3 : 2A
4 : 1.8A
5 : 1A
1k
3
2
CURRENT
SET
S2
2k
7
VAC
1.1k
4
5 IB
K A
EXTRA FAST NICAD CHARGER
K
MINUTES
S3 : 1 : 180
2 : 120
3 : 90
4 : 60
5 : 45
6 : 30
.001
TIME
SET
S3b
2
1
3 4
5
6
56 Silicon Chip
VS
6
NTC GND
3
16
TEMP SET
VR1 500k
*
DSE
R1797
100k
6
5
.033
10
47k
.001
11 220k
9
56k
27k
VREF 10
PR
8
1 TIME
SET
S3a
2
4 3
*OR 100k
Fig.2: the circuit of the charger. IC2, Q2 & Q3 together drive Mosfet Q3 and this
switches transformer T1 to form a boost converter. This steps up the 12V input
voltage to a level sufficient to charge as many as 10 nicads in a battery pack; ie,
a maximum of about 18V. The boost converter is under the control of IC1, the
TEA1100 battery monitor.
bursts for about one period on to 10
periods off.
Apart from the oscillator, the IC circuitry is also controlled by a resistor
which is connected between Vref (pin
10) and ground. This resistor sets up a
current reference (Iref) for the circuit.
The actual charge current is then set by
this reference current and the value of
an external current set resistor.
In operation, the IB pin monitors the
voltage across an external dropping
resistor which carries the nicad charge
current. This voltage is then fed to
internal op amp A1.
Any error will be amplified by A1
and compared with the oscillator
waveform in a PWM comparator. The
result is a pulse train at pin 1 with a
duty cycle varying according to the
error signal at the A1 output.
The CP input, pin 9, controls the
RN
CP
.0015
A
15
LED
4
LS
IC1
TEA1100
.01
B
E
C
VIEWED FROM
BELOW
12
V+
5
1 PWM
OSC
13
GD S
1
16VW
15k
C
4.3k
I GO
K
0.1
5W
S
E
10
16VW
LED1
N1
E 10W G
B
5
1
B
GND
100
16VW
OUT
T1
0.1
Q2
BC338
ZD1
16V
+12V 1W
0.47
+12V
0.1
IN
REG1
7808
output polarity at pin 1. In addition,
an analog output appears at pin 2 and
is used to control circuits employing
linear regulation. This latter output is
not used in this design, which employs
PWM control only.
During trickle charge, a resistor at
Rn (pin 11) controls the current into
the battery. The current is also deter
mined by the state of the PR pin (pin
8) which controls a prescaler to divide
the oscillator signal by 1, 2 or 4.
Pin 7, the VAC input, monitors the
voltage of the battery being charged.
For normal (ie, fast) operation, this
voltage must be between 0.385V and
3.85V and is set using a voltage divider
network to suit the batteries that are
to be charged.
A voltage on pin 7 that’s outside
this range initiates the trickle charge
mode. In addition, the battery full
detection circuitry initiates the trickle
charge mode when it detects a 1% fall
in battery voltage.
The NTC input at pin 3 is used
to monitor the voltage across a
thermistor. This is the temperature
sensing circuit. As shown, it drives
a couple of internal Schmitt trigger
comparators. When the temperature
of the battery pack exceeds a certain
value (ie, when the voltage at pin 3
drops below a critical level), one of
the Schmitt triggers toggles and the
current is reduced to trickle mode
(the other Schmitt trigger is used for
under-temperature sensing but this is
not a problem in Australia).
Finally, the LED output at pin 15
goes low when the IC is in fast charge
mode. Alternatively, this pin switches
between high and low (to flash the
LED) when the IC reverts to the trickle
charge mode.
Circuit details
Refer now to Fig.2 for the full circuit details. Apart from the TEA1100
This inside photo shows the general arrangement of the PC board in the case.
Note how mica washers have been used to set the gap between the transformer
cores (see text).
IC, it employs a 7555 timer (IC2) two
transistors, an N-channel Mosfet and
a 3-terminal regulator (REG1).
The resistors at pins 10 & 11 of IC1
set the reference currents for the fast
and trickle charge rates, as described
previously. The oscillator control
input is at pin 13 and this pin is
connected to ground via one of two
capacitors, as selected by S3b. When
the .0015µF capacitor is selected, the
oscillator runs at 25kHz. Conversely,
when the .001µF capacitor is selected,
the frequency increases to 37kHz.
S3a selects the prescaler value. In
positions 1 & 2, pin 8 is grounded and
the prescaler divides the oscillator
frequency by 4. Similarly, positions
3 & 4 set the prescaler to divide by 2
(pin 8 open circuit), while positions 5
& 6 set the prescaler to divide by 1 (pin
8 connected to the 4.2V bias voltage
Vs at pin 6).
Combined with switch S3b, S3a sets
the timeout period to one of six values:
30, 45, 60, 90, 120 & 180 minutes.
The NTC input (pin 3) is connected
to a thermistor and also to the 4.2V bias
voltage via VR1 and a 100kΩ resistor.
Normally, the thermistor resistance
is about 100kΩ at 25°C. However, as
the temperature rises, the thermistor
resistance falls and this reduces the
voltage at the NTC input.
If the voltage at the NTC input
falls below 0.8V, the IC immediately
switches to trickle mode and remains
there until the voltage increases to
about 0.9V. VR1 allows the temperature trip point to be adjusted, while
the adjacent .033µF capacitor prevents
false triggering by bypassing any high
frequency signals from the switchmode supply.
The PWM output at pin 1 of IC1 is
designed to drive a step-up converter
and this is based here on Mosfet Q1,
transformer T1 and Schottky diode D1.
In practice, however, the PWM wave
form at pin 1 is not suitable for directly
driving the Mosfet. This is because the
voltage does not swing sufficiently
high to fully turn on the device, nor is
the output current sufficient to charge
the gate capacitance of the Mosfet in
the time allowed.
To overcome this problem, 7555
October 1995 57
Q1
BC328
10
0.47
NICAD+
GND
T1
Q3
GND
0.1
D1
Q2
0.47
4700uF
+12V
0.1
5W
4700uF
1
56k
BC338
0.1
5W
15k
.0015.001
0.1
0.1
56k
1k
K
LED1
A
220k
27k
S3b
4700uF
1uF
S3a 1,2
10uF
REG1
4.3k
S2
WIPER
5
4
3
S2
2
1
Fig.3: install the parts on the PC board as shown in this wiring diagram, taking
care to ensure that all polarised parts are correctly oriented. Note that Q2 is a
BC338 transistor while Q3 is a BC328 type, so don’t get them mixed up.
timer IC2 is used as a buffer stage.
This is a rather unusual application
for a 7555 timer IC, since it does not
function as a timer at all. Instead, it is
used to convert the 0-7V PWM signal
to a 0-12V signal at its pin 3 output.
As shown on Fig.2, the upper
threshold of the 7555 is set to about
2V by the 6.8kΩ and 2.2kΩ resistors
at pin 5. This, in turn, sets the pin 2
threshold to 1V. As a result, when pins
6 & 2 are taken above 2V, pin 3 goes
low. Conversely, when the input goes
below about 1V, pin 3 goes high.
Because IC2 is powered from +12V,
it effectively converts the PWM
output from IC1 into a 0-12V signal.
It also inverts the signal and so to
maintain the correct output phase, the
PWM output from IC1 is inverted by
connecting a 56kΩ resistor to the CP
input at pin 9.
58 Silicon Chip
10
3.9k
.001
S3a 5,6
2.2k
47k
.01
VR1
1
2k
100k
1.1k
1
IC2
7555
ZD1
2.2k
100uF
S3a
WIPER
6.8k
NTC1
IC1
TEA1100
10
.033 NTC2
1.8k
10uF
The output from IC2 drives complementary pair Q2 & Q3 which in turn
provide the current pulses to drive
the gate of Q1.
Q1 is used to switch the N1 winding
of transformer T1. This transformer
has a turns ratio (N1:N2) of 1:1.7, to
provide sufficient voltage step-up for
recharging battery packs above 12V.
Diode D1, an MBR735 fast recovery
type, rectifies the transformer output
so that the battery is charged with the
correct polarity.
The charge current through the
batteries is sensed by the two 0.1Ω
5W resistors and the voltage developed across them is fed via one of
five resistors, as selected by S2, to op
amp A1 inside IC1 (at pin 5). This op
amp compares the voltage developed
across the current sensing resistors and
produces an error signal to control the
PWM oscillator. This, in turn, adjusts
the PWM output signal at pin 1 so that
the charging current is correct.
The .01µF capacitor at pin 5 filters
out any transient voltages which could
otherwise cause false current settings.
In addition, the output of error amplifier A1 is filtered using a 47kΩ resistor
and a .001µF capacitor at pin 4.
As the battery charges, its voltage
is monitored via a voltage divider
network (56kΩ & 15kΩ). The resulting voltage sample is filtered using
a 1µF capacitor and applied to pin
7 (VAC) of IC1. When the battery is
fully charged, the IC detects the slight
drop in battery voltage and automatically switches to the trickle mode as
described above.
Power for the circuit is fed from a
12V car battery via fuse F1. This fuse
protects against shorts and reverse
polarity connections. If the 12V battery is wrongly connected, an internal
reverse diode in Q1 will conduct and
blow the fuse.
tor and transistors Q2 & Q3 can now
be installed. Make sure that these parts
are all correctly oriented and don’t get
the two transistors mixed up. REG1
must be installed with its metal tab
towards ZD1.
Diode D1 and transistor Q1 are installed with their metal tabs towards
the edge of the board. Install them
with their mounting holes about 22mm
above the board, so that they can later
be bolted to the rear panel.
Next, install the capacitors on the
board, starting with the smaller devices and finishing with the three 4700µF
electrolytics. The temperature sensing
feature may not be needed for some
applications. If you don’t wish to use
it, connect a 100kΩ resistor across the
NTC1 and NTC2 terminals.
Winding the transformer
The PC board is secured to integral standoffs in the base of the case using four
self-tapping screws. Note the use of plastic cable ties to beep the internal wiring
neat and tidy.
The 12V rail is decoupled using
three 4700µF capacitors and two
0.47µF capacitors. These provide the
high current pulses required by T1. A
10Ω resistor and 16V zener diode ZD1
protect IC2 from high voltage transients on the 12V rail, while 3-terminal
regulator REG1 supplies 8V to IC1. In
addition, the output of REG1 supplies
power to LED 1, the other side of which
is connected to pin 15 of IC1 via a 1kΩ
current limiting resistor.
Construction
Most of the parts for the Extra Fast
Nicad Charger are mounted on a PC
board coded 14309951 and measuring
11
12
13
14
15
16
17
18
19
171 x 140mm. Fig.3 shows the parts
layout.
Begin by carefully checking the PC
board against the published pattern.
In particular, check for broken or
shorted tracks.
When you are satisfied that the
board is OK, begin the assembly by
installing PC stakes at all the external
wiring points (19 in all). This done,
install the wire links, followed by the
ICs and the resistors. Table 1 shows the
resistor colour codes but it is always a
good idea to check them using a digital
multimeter, as some colours can be
difficult to read.
The zener diode, 3-terminal regula-
Transformer T1 is wound using
0.8mm-diameter enamelled copper
wire – see Fig.4.
Begin by cutting four 1700mm
lengths of wire and soldering these to
pins 9, 8, 7 and 6 of the transformer
bobbin. This done, wind these four
wires together (ie, side-by-side) onto
the bobbin in the direction indicated
until you have completed 24 turns.
Terminate the free ends to pins 12,
13, 14 & 15 respectively (ie, 9 to 12, 8
to 13, etc), then insulate the winding
with a single layer of paper held with
insulating tape.
Next, cut two 3500mm lengths of
wire and connect these to pins 2 & 3.
These two wires are then wound on
together for 41 turns in the same direction as the previous winding – see
Fig.4. Terminate their free ends on pins
19 & 18 (ie, 2 to 19; 3 to 18) and again
CASE
20
HEATSINK
PRIMARIES
SECONDARIES
FINISH
FINISH
T1 WINDINGS VIEWED
FROM BELOW
NUT
WASHER
INSULATING
BUSH
TO3P (TO220)
DEVICE
MICA
WASHER
3mm SCREW
PRIMARY : 4x0.8mm DIA ENCW 24T
SECONDARY : 2x0.8mm ENCW 41T
PRIMARIES
START
10
9
8
7
6
SECONDARIES
START
5
4
3
2
Fig.4: this diagram shows the
winding details for transformer
T1 (see text).
1
Fig.5: here are the mounting details for Mosfet (Q1) and the
fast recovery diode (D1). They must be isolated from the case
and the heatsink using TO-220 mounting kits.
October 1995 59
F1
RED+
SOLDER
LUG
CORD
GRIP
GROMMET
D1
CORD
GRIP
GROMMET
Q1
D
RE
C1
NT
CK
LA
C2
-B
D
NT
CA
NI
BLACKNICAD+ RED
1
NTC2
NTC1
S1
B
12
1
A
5
1
2
K
LED1
S3
A
60 Silicon Chip
3
4
S2
Fig.6: use this diagram to
complete the wiring to the
switches and the PC board.
Tucked in behind the 4700µF filter capacitors are the T0220 Mosfet and fast recovery diode. The mounting details
for these devices are shown in Fig.5.
finish with a layer of paper held with
insulating tape.
The transformer can now be assembled by first inserting one ferrite core
half into the bobbin and installing its
metal retaining clip. The other ferrite
core half is then inserted and 0.5mm
spacers (eg, 4 x TO-220 mica washers)
slid in between the two halves to provide an air gap (see photo). The second
core half is then secured by installing
its retaining clip.
Once the transformer assembly has
been completed, it can be installed on
the PC board. Make sure that pin 1 is
adjacent to the 56kΩ resistor.
Final assembly
The Extra Fast Nicad Charger is
housed in a plastic case measuring 204
x 68 x 157mm. An aluminium panel
The charger has optional temperature monitoring of the
battery provided by a negative temperature coefficient
(NTC) thermistor.
measuring 194 x 65mm and a finned
heatsink (125 x 42 x 34mm) are fitted
at the rear.
Position the PC board in the case
and line up its mounting holes on the
four integral standoffs at the corners.
Use a large drill to shorten the unused
standoffs so that the PC board will sit
neatly in position. This done, secure
the PC board in place with self-tapping
screws, slide the metal panel into the
slot at the rear of the case, and mark
the positions for the Mosfet and diode
mounting holes.
Next, drill these holes in the rear
panel, along with holes for the two
cordgrip grommets and the fuseholder.
The heatsink is also secured with a
screw and nut at its centre. After all
the holes have been drilled, remove
any burrs, particularly around the
Mosfet and diode mounting holes, to
prevent punch-through of the insulating washers.
The heatsink can now be secured
to the rear panel using its central
mounting screw. Fit an earth solder
lug to this mounting screw and apply a
smear of heatsink compound between
the mating faces of the heatsink and
rear panel before the final assembly.
Fig.5 shows the mounting details for
the the Mosfet transistor and diode D1.
They each need to be isolated from the
panel using an insulating washer and
bush. If you are using mica washers,
use a smear of heatsink compound
between the mating faces before final
assem
b ly. If silicone-impregnated
glass fibre washers are used, no heatsink compound is necessary. When
you have tightened down the screw
TABLE 1: RESISTOR COLOUR CODES
❏
No.
❏ 1
❏ 1
❏ 2
❏ 1
❏ 1
❏ 1
❏ 1
❏ 1
❏ 1
❏ 2
❏ 1
❏ 1
❏ 1
❏ 1
❏ 3
Value
220kΩ
100kΩ
56kΩ
47kΩ
27kΩ
15kΩ
6.8kΩ
4.3kΩ
3.9kΩ
2.2kΩ
2kΩ
1.8kΩ
1.1kΩ
1kΩ
10Ω
4-Band Code (1%)
red red yellow brown
brown black yellow brown
green blue orange brown
yellow violet orange brown
red violet orange brown
brown green orange brown
blue grey red brown
yellow orange red brown
orange white red brown
red red red brown
red black red brown
brown grey red brown
brown brown red brown
brown black red brown
brown black black brown
5-Band Code (1%)
red red black orange brown
brown black black orange brown
green blue black red brown
yellow violet black red brown
red violet black red brown
brown green black red brown
blue grey black brown brown
yellow orange black brown brown
orange white black brown brown
red red black brown brown
red black black brown brown
brown grey black brown brown
brown brown black brown brown
brown black black brown brown
brown black black gold brown
October 1995 61
PARTS LIST
1 plastic case, 204 x 68 x
157mm
1 aluminium rear panel, 194 x
65mm
1 heatsink, 125 x 42 x 34mm
1 PC board, code 14309951,
171 x 140mm
1 self-adhesive front panel label,
190 x 60mm
1 Philips ETD49/25/16
transformer assembly:
2 4312 020 38041 3F3 cores;
1 4322 021 33882 bobbin;
2 4322 021 33922 clips
2 0.5 x 10 x 15mm spacers to
gap transformer (eg, 4 TO220 mica washers)
1 NTC thermistor (DSE Cat
R-1797)
1 3AG panel fuse holder (F1)
1 10A 3AG fuse
1 SPST rocker switch (S1)
(Altronics Cat S-3210)
1 single pole rotary switch (S2)
1 2-pole 6-position rotary switch
(S3)
1 bezel to suit LED1
2 20mm diameter knobs
1 small cordgrip grommet
1 large cordgrip grommet
1 solder lug
2 TO-220 mounting kits
1 30A red alligator clip
1 30A black alligator clip
1 15-metre length 0.8mm
enamelled copper wire
1 2-metre length automotive twin
polarised cable
1 1-metre length red hookup
wire
1 1-metre length black hookup
wire
1 1-metre length yellow hookup
wire
1 1-metre length green hookup
wire
1 60mm length of 0.8mm tinned
copper wire
19 PC stakes
2 25mm long x 3mm dia screws
6 cable ties
and nut, use a multimeter (set to a
high “Ohms” range) to confirm that the
metal tab of each device is correctly
isolated from the panel.
Work can now be done on the front
panel. Use the label as a guide for
positioning the power switch, LED
bezel and rotary switches. Drill out
the holes for these items, then affix
the label and cut out the holes with a
sharp utility knife.
This done, mount the switches and
LED bezel on the front panel and complete the wiring in the case. If the wires
passing through each grommet on the
rear panel are not gripped securely, use
some heatshrink tubing to increase the
cable diameter. Use cable ties to keep
1 IRF540 N-channel Mosfet (Q1)
1 BC338 NPN transistor (Q2)
1 BC328 PNP transistor (Q3)
1 MBR735 Schottky diode (D1)
1 16V 1W zener diode (ZD1)
1 5mm green LED (LED1)
Capacitors
3 4700µF 50VW PC electrolytic
with support pin
1 100µF 16VW PC electrolytic
2 10µF 16VW PC electrolytic
1 1µF 16VW PC electrolytic
2 0.47µF MKT polyester
3 0.1µF MKT polyester
1 .033µF MKT polyester
1 .01µF MKT polyester
1 .0015µF MKT polyester
2 .001µF MKT polyester
Resistors (0.25W 1%)
1 220kΩ
1 3.9kΩ
1 100kΩ
2 2.2kΩ
2 56kΩ
1 2kΩ
1 47kΩ
1 1.8kΩ
1 27kΩ
1 1.1kΩ
1 15kΩ
1 1kΩ
1 6.8kΩ
3 10Ω
1 4.3kΩ
2 0.1Ω 5W
Semiconductors
1 TEA1100 nicad battery monitor
(IC1)
1 7555 CMOS timer (IC2)
1 7808 3-terminal regulator
(REG1)
Fig.7 (below): this full size artwork
can be used as a drilling template for
the front panel.
the wiring neat and tidy.
Terminate the 12V battery leads
with 30A battery clips and the nicad
leads with the correct plug for your
battery. The thermistor can be permanently soldered to the NTC output
lead or a small 2-pin connector plug
connected to the lead end. In the first
case, use heatshrink tubing on the
leads to prevent shorts. In the second case, the thermistor is installed
EXTRA FAST NiCad CHARGER
60
90
45
30
1.8
120
180 1
+
2
Amps
1
Mins
3.5
4
+
180
120
1.4AH
2.4AH
1.8AH
62 Silicon Chip
TIMEOUT
(Mins)
CHARGE CURRENT
(Amps)
2
3.5
4
4AH
90
1AH
60
600mAH 1.2AH
45
500mAH 800mAH 1AH
30
POWER
1.8
2AH
2AH
4AH
1.4AH 2.4AH
1.8AH 2AH
1.2AH 1.4AH
BATTERY CAPACITY
Fig.8: this is the full size artwork for the PC board. Check your board carefully
for possible etching defects before installing any of the parts.
in the nicad battery package with a
corresponding 2-pin socket ready for
connection every time the nicad is to
be charged. Fit a short length of heat
shrink tubing over the thermistor to
prevent it shorting to the nicad case.
If the thermistor is not permanently
installed inside the nicad pack, we
recommend using either masking tape
or an elastic band to hold it in contact
with the cells during charging.
Testing
Apply 12V to the input terminals
and check that there is +8V between
pins 12 and 16 of IC1. There should
Specifications
Maximum charge current ������������������������������������������������������������������� 4A
Charge current ranges (A) �����������������������������������������������4, 3.5, 2, 1.8, 1
Charging times (mins)........................................30, 45, 60, 90, 120, 180
-dV detection ��������������������������������������������������������������������������������������1%
Trickle charge current.......................... 5% of main charge current for 30
and 45 mins timeout; 2.5% for 60 and 90 mins
timeout; 1.25% for 120 and 180 mins timeout
Thermistor cutout temperature ������������������������������������������������������<45°C
Input voltage............................................................................ 11-14VDC
also be about +4.2V at pin 6 and +12V
at pins 4 and 8 of IC2. The LED should
be glowing dimly. If not, check the fuse
and your component placement and
wiring. If the transformer makes a high
pitched squeal, check the transformer
windings – they are probably wound
with incorrect phase.
Short out the nicad battery output
leads and check that the LED flashes.
The standby current with the nicad
output leads shorted is about 16mA.
Now the unit is ready to test by
charging a battery. Switch off the
power and connect a discharged nicad
battery to the output leads. Select the
requisite timeout period and charge
current. Apply power and check that
the battery charges within the allotted
time. Note that the charger will not
operate if the NTC output leads are
disconnected from the thermistor.
During charging at the higher current levels, the heatsink and transformer windings will run hot. This
SC
is normal.
October 1995 63
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