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Notes and Errata
Multi-Purpose Fast Battery Charg
er; February and March 1998: After
testing three prototypes, we have
found that a few component changes are required to produce reliable
charging characteristics.
The 0.47µF capacitor between
pin 19 of IC1 and 0V should be
replaced with a 100µF 16VW electrolytic type. The polarity of the
component should be with the (-)
toward the outside of the PC board
and the (+) lead connecting to pin
19. This capacitance increase improves the detection of the NiCd &
NiMH fall in voltage at full charge.
The 0.18µF MKT capacitor at
pin 17 of IC1 should be reduced
to .0018µF. Its markings will either
show 1n8 or 182. The number of
turns on inductor L1 should be
reduced from 20 to 10.
The 1kΩ 0.5W resistor on the
cathode of ZD1 should be replaced
with a 2.2kΩ 0.5W type. Also the
470Ω 1W resistor between the
cathode of D3 and pin 12 of IC1
should be replaced with two 1kΩ
1W resistors in parallel.
Charging current is best determined by checking the charg
ing
this issue), you have not produced an
audio amplifier although it may well
produce an audible signal.
The normal output signal from a
CMOS chip is a switching waveform
with an amplitude almost equal to the
supply voltage of the circuit. A 5V
CMOS circuit will have 5V switching pulses. The CMOS chip cannot
deliver enough output current to effectively drive an 8Ω loudspeaker so
the usual practice is to connect a small
time of a discharged battery. If
charging time is too long, a slight
adjustment can be made to increase
the current by using a larger value
resistor at pin 2 of IC1. A 3.9kΩ
resistor should increase the current
by about 10%. If charging time is too
short, the battery is probably suffering from memory effect. Try running
the battery through a few discharge
(refresh) and charge cycles to bring
it up to full performance.
The timeout period can be increased to suit larger amp hour
batteries by increasing the value
of the 820pF oscillator capacitor at
pin 14 of IC1.
The wiring diagram on page 47
has two errors. The 1000µF adjacent to L1, between THS1 and
-VOUT should be 100µF 25VW. The
470µF capacitor between ZD1 and
D3 should be 1000µF 63VW (note
increase in voltage rating compared
to the circuit diagram).
On the circuit diagram, the 2.2kΩ
resistor at pins 12 & 13 of IC2a
should be 22kΩ to agree with the
wiring diagram. The 1kΩ resistor
feeding ZD1 should be 1/2W. There
should also be a 33kΩ pulldown
resistor at pin 6 to ground (this
resistor is on the wiring diagram).
transistor to boost the current. In fact,
the current through the loudspeaker
is generally limited by a resistor of
about 100Ω or so, to avoid destroying
the transistor.
Such a crude “amplifier” works
well when fed with CMOS or TTL
output signals but as you have found,
it doesn’t work at all, when asked to
amplify the small analog signal from
an earphone output or other source.
Not only does the amplifier need to
increase the voltage and current swing
(amplitude) of the signal, it must do so
without noticeable distortion.
This generally requires three or four
transistors, at the very least, together
with correct biasing networks, feedback and frequency compensation and
so on, in a practical audio amplifier.
Or you can do it with an IC. Either
way, the circuit will draw lots more
current than a single transistor switching stage. There just isn’t any simple
way around the problem if you want
a satisfactory audio amplifier.
Phantom power
wanted
I have a condenser microphone
but it needs phantom power to run.
My multi-track recorder has none. I
wonder if you have any project that
will give phantom power and connect
it to your mixer.
A few months ago I bought and built
the Digital Effects Unit described in
the February 1995 issue but I can’t use
it because when you talk or sing into
it, the delay has distortion. Any help
would be much appreciated. (N. M.,
Fairy Meadow, NSW).
• A preamplifier circuit featuring
“phantom power” for a microphone
was described in the May 1995 issue
of SILICON CHIP. This is available for
$7.00 including postage.
The Digital Effects Unit should not
suffer from distortion. Perhaps one of
the op amps is oscillating or the half
supply bias at pin 10 of IC1b, pin 12
of IC1a, pin 3 of IC1c and pin 5 of
IC1d is not correct. Check for about
+8V on each of these pins. Also try
100pF capacitors across the feedback
resistors of IC1a, IC1c and IC1d and a
10pF capacitor for IC1b. These should
be placed between pins 9 and 8 of IC1b,
pins 13 and 14 of IC1a, pins 2 and 1 of
SC
IC1c and pins 6 and 7 of IC1d.
WARNING!
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mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages,
you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed
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92 Silicon Chip
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