This is only a preview of the December 1996 issue of Silicon Chip. You can view 28 of the 104 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 "Build A Sound Level Meter":
Items relevant to "Build An 8-Channel Stereo Mixer; Pt.2":
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working voltages because the “tracking” carbon has a negative resistance
coefficient and therefore, only becomes visible at higher voltages, and
hence higher dissipation, when the
tracking resistance drops substantially.
(V. E., Highett, Vic).
• Australian Insulation standards for
household appliances are available
from Standards Australia – phone (02)
9746 4700 or fax (02) 9746 8450.
An insulation resistance of around
20MΩ for a washing machine is fairly
typical. The low resistance is unlikely
to be due to any mains filter but is due
to the insulation of the heating element
(if it has one) or the insulation of the
various timer switching contacts, solenoids and the motors.
In fact, washing machines with
heating elements often cause nuisance
tripping with core-balance safety
switches and it is a good idea to have
them protected separately from other
household circuits.
Smoking inductor
in power supply
We are having difficulties with the
40V/8A power supply meeting the
specifications outlined in your January/February 1992 issues. The unit is
smoking the inductor L1 which is the
17-745-22 powdered iron core wound
with 10 turns of 1.2mm enamelled
copper wire. This is heating up to the
point of it being a fire hazard if left to
draw more than about 4.5A for more
than a few minutes. (P. S., Perth, WA).
• It does seem strange that this problem is appearing after four and a half
years. However, the Neosid 17-745-22
core is not ideal for the job. The 17745-23 is better. Also reducing the
turns to four or five rather than six
will reduce core saturation.
Similarly, using a few 0.8mm wires
in parallel rather than 1 x 1.2mm
wire for winding should reduce resistive losses. Alter
natively, use an
ETD49 transformer core as per the
Nicad Charger in October 1995. Use
the 0.5mm gap and 10 turns of 4 x
0.8mm wire as shown on page 59 of
this issue. The secondary winding is
not required.
Notes & Errata
Woofer Stopper MkII; February 1996:
depending on which type of piezo
loudspeakers are used, they can pro-
Fence controller
needs more output
I would like to make some comments about two of your projects I
constructed. The first is the Electric
Fence Controller described in the
July 1995 issue. It was constructed
correctly and all components, including the ignition coil work OK.
Unlike electric fences on different
properties that I have been to, the
output of this device has very little
bite. I made changes to replace the
timing resistors so as to give a 0.1
second pulse every second and
increase the wattage of the 6.8Ω
resistor.
I also built the Engine Immobiliser described in December 1995
– a great idea. However, the 75V
zener diodes shorted after testing
and driving my car. This may be an
isolated incident but to make sure
I placed a 560Ω 0.5W resistor in
series with the new 75V zeners and
placed a 0.22µF 630VW capacitor
across the collector and emitter of
Q1. There has been no trouble since.
(D. C., Narangba, Qld).
• We should point out that Notes
duce audible clicking at the rate the
signal bursts to a high and low level.
This can be cured by adding a 47µF
16VW electrolytic capacitor between
the base and emitter of transistor Q3.
The positive side of the capacitor
connects to the base. The capacitor
effectively slows down the rate that the
burst signal rises and falls to eliminate
any audible noise in the speaker.
We should also point out that if
the tweeter drive level control (VR2)
is set too high, it can cause the same
symptom.
Minivox Voice Operated Relay; September 1994: diode D1 is shown with
the incorrect polarity on the overlay
diagram on page 33.
LPATS: Striking A Blow Against Lightning; November 1996: the text on page
8 and in Fig.1 on page 6 refers to parabolas as the paths of possible lightning
strikes. The term used should have
been “hyperbola”.
Engine Immobiliser; December 1995:
there have been reports of the zener
diodes in this circuit failing. In line
& Errata were published on the
Electric Fence Controller in the December 1995 issue. These noted that
Australian Standard AS/NZS 3129:
1993 now specifies a maximum
output voltage of 10kV instead of
5kV. In order to increase the output
voltage by the required amount, the
6.8Ω 1W resistor should be changed
to 1.2Ω 0.5W.
While the coil on-time for the
electric fence may need some minor
adjustment for different coils we
are inclined to the view that if the
coil does not give a good output
it is probably defective. We have
seen one kit version of our electric
fence controller where the circuit
was working correctly but the HT
output was non-existent. It turned
out that the coil was a dud.
As far as adding a resistor in
series with the zeners for the engine immobiliser, this cannot be
recommended as it will prevent the
zeners from protecting the MJ10012
transistor. Howev
er, in line with
our circuit practice for high energy ignition systems, the specified
zener diodes should be rated at 5W
instead of 1W.
with our circuit practice for high energy ignition systems, the specified
zener diodes should be rated at 5W
instead of 1W.
Video Transmitter/Receiver; October
1996: it has been pointed out that
some video camera modules have a DC
output instead of AC. If these are used
with the Video Transmitter it will not
work. The cure is to connect a 100µF
non-polarised electrolytic capacitor
in series with the input socket. This
can be wired directly between the
RCA input socket and the input on
the PC board.
Fuel Injector Monitor; August 1995:
we have recently seen a fuel injector
monitor in which only eight of the
LEDs would light instead of the full 16.
The problem is that differing switching
thresholds on the 4053 (IC2) can cause
faulty switching of the LM3914 dot/
bar modes.
If this occurs, the cure is to replace
zener diode ZD1 with a 1µF electrolytic capacitor, with its negative lead
SC
connected to pin 3 of IC5.
December 1996 99
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