This is only a preview of the October 1996 issue of Silicon Chip. You can view 24 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 "Send Video Signals Over Twister Pair Cable":
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Low Voltage Rails for Plastic Power Amplifier
I am about to start the “Plastic
Power Amplifier” featured in the
April 1996 issue of SILICON CHIP
and would be obliged if you would
let me know the following:
(1) Would it be OK to use trannies
I have, which are 300VA toroidals,
giving a 40 volt rail, without compromising the circuit?
(2) If OK, I guess the setting of the
quiescent would remain the same?
(3) Approximately what output
would I have – 100 watts?
(4) Do you use only off-the-shelf
devices and parts; eg, matched
pairs, o/p devices?
Many thanks for your previous
articles. I hope the Plastic Power
amplifier is as good as my SILICON
the same thing would happen if a car
thief was to leave the wires connected
after trying unsuccessfully to steal my
car. I would then return to the carpark
and find nothing but a pile of molten
metal! I am confident that the circuit
was constructed and installed correctly. It operates fine over any period of
time when the key is in accessory position. However, when the ignition is on,
+12V is present at the coil connection
and the circuit overheats.
Could this be a design fault? If so,
how could it be rectified? (T. V., North
Adelaide, SA).
• We think that your unit may be
malfunctioning. When transistor Q1 is
turned on, the current through the coil
should be no more than about four or
five amps, as determined by the coil
resistance and its associated ballast,
if it has one. Hence, the transistor
should only be dissipating about 6 or
7W, when it is turned on.
However, the transistor is only
turned on for about 0.7 seconds in 2.9
seconds (0.7s on, 2.2s off) or 24% of
the time. Therefore, even if the circuit
is powered up continuously, the power
transistor should only dissipate less
than 2W. This will make it hot but
is not likely to be enough to melt the
plastic case or your car!
It should be possible to check for
correct operation of the Immobiliser
with it out of the car. Connect a 1kΩ
resistor between the collector of Q1
and the +12V supply and then apply
94 Silicon Chip
CHIP amplifier using boards SC111287 (December 1987) which more
than compares with a Tandberg
amplifier I had (to me anyway). (H.
M., Balga, WA).
• You can run the amplifier with
40V rails; the quiescent current
setting would be the same. However, the power output would be
markedly reduced, to around 50W
into an 8Ω load or 100W into 4Ω
loads.
We have not bothered to use
matched output devices in our
circuits because they are generally
not readily available. However, the
use of matched pairs can produce
a slight reduction in the harmonic
distortion of an amplifier.
power. Use your multimeter to measure the voltage between collector and
emitter of Q1. It should be +12V (or
close to it) for 2.2 seconds, then close
to 1V for 0.7 seconds, and so on. If it
does not follow this sequence, check
the operation of Q2 and the 555.
Zener diode tester has
incorrect transformer
I purchased a zener diode test kit
from Dick Smith Electronics and have
noted a couple of changes to the original circuit as published in the March
1996 of your magazine:
(1) T1 has prewound secondary winding with 136 turns and not 40 as per
circuit. The primary winding was 18
turns and not 20 as per circuit and this
was for the constructor to wind. The
transformer in your article was stated
to be a 2:1 step up transformer, thus the
windings on the supplied transformer
made it, by my calculations, to be a
7.5:1 step-up transformer.
(2) ZD1 was a 75V 5W diode not 56V
3W as in the March 1996 circuit.
I decided to construct the circuit as
supplied and on completion the output voltage measured 470V instead of
112V. I checked and double checked
with no change to my test results, so
I decided to rewind the transformer
with 36 turns to the secondary and 18
turns to the primary, as I felt this would
make it a 2:1 step up, and also be more
in line with the original circuit. The
output voltage now read 141V. As ZD1
is higher than original specs, the higher voltage output of 141V is probably
acceptable. On testing known value
zener diodes, the circuit appears to
be measuring correctly.
Could you please advise me of the
implications of changing the ratio of
turns on windings as I have done with
my circuit. (G. M., Seven Hills, NSW).
• We are aware that Dick Smith Electronics has been supplying a different
transformer. However, the zener diode
should still be 56V as specified, in
order to be certain that the various
versions of MTP3055 which may be
used will not break down.
The turns ratio for the transformer
does not need to be precisely 2:1 since
the circuit has current feedback from
the Source of Q1 and this controls the
overall level of power delivered to the
zener diode under test. However, the
turns ratio should still be in the region
of 2:1 for correct operation, given that
zener diode ZD1 is 56V. Accordingly,
with the 136 turn secondary, the primary winding should be somewhere
between 60 and 70 turns. Alternatively, as you have found, the transformer
can be wound with lesser turns on
the primary and secondary and with
a ratio of 2:1.
We have advised Dick Smith Electronics to this effect.
Making Clifford work
in daylight
I built your Mini Electronic Cricket
(Clifford) as described in the December
1994 issue of SILICON CHIP. I would
like to know how to reverse the LDR
so it comes on in the light. (I. B.,
Strathpine, SA).
• It should be possible to make the
circuit operate in ambient light by
swapping the positions of the LDR and
the associated 47kΩ resistor.
Notes & Errata
Fluorescent Lamp Starter, August
1996: the circuit diagram on page 16
shows D1-D4 as 1N4004 diodes. They
should be 1N4007 1000V types, as
specified in the parts list.
2-Amp SLA Battery Charger, July
1996: the wiring diagram on page
57 has reversed polarity signs on the
output cable crocodile clips. The cable
coming from the lefthand side of the
SC
PC board should be positive.
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