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250VAC capacitors
for LED circuits
We had some correspondence
last year on the danger of using
DC-rated capacitors as the voltage-dropping elements in 240VAC
circuits (Ask SILICON CHIP, November 1994) with particular reference
on my part to driving LEDs from the
mains. However, I notice that since
that time, mains-rated capacitors of
suitable size have become readily
available; eg, from DSE and Jaycar.
Using these, would you now
regard such applications as “safe”
(I suppose nothing connected to
the mains can be completely safe)
two requirements which make the
design somewhat more complicated.
If you have the timer on and then
you decide to overrule it by manually
switching to LPG or gas, then that
setting will be maintained until you
remember to switch over to the timer
– which rather defeats the purpose of
the exercise. We are also concerned
about the possibility of the engine
faltering if the timed changeover
takes place at a critical time, such as
when pulling out to pass another car
at high speed.
With these thoughts in mind, we are
reluctant to publish a circuit without
more information on the subject.
Uninterruptible power
supplies for PCs
I would like to comment on the
matter of making up an uninterruptible
power supply for computers as raised
in the “Ask SILICON CHIP” pages of
the June 1995 issue. I have had a
UPS running for over one year. It was
interrupted once when Featherfoots
the cat jumped on a charging lead!
It consists of a home made battery
charger, a 10A Variac to adjust the
charging voltage via the battery charger
primary, a 1kW 24V modified square
wave inverter and a 24V 80A/hr deep
draw battery set.
The battery charger consists of a
300 watt toroidal power transformer
driving a 40A bridge rectifier with
suitable heatsink and transformer
cooling is by small computer fan. A
steel case was used as a plastic one
92 Silicon Chip
or would there still be some reservations about it? (J. K., Kenmore,
Qld).
• Provided these capacitors do
have the correct 250VAC rating
and also a suitably rated limiting
resistor is placed in the circuit,
such circuits should be safe. The
resistor should be designed to fuse
in the event that the capacitor becomes a short circuit and thereby
fail-safe.
However, we are still not really
happy with LEDs running directly from the mains and prefer the
old-fashioned and reliable neon
indicator lamp with inbuilt current-limiting resistor.
may melt if the components overheat.
A small value high wattage resistor is
used in series with the charging lead
to limit the possible current draw to
that rated for the transformer; in my
case, 8A.
Don’t be an idiot like me and use
a number of power resistors in paral
lel; the whole lot will fail in cascade!
The batteries were specified with bolt
type connections and were strapped
together with aluminium bar, 3mm x
25mm. Plenty of petroleum jelly was
used over the terminals and lead ends
at the batteries. Charging and inverter
wiring was heavy duty electrical earth
cable.
In practice, the battery is charged
to its rated maximum voltage and
the Variac is adjusted, over a day or
so, until with the computer load, the
batteries stay fully charged. My rather
extensive 386 system, less monitor,
draws just on 3.4A at around 27V. I
can plug the monitor in when I need
to; the supply stops my Unix system
crashing and on test kept it running
for 19 hours continuously. After that,
it does take a day or so for the setup
to drop right back to 3.4A.
I aimed at a charging capacity about
double the load draw. The 1kW inverter means that I can run everything
except the laser printer for reasonable
periods, about six hours if the 20-inch
monitor uses about 200 watts.
The batteries have needed no maintenance whatsoever since installation
thirteen months ago. They have an
expected life of five years, limited by
internal terminal corrosion.
The cost to me of this setup was
about $1000 plus an afternoon or so
of work and a bit of experimentation.
Nothing has had to be done to the unit
for many months. It is deliberately
made simple for high reliability, hence
the Variac rather than automatic voltage control. I have had a big regulated
power supply fail to regulate. The
device it was driving caught alight
when the power Mosfets blew! (R. H.,
Tranmere, SA).
• While we understand your desire
for simplicity, using a Variac to adjust
the charging current and hence, the
final charging voltage, is a little crude.
There is a risk that the mains voltage
could run at a high level for several
days and you could be seriously overcharging without knowing it.
Variable rejection
filter wanted
I was interested to see your simple
2-transistor CW filter in the March
1995 issue of SILICON CHIP and wondered if this could be easily modified
to act as a variable frequency rejection filter. I want one which could be
used to reject the 9kHz whistles on
the broadcast AM band and also the
various whistles on the international
short
wave bands. (A. S., Denmark,
WA).
• The CW filter in the March 1995
issue is not suitable as a variable
rejection filter. To effectively reject
9kHz whistles without removing too
much of the wanted audio signal,
you need a filter with a very deep
and sharp null, exactly at 9kHz. If
the filter is just 10Hz or more off
the exact frequency, the rejection is
greatly reduced. For that reason, a
variable rejection filter is unlikely to
be effective as it is too difficult to set
it to the exact frequency.
We did publish a 9kHz whistle
filter as part of the circuit for the
wideband stereo AM tuner described
in the February, March & April 1991
issues.
Notes & Errata
Walkaround Throttle, Ask Silicon
Chip, page 93, May 1995: the suggested
wiring diagram for a centre-tap transformer shows the 10µF capacitor near
the 7812 reverse biased. The capacitor’s negative connection should go to
SC
the centre pin of the 7812.
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