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More current for
white LED torch
I recently purchased a kit for the
6 white-LED torch (published May
2001) from Dick Smith Electronics.
I notice that the LEDs are rated for
a continuous forward current of up
to 30mA (and can tolerate spikes
of up to 200mA for less than 10
microseconds) but that the circuit
delivers 18.5mA.
You say that in practice the current is slightly higher than this but
you don’t specify by how much.
Am I likely to destroy the LEDs if I
replace the 27Ω resistors in series
with the LEDs with 20Ω or 18Ω
resistors, giving a current of 25mA
and 28mA respectively to each
LED? I would like to make the torch
over them which you have disturbed,
allowing corrosion to proceed. Possible tip: put clear nail varnish over the
screws that have been disturbed.
Surround sound amplifiers
I recently built the surround sound
decoder and digital delay board from
the May 1999 issue of “Electronics
Australia” and would like to build
suitable amplifiers for the centre and
rear surround channels. The rear channel usually has two speakers in parallel which would result in a 4Ω load.
My main amplifier is capable of
50 watts RMS per channel. Could
you advise me of the power output
required for the rear and centre channels that would be a suitable mix for
my current main amplifier? (Y. C., via
email).
• Have a look at the 50W module
described in the March 1994 issue of
SILICON CHIP. Kits are available from
DSE, Jaycar, etc. We can supply the
March 1994 issue for $7.70, including
postage.
Glue gun
temperature control
I am using a 10W 240VAC glue gun
to make candles and I find it gets far too
hot for the wax I am using. What is the
best way to control the temperature?
(A. Q., Adelaide, SA).
www.siliconchip.com.au
as bright as possible but I do not
want to put the LEDs in jeopardy.
(S. H., via email).
• We do not recommend driving
the LEDs with more current than
that afforded by the 27Ω resistors.
This is because the turn on voltage
for the LED could be lower than
the typical 3.5V, causing a higher
current flow.
The only realistic way to measure and reset the LED current to
a higher value is to measure the
voltage across each resistor and the
replace the resistor with a value
which will provide the desired current.
Also the inverter driving the
LEDs is not suitable for driving all
six LEDs at 30mA as it does not have
this much power output.
MicroZed
Computers
HAS MADE THESE NAMES
FAMOUS IN AUSTRALIA
micro Engineering
Labs, Inc.
NOW WE INTRODUCE
•
Have a look at the article entitled
“Power Control With a Light Dimmer”
in the October 1996 issue. Although
this article described power control
for soldering irons, the idea is equally
applicable to glue guns.
Leakage on engine immobiliser
I am enquiring about the Engine Immobiliser described in the December
1998 issue. The immobiliser board
has about 0.4MΩ across collector and
ground of Q1 when in the idle state.
This is enough to affect the points and
not allow starting when the unit is not
turned on.
Everything works well with the overall kit and all is OK once the standing
12V is removed from the board. Is
there a correction for this or do I need
to look at my own handiwork? (B. M.,
via email).
• Try shorting the base and emitter
of Q1. If this eliminates the 0.4MΩ,
it suggests that you may have leakage
in the earlier stages or across the PC
board itself.
Notes & Errata
LP Doctor, January 2001: in order to
avoid residual mains voltage across the
transformer when the power switch
is off, the associated .01µF/250VAC
capacitor should be reduced to
SC
.001µF/250VAC.
Basic Micro offer ATOM:
a Stamp2 lookalike with
extended capabilities available
in an IC or OEM format.
A development board with
sockets for IC and OEM version
with a breadboard space
instead of solder pads.
Basic Micro also offer BASIC
compilers, programmers and a
range of development boards
for 18, 28 and 40 pin PIC Chips.
MicroZed Computers
PO Box 634, Armidale, NSW 2350
(296 North Cooke’s Rd)
Tel: (02) 6772 2777 Fax: (02) 6772 8987
Mob: 0438 277 634
http://www.microzed.com.au
January 2002 93
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