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Power control with
a light dimmer
By LEO SIMPSON
In this article we show you how to wire a
standard light dimmer in a plastic case to
do low temperature soldering, control an
electric blanket or to dim a table lamp. No
electronics assembly is required, just some
drilling of the plastic case and a little wiring.
This article was prompted by a reader who wanted to do some low temperature soldering of white metal model
railway kits. White metal is an alloy
of tin and lead, with small amounts
of antimony and copper (typically
19% antimony, 1% copper, 75% lead
22 Silicon Chip
and 5% tin). While white metals have
long been used in the manufacture of
bearings, they are also used in small
castings for model railway locomotives, rolling stock and structures. The
reason that white metal is used is that
it has a low melting point, typically
around 200-250°C. That low melting
point means that normal tin-lead solders cannot be used; the casting melts
before the solder!
There are a variety of low temperature solders available but the one
normally used for soldering white
metal castings is based on cadmium
(tin-lead-cadmium). The most popular
is made by Carr’s Modelling Products,
a UK company. Carr’s 70 is a solder
that melts at 70°C.
This cannot be safely handled or
worked with using an ordinary soldering iron or even one that is temperature-controlled, for that matter.
Why not? Most temperature controlled irons are not designed to op-
Fig.1: here's how to wire the dimmer unit to the mains cord
and the 3-pin socket.
erate reliably with a tip temperature
below 200°C. So there is a need to run a
soldering iron at much reduced power;
sufficient to melt and work the solder
but not hot enough to cause heavy
metal gases to be evolved or damage
the white metal casting. In fact, it is
absolutely imperative that solders con
taining cadmium (or bismuth and antimony) should not be overheated by the
tip of the soldering iron – otherwise
you will end up breathing poisonous
metal fumes.
One way to reduce the power to a
standard soldering iron to a low level
is to use the 5A heavy duty drill speed
controller published in the September
& November 1992 issues of SILICON
CHIP. That will certainly work but it
is rather like using a sledge hammer
to crack a walnut. It is also more expensive than the dimmer approach
described in this article.
Our suggestion is to purchase a
low power soldering iron rated at
between 15 and 30 watts and use it
in conjunction with the dimmer as
described here. Use the dimmer at a
setting just hot enough to make the
solder workable and mark that setting
on the dimmer plate so you can repeat
it in the future. Second, use the iron
only for low temperature work. Do not
October 1996 23
55 x 85mm but there are a number of
alternatives available.
Whichever plastic case you use,
it needs to be big enough to accommodate the dimmer panel on its top
surface and a surface-mount 3-pin
socket at one end. As well as this
socket, you will need a 3-core mains
flex with moulded 3-pin plug and a
cordgrip grommet.
You will need to drill holes in
one end of the case for the surface
mount socket – two 3mm holes for
the mounting screws and three 5mm
holes for the lead entry. At the other
end of the case, you will need to drill
and file an elongated hole to take the
cordgrip grommet.
Finally, you will need to drill two
3mm holes and cut a rectangular hole
in the lid so that the dimmer can be
mounted.
Wiring
This view shows the wiring inside the case. Make sure that the mains cord is
securely clamped and note that plastic cable ties are used to secure the internal
wiring. The Neutral and Earth leads from the mains cord go direct to the socket.
use it for normal soldering because if
you do, when the iron is hot enough
to melt normal solder it will boil off
any cadmium residue and you could
end up breathing it!
Assembly work
First, purchase your dimmer. Shop
around for it as you will find a wide
range of prices. At the time of writing
we found dimmer prices to range from
$12 at Woolworths to more than $28
at some electrical wholesalers. The
one we used is made by HPM (Cat
500A/500VA) and was purchased for
$16.60. Other brands of dimmer, made
by GAF and Arlec, are cheaper.
Second, you will need a suitable
plastic box. The one we used came
from Altronics and measures 125 x
The yellow/green Earth lead from
the mains cord is terminated directly to the earth terminal of the 3-pin
socket. The blue Neutral wire from the
mains cord also terminates directly to
the Neutral terminal of the 3-pin socket. The brown Active wire from the
mains cord goes to one of the switch
terminals. The other switch wires goes
to the dimmer module.
Finally, the second wire from the
dimmer module goes to the Active
terminal on the 3-pin socket. Fig.1
shows the details.
When you have completed the wiring, check it against Fig.1 and then
test the dimmer on a table or desk
lamp. Don’t forget to screw the lid
on the case before you do the test. If
everything works as it should, push
the plastic screw covers into the
dimmer mounting screw holes and
SC
you are finished.
Using A Standard Light Dimmer
What they can do: most standard
wall-mounting light dimmers are rated
at between 300 and 500 watts but
there are conditions applied to this
rating. Apart from dimming lights,
most standard dimmers can be used
to control fans and low powered
heating appliances such as soldering
irons and electric blankets.
What they can’t do: because they
24 Silicon Chip
have such a modest rating, light
dimmers cannot be used to control
the speed of a typical power tool or
food mixer. If you do attempt to use
a light dimmer, it will fail immediately.
Most light dimmer manufacturers
also warn against dimming lights
where the individual lamps have
a rating in excess of 150 watts or
where the lamp is upright rather
than hanging down from the fitting. In
both cases, when the lamp fails the
broken filament is likely to flail around
and come into contact with one of
the stem supports. This will cause a
brief but very large fault current which
often blows the Triac in the dimmer.
The good news is that such dimmers
can generally be fixed by replacing
the Triac with an SC141D.
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