Fullscreen HTML5Med Res (??MB)HTML4

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.