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Salvage It! BY JULIAN EDGAR A $5 variable voltage power supply Want to be able to dim lights in, say, a model rail layout? Or what about varying the speed of a low-powered motor or regulating the voltage going to a bike headlight, so that its brightness stays the same as the battery voltage falls? It’s all possible for less than $5.00. T HIS DESIGN uses a slightly modified car phone charger. The idea is based on a Circuit Notebook contribution from Timo Mahoney in the November 2003 issue of SILICON CHIP. Most car phone chargers use a DCDC switching power supply to reduce the voltage from the car’s 13.8V to what ever the phone requires. However, it’s easy to modify the PC board to give an adjustable voltage output. Apart from the car phone charger (the electronics are normally built into the cigarette lighter plug), all you need is a 10kW potentiometer. Car phone chargers are available in secondhand shops, at the tip and in the junk rooms out the back of phone retailers, while a 10kW pot can be scrounged from some old gear or purchased from your local electronics parts retailer. Building it Fig.1 shows a typical circuit for a car phone charger. The IC controls the output voltage, using voltage feedback provided by resistors R1 and R2. So if you change the values of R1 and R2, Inside each of these car phone chargers is a sophisticated DC/DC converter than can be easily modified to provide a variable output power supply. 88  Silicon Chip the voltage output will also change. However, rather than have a fixed output, we can vary the output at will by installing an adjustable pot in place of the two resistors. Then it’s just a case of measuring the output voltage and adjusting the pot until the required voltage is obtained. If you want the voltage to be set and then forgotten, use a trimpot. Alternatively, if you want to be able to vary the voltage at any time, wire a full-size pot to the board with flying leads. The most difficult part is finding the correct resistors to change. Individual chargers vary quite a lot, so the com- Rat It Before You Chuck It! Whenever you throw away an old TV (or VCR or washing machine or dishwasher or printer) do you always think that surely there must be some good salvageable components inside? Well, this column is for you! (And it’s also for people without a lot of dough.) Each month we’ll use bits and pieces sourced from discards, sometimes in mini-projects and other times as an ideas smorgasbord. And you can contribute as well. If you have a use for specific parts which can easily be salvaged from goods commonly being thrown away, we’d love to hear from you. Perhaps you use the pressure switch from a washing machine to control a pump. Or maybe you salvage the high-quality bearings from VCR heads. Or perhaps you’ve found how the guts of a cassette player can be easily turned into a metal detector. (Well, we made the last one up but you get the idea . . .) If you have some practical ideas, write in and tell us! siliconchip.com.au Silicon Chip Binders REAL VALUE AT $12.95 PLUS P & P Fig.1 – the circuit of a typical car phone charger. The output voltage is set by R1 and R2. If these are replaced with a 10kW pot, the output voltage can be adjusted over a wide range. H SILICON CHIP logo printed in goldcoloured lettering on spine & cover H Buy five and get them postage free! Price: $A12.95 plus $A7 p&p. Available only in Australia. Just fill in the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. The charger is modified to produce a variable output by substituting a pot (arrowed) for two resistors. If external adjustment is needed, a full-size pot can be wired to the board with flying leads. In this case, a multi-turn pot has been used, which allows for very accurate setting of the output voltage. ponent designations marked on the board (R1, R2, etc) will probably not coincide with Fig.1. Hmmm. So how do you find the right resistors? The answer is to closely look at the top and bottom side of the board. Somewhere, there will be two resistors that join to a common track at one end Each of these bike headlights is run from a modified phone charger. The chargers are used to drop the voltage from the available 12V (provided by sealed lead-acid batteries that also power the electricassist bike) to a regulated 6V. siliconchip.com.au but go to different tracks at their other ends. The track that both resistors join to will also connect to pin 5 of the IC. Given that there will only be three or (at most) four resistors on the board, it shouldn’t be all that hard to find the resistor pair in question. The next step is even easier – re- move these two resistors and solder the pot to the board in their place. The centre terminal of the pot goes to the track where the two resistors were originally joined together, while the outer pot terminals connect to the remaining two vacant pads. Note that it, in some cases, it’s easier to make the connections by soldering the pot to the track side of the board. Using it To test the modified power supply, you’ll need a source of 12V power and a multimeter. First, connect the power supply to the 12V source (eg, a plugpack), making sure that you get the polarity the right way around (the tip of the cigarette lighter plug is positive). That done, connect the multimeter to the output leads of the adaptor and measure the voltage as the pot is adjusted – the output voltage should vary. Finally, place a load on the output (eg, a small 12V bulb) and check that you can alter the brightness of the lamp over a wide range. Typically, you can draw about 0.5A from the unit (depending on the plugpack). If the output voltage is critical, the pot should be set SC with the load attached. April 2005  89