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Power Supply Demo Design Fig.8: a complete and accurate circuit diagram is required before you attempt even the simplest of layouts. Here’s the circuit for a simple DC power supply that we’ve used as our demo design. It uses a conventional 3-terminal regulator, with the output voltage programmable via resistors R2 & R3. A lthough Autotrax includes a demonstration design (DEMO. PCB), it is far too complex to be of use to the first-timer. We decided instead to create our own simple design, the layout for which appears in various stages throughout this article. The complete circuit and overlay diagrams appear in Figs.8 & 9. You can download the design (PSU.ZIP) from the Silicon Chip web site at www.siliconchip.com. au – look in the software download area. This file also includes the SIMPLE.LIB library referred to in the text. Unzip PSU.ZIP into your C:\AUTOTRAX directory. How it works The Simple DC Power Supply is based around the well-known LM317T 3-terminal adjustable voltage regulator. These devices are Autotrax automatically saves a back-up copy of your work for disaster recovery purposes. You can change the backup interval (in minutes) and the filename used via the Setup -> Options menu. An interval of between 10 and 20 minutes is typical. Loading the demo design With the information presented 70  Silicon Chip extremely robust, having in-built over-temperature and over-current protection. The supply can accept an input of up to 28VAC or 40VDC and provide a well-regulated DC output in the range of 1.2V to 37V. Output current is 1A maximum and depends on the input to output voltage differential. Using the specified heatsink and at room temperature (25°C), The LM317 can safety dissipate 2.5W of power. You can use this power level to calculate the maximum output current for a given input to output differential. For example, with 16V at the input to the regulator and 5V at the output, the maximum current is: IOUT(MAX) = PDMAX/(VIN - VOUT) = 2.5W/16V - 5V = 0.227A The output voltage can be programmed by selecting appropriate thus far, you should be well on your way to completing the demo design. Alternatively, if you’d rather load the “one we prepared earlier” and experiment with that instead, then follow the instructions in the “Power Supply Demo Design” panel to download and install the relevant files. So you’ve finished the board layout – what now? Well, the following R2 & R3 Values For Common Output Voltages Output Voltage R2 R3 3V 5V 6V 7.5V 9V 12V 15V 1.2kΩ 3kΩ 11kΩ 1.2kΩ 3.3kΩ 3.3kΩ 3.9kΩ 470Ω 2.7kΩ 5.6kΩ 8.2kΩ values of R2 & R2, according to the formula: VOUT = 1.25 x (1 + (R2||R3)/R1) A list of commonly used voltages and the corresponding values for R2 and R3 appear in the above Table. Alternatively, you can install a miniature 5kΩ multi-turn potentiometer in place of R2 & R3 for a 1.2V to 27V half of this article describes several concepts and features of Autotrax that will help you to get started with your own creation! Multiple layers or wire links? A good single-sided PC board design is one that requires no wire links – or so we’ve heard. The reality is that no matter how proficient you become, www.siliconchip.com.au Parts List 1 PC board, code 04103041, 36.8mm x 68.6mm 1 LM317T adjustable positive voltage regulator (REG1) 6 1N4004 1A diodes (D1-D6) 1 5mm red LED (LED1) 2 2-way 5.08mm-pitch terminal blocks (CON1, CON2) Capacitors 1 2200µF 50V PC electrolytic 1 100µF 63V PC electrolytic 1 10µF 50V PC electrolytic 1 100nF 63V MKT polyester Resistors (0.25W 1%) 1 1.5kΩ R2 (see table) 1 240Ω (R1) R3 (see table) not straddle or otherwise interfere with them!). If you wish, you can disguise you links by using zero ohm resistors instead of plain old tinned copper wire. These are available in standard “1/4W” package styles from the usual electronics outlets. Fills and arcs Large copper areas are easily created with the Place -> Fill command and edited in a similar manner to the previously described “primitives” (pads, tracks, strings, etc). Fills should be used in place of multiple overlapping tracks wherever possible, as editing is far more efficient. Autotrax supports arcs of any diameter and width with one to four quadrants. Avoid these on the copper layers unless you know what you’re doing. Libraries Fig.9: companion overlay diagram for the completed design. You can purchase a ready-made PC board from RCS Radio at www.rcsradio.com.au if you would like to build one, or wait until next month to find out how to make the board yourself! adjustment range. Note that the voltage at the input terminal of the 3-terminal regulator some of your designs will require links to make those last few connections. Of course, depending on complexity, a two-layer (or more) design might also be the answer, especially if you have limited space to work with. Multiple-layer designs are for experienced designers only, so we won’t cover them here! Typically, a link is just a straight www.siliconchip.com.au (REG1) must be at least 2V higher than the programmed output voltage. piece of wire with a pad at either end. We recommend a minimum pad size of 70 thou (85 thou preferred) with a 28 or 32-thou hole. Draw a track between the two pads on the component overlay to indicate the link position. To give the assembled board a professional appearance, wire links should be oriented and aligned with surrounding components (they should As mentioned previously, the standard Autotrax library (TRAXSTD.LIB) is unsuitable for use without major editing. One option is to obtain a complete set of libraries on CD-ROM from RCS Radio. These are supplied “ready to go” and are optimised for use on non-plated through board layouts. Contact Bob Barnes on (02) 9738 0330 or check out www.rcsradio.com.au for more information. An excellent component library is also available from Airborn Electronics at www.airborn.com.au/layout/ autolib1.html. Note that this library is optimised for plated-through (double -sided) board design. This means that the pad sizes (for through-hole components) are too small for use on single-sided boards. However, you can readily use it as your reference library, editing footprints as required and adding them to your own library. Building your own library Library components are made up of all the familiar primitives. However, their individual elements are not free to move; they’re bound together in a fixed relationship to one another. We can break that relationship, edit the individual primitives and then regroup them again at will. Let’s experiment with an existing component from SIMPLE.LIB. First, find some free space (anywhere outside the border) of the power supply demo design if you have it March 2004  71