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Advertising Index ADM Instrument Engineering....... 39 Altronics.................................. 68-71 Australasian Exhibitions & Events.. 27 Elcomtel..................................... 103 Element14...................................... 3 Embedded Logic Solutions.......... 80 Emona Instruments........................ 5 Futurlec........................................ 10 Hare & Forbes.......................... OBC Ask SILICON CHIP . . . continued from page 103 that this circuit would meet my needs. Accordingly, I built the circuit on a Veroboard layout. I have doubled-checked, even-triple checked, my layout and can find no fault, however even with a modest load of half an amp the 7812 becomes so hot it is impossible to touch it within 15-20 seconds. Surely this is not normal? I have checked the pinout of the TIP2955 and the 7812 and as I mentioned earlier, cannot fault my layout connections. I know this is presumptuous of me but is the circuit and component listing correct to provide the resultant 12V at up to 3A? Could you perhaps suggest an alternative power supply, published by SILICON CHIP, that I could use? (K. J., via email). •  That Jaycar current boost circuit is fairly standard (see www.eleccircuit. com/boosting-regulator-current-foric-78xx-by-mj2955/). The 3.3Ω resistor between the base and emitter of the TIP2955 means that there will be 0.6V across the resistor when the transistor is conducting, allowing the transistor to bypass some of the current that would otherwise pass through the regulator to the output. When the transistor is conducting, the current supplied by the regulator will be 0.6V/3.3Ω or 182mA. Whether the regulator runs hot or not when delivering current depends on the voltage difference between the input and output. If you are feeding in, say, 15V to the input for a 12V output, there is about 3V minus the 0.6V drop across the 3.3Ω resistor. The regulator then dissipates 2.4V x 182mA or 437mW. 104  Silicon Chip DOWNLOAD OUR CATALOG at www.iinet.net.au/~worcom WORLDWIDE ELECTRONIC COMPONENTS PO Box 631, Hillarys, WA 6923 Ph: (08) 9307 7305  Fax: (08) 9307 7309 Email: worcom<at>iinet.net.au Harbuch Electronics..................... 95 Instant PCBs.............................. 103 Jaycar .............................. IFC,49-56 Keith Rippon .............................. 103 KitStop.......................................... 10 LED Sales.................................. 103 Notes & Errata Low Energy Developments........ 103 CLASSiC DAC, February-May 2013: the element14 part number listed for the AP5002S IC was wrong (page 41, April 2013). It should be 1825351. Mikroelektronika......................... IBC Demonstration Circuits for Human Colour Vision, July 2013: The RGB LED specified for the circuit of Fig.3 is a common cathode type and its Jaycar catalog number is ZD-0012. Do not use the ZD-0010 which is a common anode type. Radio, TV & Hobbies DVD.............. 8 Microchip Technology................... 23 Oatley Electronics........................ 79 Ocean Controls............................ 37 Quest Electronics....................... 103 RF Modules................................ 104 Rockby Electronics....................... 89 Sesame Electronics................... 103 Silicon Chip Binders..................... 95 Silicon Chip Online Shop........ 96-97 The thermal resistance of a TO-3 package is typically 4°C/W junction to case and 35°C/W case to ambient. The thermal resistance of a TO-220 package is typically 4°C/W junction to case and 50°C/W case to ambient. So for a TO-3 package, the regulator will run some 39°C/W x 437mW. That’s 17°C above the ambient temperature. For a TO-220 style package, the regulator will run at 54°C/W x 437mW or 23.6°C above ambient. The 23.6°C above ambient (for a TO-220 device) would mean that the regulator tab will be 48.6°C and that is fairly hot. If you have a higher input voltage, then the regulator will run hotter again. One way to reduce temperature is to reduce power dissipation within the Silicon Chip Subscriptions......... 102 Syndetic Pty Ltd........................... 11 Tekmark Australia...................... 7,59 Tenrod.......................................... 81 Verbatim Australia.......................... 9 Wiltronics...................................... 33 Worldwide Elect. Components... 104 xLogic............................................. 8 regulator. To do that, the input voltage needs to be reduced. Note that you do require at least 2V between the input and output of the regulator in order to maintain output regulation. Otherwise, use a heatsink to keep the regulator SC running cooler. siliconchip.com.au