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MARKET CENTRE Cash in your surplus gear. Advertise it here in SILICON CHIP FOR SALE KIT ASSEMBLY & REPAIR PCBs MADE, ONE OR MANY. Any format, hobbyists welcome. Sesame Electronics Phone 0434 781 191. sesame<at>sesame.com.au www.sesame.com.au PCB MANUFACTURE: single to multi­ layer. Bare board tested. One-offs to any quantity. 48 hour service. Artwork design. Excellent prices. Check out our specials: www.ldelectronics.com.au PCBs & Micros: Silicon Chip Pub­ lications can supply PCBs and programmed micros for all recent (and some not so recent) projects. Order online or phone (02) 9939 3295. Audio + Video: Professional quality Quest AV brand equipment is made and sold in Australia exclusively by Quest Electronics. Ph 0431 920 667. sales<at>questronix.com.au KEITH RIPPON KIT ASSEMBLY & REPAIR: * Australia & New Zealand; * Small production runs. Phone Keith 0409 662 794. keith.rippon<at>gmail.com VINTAGE RADIO REPAIRS: electrical mechanical fitter with 36 years experience and extensive knowledge of valve and transistor radios. Professional and reliable repairs. All workmanship guaranteed. $10 inspection fee plus charges for parts and labour as required. Labour fees $35 p/h. Pensioner discounts available on application. Contact Alan on 0425 122 415 or email bigalradioshack<at> gmail.com WANTED WANTED: EARLY HIFIs, AMPLIFIERS, Speakers, Turntables, Valves, Books, NIXIE CLOCK KITS SILICON CHIP July-Aug 2007 Full kits & spare tubes still available (For a limited time only) Phone 0403 055 374; Email glesstron<at>msn.com Television Replacements Your one-stop shop for all your electronic parts from aerials to zener diodes. 134a Ayr Street, Doncaster 3108 03 9850 4144 sales<at>tvr.com.au For Capacitors, Transistors & Integrated Circuits Call or email for details For our specials, like us on Facebook. Quad, Leak, Pye, Lowther, Ortofon, SME, Western Electric, Altec, Marantz, McIntosh, Tannoy, Goodmans, Wharfe­ dale, radio and wireless. Collector/ Hobbyist will pay cash. (07) 5471 1062. johnmurt<at>highprofile.com.au ADVERTISING IN MARKET CENTRE Classified Ad Rates: $32.00 for up to 20 words plus 95 cents for each additional word. Display ads in Market Centre (minimum 2cm deep, maximum 10cm deep): $82.50 per column centimetre per insertion. All prices include GST. Closing date: 5 weeks prior to month of sale. To book, email the text to silicon<at>siliconchip.com.au and include your name, address & credit card details, or phone Glyn (02) 9939 3295 or 0431 792 293. the rule of thumb is that the ripple current rating should at least be equal to the total current drain from the supply when the amplifier is delivering maximum power. In your case, the maximum power is 150W. Divide that by 0.6 to allow for the total power of a class-B output stage and you have a figure of 250W. Divide that by the total supply voltage of 90V and the current is 2.77A. So if you purchase capacitors with a ripple rating of 7.5A, you have that point covered. The second point to consider is power supply ripple (ie, the 100Hz ripple superimposed on the supply rails) and this needs to be looked at when the amplifier is quiescent (ie, delivering no audio signal) and when siliconchip.com.au it is delivering full power. You need the ripple to be quite low when the amplifier is quiescent otherwise there may be audible hum, particularly if you are listening via headphones. Or maybe the hum won’t be particularly audible but it might lead to higher than wanted THD +N (total harmonic distortion + noise). Given the fact that you cannot do anything about the fixed load on the amplifier power supply when it is quiescent, the two factors which determine the ripple voltage are the total filter capacitance and the total ESR (equivalent series resistance at 100Hz). If you purchase good quality capacitors you can assume that the ESR will be low. Finally, a designer of an audio amplifier would want to look at the sup- ply voltage ripple when the amplifier was delivering maximum power. This will be a much higher figure, typically several volts, superimposed on the nominal supply voltage. This will act to reduce the maximum power available at clipping. In this discussion, we have not mentioned PSRR (power supply rejection ratio) which is a determining factor as to how much ripple can be tolerated but since the PSRR of your amplifier is already fixed, we can neglect it. In general, the more filter capacitance the better, but the in-rush current at switch-on can then present a problem for the bridge rectifier and fuse ratings. In practice, if you fit 15,000µF of capacitance to each supply rail, the overall SC performance should be fine. April 2014  95 Advertising Index Altronics.................................. 75-79 Apex Tool Group............................. 5 Bitscope Designs......................... 13 Control Devices Pty Ltd.................. 7 Electrolube................................... 39 Emona Instruments...................... 11 Enertel Pty Ltd............................. 29 Front Panel Express....................... 8 Futurlec.......................................... 8 Gless Audio.................................. 95 Switchmode Bench Supply . . . continued from page 37 The 10MΩ and 2.2MΩ resistors provide a small bias current to the two panel meters so that they do not give a negative reading when the output voltage is 0V or no current is being drawn. The two remaining trimpots, VR4 and VR6, are used to trim out any offset error in the voltage feedback and current limiting circuitry respectively. These inputs have a low impedance to ground so the adjustment ranges span just a few millivolts either side of 0V. Remaining circuitry The circuit is protected from a reversed input supply polarity by Mosfet Q1. When the supply is connected the right way, Q1’s gate is pulled positive by the 100kΩ resistor and clamped at a safe level by the 15V zener diode. This switches it on and allows ground current to flow from the circuit back to the supply. If connected backwards, the gate is pulled negative and so Q1 remains off. Its body diode is also reverse-biased and thus very little current will flow. The 100nF capacitor from its gate to ground slows its turn-on to avoid large current spikes charging the input capacitor bank when power is first supplied; IC1 has a soft-start feature, so it’s just this input bank that can draw a high current initially. A 10A fuse protects the circuit against serious faults while 27V zener diode ZD2 conducts if the input supply voltage becomes too high. If that excessive voltage is maintained for very long, it will blow the fuse. The clamping voltage is above the 25V rating of the input capacitor bank but they are unlikely to fail due to a brief 96  Silicon Chip Harbuch Electronics..................... 91 DOWNLOAD OUR CATALOG at www.iinet.net.au/~worcom Hare & Forbes.......................... OBC High Profile Communications....... 95 WORLDWIDE ELECTRONIC COMPONENTS PO Box 631, Hillarys, WA 6923 Ph: (08) 9307 7305 Fax: (08) 9307 7309 Email: worcom<at>iinet.net.au Icom Australia................................ 9 Notes & Errata LD Electronics.............................. 95 Soft Starter for Power Tools (July 2012): the 10mΩ SMD shunt was left off the parts list. This should be a 6332 (metric)/2512 (imperial) size SMD chip resistor with a rating of at least 2W, such as CRA2512-FZ-R010ELF (element14 Cat. 2394421). Microchip Technology................... 71 Jaycar ......................... IFC,45-52,92 Keith Rippon ................................ 95 KitStop.......................................... 10 Master Instruments...................... 41 Mikroelektronika......................... IBC Ocean Controls............................ 12 Quest Electronics......................... 95 RF Modules.................................. 96 Rohde & Schwarz.......................... 3 Sesame Electronics..................... 95 Silicon Chip Binders..................... 72 over-voltage of just a few volts and we don’t want ZD2 to conduct any significant current with the supply below 25V. A 4.7µF capacitor and 3.3µH inductor L2 prevent much switching noise from passing back through the input leads, which could lead to electromagnetic interference being radiated from them. Power switch S1 enables the switchmode regulator and at the same time, applies power to the rest of the circuit. When LK1 is moved to the “Test” position, the linear regulator remains off and power can bypass it from S1 straight to the output. This is so that the constructor can check the linear regulator and other circuitry is working before activating the switchmode portion; otherwise troubleshooting could be very difficult. Finally, there is a Schottky clamp diode (D16) at the output of the switchmode regulator so that its output can not be pulled very far below ground by Silicon Chip Online Shop........ 88-89 Silicon Chip Subscriptions........... 75 Television Replacements............. 95 Verbatim....................................... 55 Vicom Australia............................ 43 Wiltronics........................................ 6 Worldwide Elect. Components..... 96 the linear regulator at start-up. There is also a clamp consisting of two 27V zeners (ZD5 & ZD6) in series after filter inductor L3, so that if the switchmode regulator feedback fails (including the ZD7/ZD8 voltage clamp), its output will not go high enough to damage the 63V filter capacitors or any part of the linear regulator circuitry. LDO operation & construction That’s all we have room for this month. Next month, we’ll describe the linear regulator section and begin the construction. SC siliconchip.com.au