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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