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Optocoupler breaks down in welder If possible, could you please help me with the following problem? We have an inverter welder that has been imported from Europe. The machine has an input rating of 230VAC and the manufac­turer has stated that running the welder at 240VAC would pose no problem. The problem is that a MOC3023 optocoupler is breaking down and letting AC flow back through the circuit and cause the solenoid to chatter. The optocoupler drives a gas solenoid rated at 230VAC 13.5VA and incorporates an RC circuit which consists of a .022µF capacitor and a 22Ω watt resistor connected in parallel. My question is, would the 10V Question on modifying PC power supply With respect to the article entitled “Use your old PC power supply for high current outputs” in December 1998, how high is the “high current”. Also, have you ever described the theory behind high power Mosfet auto amplifiers? Can the output current of the Power Supply for Amateur Transceivers described in May/June 1991 be increased to around 30A relatively easily? (T. C., via email). • The December 1998 article dealt with modifying a standard PC power supply to deliver slightly higher voltage; eg, 13.8V in­stead of 12V. Typically, you can get 8A from the 12V rail and 20A from the 5V rail. We have not published any theory behind Mosfet car amplifi­ ers. They increase in the primary generate a sufficient increase in the breakdown voltage to cross the protection threshold and blow the optocoupler? And is there any formula to work out what the break-down voltage would be? (P. D., via email). • The MOC3023 only has a 400V blocking rating so depending on what it is driving, it could easily be fragile, especially in a welder. Given that its load is only a low current, the designers probably thought that it would easily handle the job. Our ap­proach would be to use the MOC3023 to drive a 600V Triac but one with a low holding current or alternatively, shunt the solenoid with an incandescent lamp to ensure reliable operation. You may also need Varistor protection in the circuit. are just standard Mosfet amplifiers powered by a DC-DC inverter. The output of the 13.5V 25A supply cannot be increased without substantially upgrading the major components. How to connect a subwoofer amplifier I’d be the first to admit that I am a novice in the field of electronics. With perseverance and a lot of reading I had thought that I had developed a basic understanding of audio amplifiers, until now. I have built a couple of amplifier modules supplied by Dick Smith Electronics, namely the 100W module (Cat. K3442) and the more powerful 300W beast, primarily to power subwoofers for my home-theatre system. Each amplifier module has the Sub Bass Processor preamp module (DSE Cat. K5403). My dilemma is this: if I use the subwoofer out jack on my surround receiver I get virtually no signal through either amp module. If I connect the same subwoofer output to my small 30W bookshelf stereo and get this to power my subs, I get thumping bass but only 30 watts worth. Obviously there is signal coming out of the sub out jack, so why is it lost between here and my speakers? (S. F., via email). • The sub-bass processor should not be connected to the sub­woofer output of your surround sound receiver. The sub-bass processor is intended for those people who do not have a subwoof­er output from their receiver. Try connecting the subwoofer signal directly to your 100W amplifier. You should get heaps of bass. Bridging audio amplifier modules I’ve bought two power amplifiers from Dick Smith Electron­ ics (Cat K3442) and I would like to bridge them. What do I need and can it be done? (E. P., Vermont, Vic). • There is a problem in bridging these amplifier modules. As they stand, they will deliver 100W into 4Ω or 50W into 8Ω. If they are bridged, they will only deliver 100W into an 8-ohm load. They cannot drive a 4-ohm load in bridge mode, since each module cannot drive a 2-ohm impedance. Notes & Errata Touch/Infrared Light Dimmer, January & February 2002: the circuit diagram (Fig.3) should show the .01µF capacitor and 1MΩ resistor connected to pin 6 (RB0 input) of IC1 and not to the A2 terminal of TRIAC1. The PC board pattern and overlay diagram SC are correct. WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable. 90  Silicon Chip www.siliconchip.com.au