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fore repair any faults in the circuit. (G.W., Braddon, ACT). • Quite a few components would need to be changed to make the circuit capable of delivering 4A. You would need to change the transformer, bridge rectifier, the sensing resistor (halve it) and the transistor heatsink needs to be at least twice as large. Frankly, we’re not keen on the idea, especially if you’re not confident about troubleshooting the circuit if it doesn’t work first time. Damaged speed controller killed the IGBT We have built one of the 240VAC speed controllers from the November 1997 issue to drive a new 2hp Hitachi router. The unit tested brilliantly with incandescent bulb and electric drill. We then tried it on the above router and excellent results even when “hogging” into timber with a 12mm cutter. So to the real reason we built the controller: we needed a small special purpose centrifuge and on the cheap. Essentially, the same router is mount­ed as for a router table with a carefully balanced “pot” weighing 50g mounted in the collet. The router is run slowly up to about 8000 rpm over about one minute, held at that speed for two minutes, then switched off. The cycle is repeated after about five minutes. So the router is not working under any appreciable load. We completed 10 cycles and then switched the lot off. Next day when we went to repeat the process the unit would only run at full speed and tests show that the IGBT is low resistance across source and drain. The DC supply is also down to about 5V but I guess this is because there is an appreciable current through the IGBT to ground. The 4050 gets hot and removing it puts the DC back up to 15V. The unit and the router have done only about four hours work in total; the brushes appear new, as does the armature. There has been no breakdown of the insulation between box and IGBT. The kit was supplied by Jaycar but I am not sure if the IGBT is a genuine Siemens and at $39 a shot, it’s not cheap. As we only want to run up to about 10,000 rpm, could the gate cur­rent be limited? How heat sensitive is the IGBT? The unit was warm to touch but it seemed well within usual limits. Would adding a heatsink and/ or fan help? Sorry about the long and involved story but do you have any suggestions? (I. S., via email). • The IGBT is well heatsinked with the diecast case and is operating well within its ratings even at 10A. Failure of the device is most likely due to an accumulation of heavy transient current or excessive voltage across it from inductive loads. It would be prudent to check the MOV (MOV1), the fast recovery diode (D1) and the snubber components (82Ω resistor and the .01µF 250VAC capacitor) which are mounted across Q1. Note that poor solder connections around any of these critical components could cause the IGBT to fail because if any one of these is open circuit while the unit is working, the IGBT has no protection at all. We’ve also heard of one user assembling this unit with “high tin” solder. This invariably causes cold solder joints or, if the soldering iron is hot enough, it can cause damage to the compon­ents. Needless to say, his controller stopped working while powered up although luckily no serious damage was done. The BUP213 IGBT will have a Interface card draws high current I built the “Flexible Interface Card For PCs” as described in the July 1997 edition. Could you tell me what current it should draw from the +5V line? It seems to be drawing about half an amp and is burning out the power supply we have. (J. A., via email). • The current drain from the 5V rail should be quite modest, no more than 50-100mA at a guess; nothing like 0.5A. You have a fault there somewhere. Siemens logo on it if it is a genuine component. The Siemens logo is a large S which is sloped anti-clockwise by about 45°. At the centre of the S is a H sloped with the same angle. We suspect that the 4050 (IC2), and the 15V zener (ZD2) are also faulty and should be replaced along with the BUP213. Increasing the value of the gate resistor for Q1 will im­prove its short circuit rating but at the expense of increased dissipation due to slower turn on and turn off times. The 10Ω resistor could, however, be increased to 47Ω without any undue effect on its temperature rise. Gate current limiting will not limit the router speed. You would need a tachometric circuit to achieve that. Notes & Errata Turbo Timer, November 1998: The 100µF capacitor shown con­nected to pin 6 of IC1 on Fig.2 (page 27) should be 220µF to agree with the circuit diagram on page 26. 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. February 1999  93