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was also 6.8V. Then I borrowed another 6V SLA battery charger from a friend and both the charging and standby voltage of this charger were 7.25V. I wonder which charger is working correctly or neither. I would appreciate it if you would enlighten me as to how to make the vol­tages of my charger within specifications. (A. H., via email). •  We have obtained the kit information on the 6V version of the SLA charger from Altronics and we can confirm that all the component values are exactly as they should be, according to the data on the UC3906 chip. Its over-charge voltage is 7.4V and its float voltage is 6.9V. We published a detailed article on the UC3906 in the March 1990 issue. Provided all the resistor values are as specified on your circuit, the charger should work cor­rectly. Notes & Errata Voice Direct Speech Recognition, September 1999: both the circuit on page 38 and the PC board overlay diagram incorrectly show the 4081 AND gate packages connected to the +12V rail rather than the +5V rail as they should be. If you have built the board as published, the +12V rail from the relay to pin 14 of both 4081s should be broken and the line connected to +5V instead. An amended PC pattern has been produced and can be down­loaded from our website. Autonomouse Robot, September 1999: there are number of errors on the circuit on pages 20 & 21. The 1N914 below D3 should be Mailbag: continued from page 44 of temperature rise. When mounted on a PC board, the copper track of the source lead is used to dissipate this heat as is the drain track. When trying to mount TO-220 packages on any standard off-theshelf heatsink, where multiple transistors are required to be placed in parallel, then the wiring of these devices becomes quite painful, tedious and delicate and looks disastrous. Problems that can occur during testing include the difficulty in probing with a CRO safely due to the close proximity of the drain and gate leads. One slip of the CRO probe and you can kiss the FETs good­bye. If paralleled (28 per side on the inverter that I am pre­sently repairing), then they all fail. Also it is quite a common occurrence to have the FETs burn the PC board to a crisp which renders it extremely difficult to repair, with a 1500W inverter costing $1000 to replace the one and only PC board. What I am leading to is that in applications where high powers are required then maybe the TO-220 package is not the most desirable but it is used quite commonly as its cost is most attractive. What I am advocating here is that a transistor manu­facturer should consider making a true 50A transistor. A typical example would be perhaps three or four 60NO6 FETs mounted in a reverse TO-3 package with spade terminals, such that the drain and source leads can actually carry the 50A and be mechanically and thermally stable at the same time. Another alternative is the package used for bridge rectifi­ers. Here we have a package, usually alumini- D4, not Q4. The collector leads of Q5 & Q13 are labelled “B” instead of “C”. Finally, the text on page 23 refers to a 100kΩ resistor associated with IC3. The value is 390kΩ, as indicated on the circuit and wiring diagrams. Surveillance Lights With Buzzer, Circuit Notebook, September 1999: NAND gate IC1a is shown reversed. Input pins 1 & 2 should connect to the PIR output. Switching Temperature Controller, August 1999: the reference on page 55 to the Seeburg effect is wrong; it should be the Seebeck effect. Seeburg is a brand of jukebox! um-based, with four spade connectors and four diodes encapsulated. It is cheap, and it will fit down the centre of most grunty heatsinks and has a good base for heat transfer. It also uses a single-bolt mount and is easily insulated if required. The lead configuration would be 10mm spade lug for drain on one side and source on the opposite side with the 3mm spade lug for the gate on the third side. This configuration would clarify that it was not a bridge rectifier. This basic configuration would lend itself extremely well to the manufacturing of bridge transistor circuits or parallel configuration as it would leave the running of copper busses wide open, with unlimited flexibility in connection methods; eg, spade lugs or copper braid or copper wire or strip busses. T. C. Thrum, Para Hills West, SA. 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. October 1999  93