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Notes & Errata 40W Fluorescent Inverter, September 2002: Mosfets Q1 & Q2 have been found to be prone to overheating when the PC board is placed within the confines of a slimline 36W fluorescent tube batten. To correct this, the inverter has been altered to improve efficiency without reducing the lamp brightness. Changes include reducing the 334V DC supply to 280V and winding transformer T1 differently. The voltage change requires replacing one of the 270kΩ resistors leading to pin 1 of IC1 with a 180kΩ resistor. The changes to T1 involve using 130 turns on the secondary. For the primary use figure-8 7.5A wire with a polarity stripe. Insert one end of the figure-8 wire in the S1 & F1 holes nearest to Q2 and wind on five turns, starting up through the centre of the core and anticlockwise towards S2 & F2. Insert the wire ends into S2 & F2 with continuity (same wire) between S1 and S2 and the second wire between F1 and F2. This means that if the polarity stripe on the figure-8 wire is at S1, it then terminates into S2. rating of more the than load current being switched. This should suppress any contact arcing across the relay contacts. When switching 240VAC with a solenoid, it is usual to con­nect a 250VAC-rated capacitor across the (solenoid) contacts. Try a 47nF (.047µF) 250VAC capacitor across the contacts. High input impedance amplifier wanted I would like a suggestion for a high input impedance ampli­ fier circuit. What I need is to be able to intercept a line level signal and basically split it in to two line level signals. I need it high impedance so that it doesn’t load the line too much. I want to build five of these amplifiers and then use them to provide line-level outputs from each channel of my Boston Acoustics BA7500G Speak­er system so I can feed external amps, without disturbing the internal 92  Silicon Chip Set the current drain from the battery at 3A (300mV across the 0.1W resistor used in the current measurement set-up in Fig.8). As noted in December 2002, it is recommended that the maximum current delivered to the fluorescent tube be adjusted using a trimpot. The 100kΩ resistor connecting between pin 2 of IC3 and the top of the dimming potentiometer (VR1) should be replaced with a 50kΩ trimpot and series 82kΩ resistor. The 1.2Ω resistor between the source of Q4 and ground should be changed to 2.2Ω to allow the full dimming range available from VR1. Using the current measuring setup of Fig.8, the trimpot should be adjusted for the 300mV, corresponding to 3A when the dimming pot (VR1) is turned fully clockwise. Note that this adjustment should be made after the inverter has been running for some time and is fully warmed up. Once adjusted, the trimpot and 82kΩ resistor can be swapped for a single resistor that is the same value as the total series combination. When testing the current (using amplifier. Any help would be greatly appreciated. (M. R., via email). • You really don’t need a high input impedance. What you need is a buffer amplifier that can drive a series of low impedance outputs. We have already published a suitable circuit – the audio portion of the Audio/Video Distribution Amplifier in November 2001. This circuit is based on an LM833 dual op amp. If you decide to use it, make sure you install 1kΩ resis­tors in the outputs instead of the 47kΩ resistors mistakenly specified in the original article. Speed control in cordless drills I recently bought a variable speed cordless drill/screw­ d river (Black & Decker) and would like to know what method is used to provide the variable speed feature. I guess I could open the drill and look inside but the setup of Fig.8), it is important not to have the 0.1Ω 5W resistor in series with the supply for any appreciable length of time as the current drain will begin to increase. To prevent this, short out the 0.1Ω resistor (with a clip lead) when not making the measurement. Remove the clip lead briefly to make the current measurement. In addition, use heavy gauge wire rated at 7.5A or more to connect the inverter to the 12V battery. It is recommended that the inverter not be used while the battery is being charged from a high current charger such as an automotive alternator or mains-powered unit. If the inverter Mosfets still run excessively hot, it is recommended to reduce the current drain to 2.5A (250mV across the 0.1Ω resistor) which will reduce the lamp brightness slightly. With slimline batten holders, ventilation can be improved by drilling some holes in the side of the batten adjacent to the Q1 and Q2 heatsinks and some extra holes at the other end of the batten to SC allow air flow. that will void my warranty. (H. P., via email). • Cordless drills are usually permanent magnet motors with a Mosfet switchmode power control, usually running at about 1kHz or so. In fact, you can usually hear the 1kHz tone at very low speed settings. Temperature controlled fan I want a circuit for a temperature-controlled fan for the heatsinks of an amplifier I’m working on. I am positive SILICON CHIP published one in the not-too-distant past but for the life of me I can’t find it in my back issues, nor can I locate it on your website. Can you recall where the circuit appeared please? (G. B., Scotland Island, NSW). • The circuit was associated with the Ultra-LD Amplifier in the August 2000 issue. There were two options: a SC thermistor or thermal cutout. www.siliconchip.com.au