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Notes and Errata Multi-Purpose Fast Battery Charg­ er; February and March 1998: After testing three prototypes, we have found that a few component changes are required to produce reliable charging characteristics. The 0.47µF capacitor between pin 19 of IC1 and 0V should be replaced with a 100µF 16VW electrolytic type. The polarity of the component should be with the (-) toward the outside of the PC board and the (+) lead connecting to pin 19. This capacitance increase improves the detection of the NiCd & NiMH fall in vol­tage at full charge. The 0.18µF MKT capacitor at pin 17 of IC1 should be reduced to .0018µF. Its markings will either show 1n8 or 182. The number of turns on inductor L1 should be reduced from 20 to 10. The 1kΩ 0.5W resistor on the cathode of ZD1 should be re­placed with a 2.2kΩ 0.5W type. Also the 470Ω 1W resistor between the cathode of D3 and pin 12 of IC1 should be replaced with two 1kΩ 1W resistors in parallel. Charging current is best determined by checking the charg­ ing this issue), you have not produced an audio amplifier although it may well produce an audible signal. The normal output signal from a CMOS chip is a switching waveform with an amplitude almost equal to the supply voltage of the circuit. A 5V CMOS circuit will have 5V switching pulses. The CMOS chip cannot deliver enough output current to effectively drive an 8Ω loudspeaker so the usual practice is to connect a small time of a discharged battery. If charging time is too long, a slight adjustment can be made to increase the current by using a larger value resistor at pin 2 of IC1. A 3.9kΩ resistor should increase the current by about 10%. If charging time is too short, the battery is probably suffering from memory effect. Try running the battery through a few discharge (refresh) and charge cycles to bring it up to full performance. The timeout period can be increased to suit larger amp hour batteries by increasing the value of the 820pF oscillator capaci­tor at pin 14 of IC1. The wiring diagram on page 47 has two errors. The 1000µF adjacent to L1, between THS1 and -VOUT should be 100µF 25VW. The 470µF capacitor between ZD1 and D3 should be 1000µF 63VW (note increase in voltage rating compared to the circuit diagram). On the circuit diagram, the 2.2kΩ resistor at pins 12 & 13 of IC2a should be 22kΩ to agree with the wiring diagram. The 1kΩ resistor feeding ZD1 should be 1/2W. There should also be a 33kΩ pulldown resistor at pin 6 to ground (this resistor is on the wiring diagram). transistor to boost the current. In fact, the current through the loudspeaker is generally limited by a resistor of about 100Ω or so, to avoid destroying the transistor. Such a crude “amplifier” works well when fed with CMOS or TTL output signals but as you have found, it doesn’t work at all, when asked to amplify the small analog signal from an earphone output or other source. Not only does the amplifier need to increase the voltage and current swing (amplitude) of the signal, it must do so without noticeable distortion. This generally requires three or four transistors, at the very least, together with correct biasing networks, feedback and frequency compensation and so on, in a practical audio amplifier. Or you can do it with an IC. Either way, the circuit will draw lots more current than a single transistor switching stage. There just isn’t any simple way around the problem if you want a satisfactory audio amplifier. Phantom power wanted I have a condenser microphone but it needs phantom power to run. My multi-track recorder has none. I wonder if you have any project that will give phantom power and con­nect it to your mixer. A few months ago I bought and built the Digital Effects Unit described in the February 1995 issue but I can’t use it because when you talk or sing into it, the delay has distortion. Any help would be much appreciated. (N. M., Fairy Meadow, NSW). •  A preamplifier circuit featuring “phantom power” for a microphone was described in the May 1995 issue of SILICON CHIP. This is available for $7.00 including postage. The Digital Effects Unit should not suffer from distortion. Perhaps one of the op amps is oscillating or the half supply bias at pin 10 of IC1b, pin 12 of IC1a, pin 3 of IC1c and pin 5 of IC1d is not correct. Check for about +8V on each of these pins. Also try 100pF capacitors across the feedback resistors of IC1a, IC1c and IC1d and a 10pF capacitor for IC1b. These should be placed between pins 9 and 8 of IC1b, pins 13 and 14 of IC1a, pins 2 and 1 of SC IC1c and pins 6 and 7 of IC1d. 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. 92  Silicon Chip