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Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097; or send an email to silchip@siliconchip.com.au

Charging 4.8V batteries

I want to build the Multi-Purpose Fast Charger featured in the February & March 1998 issues of SILICON CHIP. My problem is that I want to be able to charge 4.8V NiCd battery packs (for R/C model aircraft) of 600-1200mA.h. Have there been any updates to the circuit to allow this? If not, what modifications would need to be done? (B. V., Tindal, NT).

 The value of divider resistance for a 4.8V battery pack will be 33.3kΩ and will need to be used instead of the dividers used for a different voltage selection. For example, you could remove the 150kΩ and 12kΩ paralleled resistors for the 12V NiCd position and place the 33.3kΩ value in their places. This will change the 12V NiCd setting to 4.8V. A 47kΩ and 120kΩ resistor in parallel will be close enough to the correct value.

13.8V regulated power supply

I have the November 1997 issue which describes the Regulated Supply for Darkroom Lamps. It has very good regulation but I am interested in making a 13.8V version. Can you suggest what modifications will be required? (I. S., via email).

 The circuit should provide 13.8V with no modifications being necessary. Just set trimpot VR1 for 13.8V instead of 12V. By the way, you do realise that the output is not smooth DC but pulsed. It is OK for driving motors or lamps but not suitable for other loads.

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