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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 silicon@siliconchip.com.au

NiMH cells can
charge up to 2V

I have built the Fast NiMH Charger featured in the September 2007 issue and I have a question regarding the output voltage. Nowhere in the article does it indicate where one sets the output voltage for the unit.

When I hook it up to my NiMH cells it basically charges a 7.2V bank at 12V. It seems to follow all the correct cut-off procedures etc but it still worries me that I am charging the batteries at this level. Any suggestion will be greatly appreciated. (D. B., via email).

• The charging voltage depends on charge rate and the amount of cell charge. Typically, the cell voltage can rise to 2V when charging. So for your 7.2V battery with six cells it is normal for the battery to be at 12V while charging. End of charge is detected by the temperature rise.

Note that NiMH and Nicad cells are very different to lead-acid batteries where it is the voltage that determines end of charge.

Residual voltage in Screecher car alarm

I completed the Screecher car alarm kit described in the January 1999 issue of Electronics Australia. The alarm is connected to IN1 to the door switch and is working but I have one issue.

There is a residual voltage of 1.7V which lights the LED when the unit is not powered and the doors are closed. Currently the alarm is working with the LED disconnected.

Could you please provide me guidance on what could I do to remove this residual voltage and allow me to use the LED as per normal operation for the alarm kit? (C. L., via email).

• Try connecting a 4.7kΩ resistor between the base and emitter of transistor Q1, the LED driver.

Advice Wanted On HDTV Antenna Installation

I have recently purchased a HDTV and need to install an antenna plus cabling. Can you please advise as to what I may need to do an installation, ie, antenna, cabling, terminations etc? I live Canberra. (M. F., Chifley, ACT).

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