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Changing the Zapper to suit 8V batteries

I recently purchased and built the Battery Zapper (SILICON CHIP, May 2006) and it works perfectly. However, is it possible to change the 6V setting to 8V or 9V. There are a lot of 8V batteries used in golf carts etc. I feel that it would be better to treat batteries independently instead of in 24V banks, don’t you?

I would use a 9VDC 1A plugpack during zapping and then revert to the normal 48V charger when zapping is completed. (T. Q., via email).

• We would suggest treating 8V lead-acid batteries independently instead of in 24V banks.

Only one modification is needed to change the 6V range for checking 8V batteries. This involves swapping the connection from the “6V” position of switch S2b from the junction of the 270Ω and 220Ω resistors in the metering voltage divider down to the other end of the top 220Ω resistor (ie, to the junction of that resistor and the 220Ω resistor below it).

However, for improved operation of the Condition Checker section of the project, we suggest that you also make some of the changes incorporated into the “Mk3” battery checker which was described in the August 2009 issue.

Manual controller for thermoelectric cooler

I want to build a simple controller to manually adjust a 96W 12V thermoelectric cooler. Your 12V Speed Controller/Lamp Dimmer project (SILICON CHIP, November 2008) seems ideal but only handles up to 50W. The earlier 12V/24V 20A design featured in the June 1997 issue is more than I need (and costs twice as much).

Can the 12V Lamp Dimmer be upgraded to handle a 100W cooler by simply changing the Mosfet? If so, which component should I use in place of the one included in the kit? (R. M., via email).

• For an 8A load, the MTP3055 is not suitable (as you suggest) because it is only rated at 12A and has an on-resistance of 0.15Ω. Its dissipation would be just under 10W at full power to the motor. It therefore should be changed to an IRF1405 (169A, 5.3mΩ). This has a much lower on-resistance, so there will be less heat generated within the Mosfet.

The fuse should be rated at 10A and the tracks leading from the screw connectors to the fuse, the Mosfet’s drain and source and to diode D2 should be thickened by running solder along the tracks or add some tinned copper wire between the connections on the PC board if it is solder-masked.

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