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Laptop needs
more power

Way back in 2003 you published a DC-DC Converter For Cars. I built one and I now have a newer laptop PC and it draws 3A. I can charge it back up again but it will not power it for use. It requires 19V @ 2.85A.

How can I modify it to do the job? (L. S, Ultimo, NSW).

• The 1000µF 25V and 470µF 50V low-ESR capacitors will need to be paralleled with duplicate capacitors to double the ripple rating. In addition, Mosfet Q1 and diodes D1 & D2 will require more heatsinking, while inductor L1 will need to be increased in size by stacking two of the 17-742-22 cores together and winding on about 28 turns of wire around both cores.

The windings are made up of 1.25mm enamelled copper wire or two strands of the 1mm wire twisted together. The 0.1Ω 5W resistor would need to be shunted with another resistor of the same value to double the peak current for L1. The fuse will need to be 7.5A and the power switch rated at 10A.

On the PC board, the current carrying tracks will need to be beefed up by running a layer of solder over them or by soldering several layers of 0.7mm tinned copper wire over the tracks.

Flasher unit for
vintage cars

I wish to build three turn signal flashers for vintage cars using 6V batteries (modern 12V flasher units cannot be used). Only mechanical 6V flashers are presently available and they are not satisfactory. Can you help? (R. G., Kayena, Tas).

• A flasher unit for cars was published in Circuit Notebook for August 1989. It can run on 6V, providing a 6V relay is used. Unfortunately, you cannot access this old circuit on our website but we can supply a photostat copy for $12.00, including postage.

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