Cornwall
Austin Seven Club
creating
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Cornwall
Austin Seven Club
creating
interest in Austin Sevens
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A 6v to 12v Converter (if you wish to fit a Sat-Nav)Many of us enjoy our 'Sevens' on unfamiliar roads or over considerable distances, both at home and abroad. Maybe to some distant rally, maybe over a new route. Until recently, this necessitated a pile of good maps and a passenger to read them. However, we now have a few more options. A member contacted me to ask if it were possible to make a device that would keep a sat-nav system charged up when used in a car running on 6V. Sat-Nav devices work fine for a few hours, but if you are out all day, rarely have enough internal battery power to keep going. I had made a small converter with enough power to charge up a mobile phone, but a sat-nav is an altogether hungrier beast. A quick surf on the net revealed that there is a device on the market that can be adapted to generate 12V from 6V. This is a “Simple Switcher”® step-up voltage regulator made by National Semiconductor, (part number LM2577-ADJ). Together with a few extra components, a heat sink and some wiring, it is possible to generate over 1A from the battery of an Austin. The output connector is taken from a car charger extension lead, so that a phone or a sat-nav just plugs straight in. So as to best replicate the power available from a modern, I chose to generate 13.5V rather than 12V, which has the benefit of being a better match to sat-nav chargers, mobile phone chargers and even SMALL car inspection lamps. If you really wanted, it would even power a CB set or a normal car radio – not that you’d be able to hear it on the move! A prototype was assembled, and has been on trial in a number of cars during recent trips. I have to say I’m pretty pleased with how well it works, and so now offer it to other members. For those with a technical bent, I have included the circuit below.
There are a few important precautions to observe to get the best out of it. Firstly, the regulator will need a heatsink, preferably one with some fins. Secondly, the input capacitor must be as close as possible to pin 5 of the chip – not remote by a length of wire! The inductor should be a toroidal type, rated to at least 4A, and the output capacitor must have a low ESR. The parts supplier (Farnell Components) has a minimum order charge, and the approximate cost for parts is £25. It all fits into a small plastic box, about 3½" x 2" x 1½", with a trailing socket like the one in the dash of your modern. On the other end will be two lengths of wire, one red (positive) and one black. In addition you will need the appropriate terminal, (bullet terminal, ring tag or whatever - chosen to suit your car) and an in-line fuseholder. I had a spare hole in the dashboard, so I popped in a switch so it’s only in use when I need it. Fitting is very simple. Assuming the car is fitted with a battery isolator, (as all cars should be to prevent risk of fire), the module is simply connected to the output of the isolator via a 5A fuse. This is necessary as the converter consumes a small amount of power all the time, about a tenth of an amp, so would otherwise flatten the battery whilst the car is not in use. If you have no isolator, the (very second best) place is on the output of the ignition switch. Don’t connect the supply to the IG side of the ignition coil as there are too many spikes on the supply and the chip will not survive. As for mounting, mine lays in the glovebox, so that the wire to the sat-nav is not in the way.
This article, written by Geoff Hardman, originally appeared in CA7C Seven Focus in Jul 2008 pp25-26.
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