DD>> National Semiconductor claim 70-80% efficiency....

BL> ROFL! As the Arab replied whn asked the price of his camels...
BL> it depends on whether you are buying or selling.

DD> Hmmm - false advertising... in this country. Isn't that
DD> illegal?

 There are no local manufacturers to take the lying bastards to
court. Haven't you noticed that your 17" computer screen is actually a
bit less than 16"? Have you measured a TV screen lately? My "72cm" is
actually 68. In the old days, if someone advertised an outright lie
like that, all hell broke loose. Now they claim car stereo  and
computer audio power that is ridiculous. A 2-inch speaker rated at
40W? In a pig's arse. Toyota engines seem to have grown tiny horses
when they convert to kilowatts.

 The laws are on the books, but who complains? It's reached the
stage where a specification is like reading science fiction. when I
bought my TV, I carried a tape measure! The funny part was that the
salesman admitted the lie as soon as he saw it.

BL> NOW, I understand why you think it won't run off a 6V
BL> battery... you think it may be AC! I'd be surprised if it was.

DD> No, the PSU definitely puts out DC. It was the rating sticker
DD> on the bottom saying "6.3C 3A" that made me think it might need
DD> 6.3V 

 For God's sake, David. Don't you know that a "6V" lead-acid battery
is actually 6.3V, or to be precice, 6.0V flat, 6.3V nominal, 6.7V full
charge, 7.0V maximum standby charge, 7.4V maximum cycle charge. It
varies a bit with temperature too, and an old battery tends to increase in
voltage a bit.

BL> An extra 6V battery looks better all the time! Or just tap into
BL> half of your 12 volts. You can usually see where the connectign
BL> bars are... just screw in a self-tapper. 

DD> I have 6 x 2V cells. Each cell has a "bolt-on" +ve and -ve
DD> terminal.

 How easy is that? Just run an extra wire from the bottom three cells
and you're laughing. They *are* lead-acid, I suppose?

BL> How do you charge your "country mansion" battery?

DD> Solar panels, and a small petrol generator for when I get too
DD> many cloudy days in a row. 

 Terrific! That's a great setup. What's the ampere-hours? About 80?

DD> In the winter, when the sun is at a lower angle, the panels are
DD> shaded by trees a bit too. If we're there more than a few days
DD> I need to run the generator to bring the state-of-charge up. In
DD> the summer the sun passes almost straight overhead and I get a
DD> shitload of amp-hours from the panels each day. The 12V fridge
DD> draws more power in the summer. 

 What a great setup.

 IMO, you'd be mad if you didn't just run a 6V point out of your
batteries. Lead-acid batteries are quite tolerant of unequal charging
(over-voltage), so if you ran one half a bit flat and then charged
the whole thing together it wouldn't matter too much. If it's not
lead-acid, you'd have to be a bit more careful about running one half
flat.

DD> Yeah - I've now been looking at their adjustable 3A series
DD> regulator chip. Much simpler circuit with only a few
DD> components. 

 No! Just run it straight off three cells! If you're worried about
it, there'll be a series regulator inside the DVD. Just put your
finger on it and see how hot it gets. My guess is that the
DVD actually runs off 5V (maybe the motor has its own regulator).
Worse case, with the battery charged right up to 7V, you're looking at
a total loss of (7-5)*3A = 6W. That's bugger all.

 Plus, I'd be really surprised if the DVD takes as much as 3A (once
the motors are up and running). 1A sounds more likely... and 2W is nothing.

Regards,
Bob
    


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