From: Marcus Andree S. Magalhaes (marcus.magalhaes_at_vlinfo.com.br)
Date: Thu Jun 17 2004 - 08:07:14 PDT
<snip>
> Incidentally, I recently tried a science experiment in my car,
> attempting to run one of my MP 2X00s off of a Radio Shack cigarette
> lighter adapter. The RS car adapter has a switch to adjust the voltage
> output from 3-12 volts (I set it at 7.5 volts, of course). Output
> current for the device is rated at a maximum of 300 mA. My MP ran just
> fine off of the adapter -- for about two minutes. I blew one of those
> dinky-assed fuses inside the body of the adapter plug.
>
Very predictable. My 9W AC adapter suggests a current of about 1.3 Amps.
300mA can't do the job.
>
> I'm trying to figure out why the fuse blew, and the best answer I can
> come up with is that the Newton's load on the circuit was too great for
> the adaptor to handle (Reminded me of the old garage band days, when I
> discovered that one could not connect an infinite number of speakers in
> a series circuit to a guitar amplifier rated at 8 ohms! But I
Yes. It's the best answer. There are two kinds of fuses: slow and fast.
Fast burning fuses tend to accept large currents for a very short time
and, then, do its smelly job of opening the circuit, protecting more
vital components. Slow fuses hold a large current for a bit more time.
They're useful with, say, inductive machines (like electrical motors)
that, right when turned on, tend to generate a large peak of current.
> digress...) I suppose that I could place a LARGER fuse into the RS car
> adapter, but that would probably mean that a secondary fuse under my
> car's dashboard would then fry out. If any electrical engineers out
> there have a suggestion, please post a response. In the meantime, I'm
> thinking a DC 12 - AC 120 power inverter is the kludgy solution to
> charging the Newton on camping trips this summer.
>
Well, guess what!? I'm an electrical engineer... If you put a larger fuse
into the car adapter, you'll probably fry your adapter, not your dashboard.
I really don't know the "standard" current consumption of newton devices,
but you can, say, replace a 300mA fuse with a 500mA one. If your MP needs
more than that, you'd better try another option (see below). This also
will likely reduce your adapter lifetime.
Since we're talking about a source of 12V DC (your car battery), you
could do a simple voltage drop by using common diodes. Silicon diodes
provide a voltage drop of about 0.7V each. You can solder, say, 6 then
in series, resulting on a voltage drop of about 4.2 V. Check the
output with a voltimeter, just to be sure that it is between acceptable
and safe values.
Don't forget to enclose your diodes in a well ventilated case, since you're
getting a heater for free: they'll dissipate around 5W. Diodes from a
family called 1N400X can do this job nicely. Buy 6 1N4004 diodes
and you can even use it in your home, since they can hold up to 400V.
There are better options, by using voltage regulator chips of a family
called 78XX, but they're a bit more complex....
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