car charger

[Joel Newkirk]

On Fri, 10 Oct 2008 20:01:47 +1300, "Robin Paulson"
<robin.paulson at gmail.com> wrote:
> 2008/10/10 Joel Newkirk <freerunner at newkirk.us>:
>> Just wanted to post that I received
>> http://www.cross-mark.com/charger-adapter-black-1000mah-p-962.html in
> the
>> mail today.  The FreeRunner automatically kicks into 1000ma charge mode.
>> :)  Best $3.00 (delivered) I've spent in a while.
> 
> $3? bloody hell, i'd be very wary of that powering a $300 phone. car
> power is notoriously unstable; i can't remember where i read it now,
> but it's an iffy power source at the best of times. there's a bit of
> info on mini-itx.com:
> 
> http://www.mini-itx.com/store/?c=10#int
> 
> the implication being that it spikes/troughs a lot, particularly at
> ignition

Yes, I'm aware of the vagaries of DC in a car.  I tested the output of the
adapter, and it's a very steady 5V at all times, except when cranking the
engine - at which time it drops rapidly (but not instantly) to zero or
near-zero.

Now, it may just be that my car has particularly stable 12V... at least on
accessory/lighter jack and likely the stereo.  But since we're only using
5V then significant fluctuations in the 12V supply tend to (hopefully)
disappear within the adapter.

The concern in the case of mini-ITX and similar systems (I have one as a
mini-workstation ATM, plan to mount indash in a month or two when I can
spare the cost of the 7" touchscreen and mobile PS) is that most power
supplies for them are powered by 12V, and people think they can just run
the car's 12V straight in with little or no cleanup.  mini-itx.com,
logicsupply.com, and other vendors emphasizing mini-ITX and automotive
computing offer usually a few different DC-DC power supplies - one of which
IIRC expects regulated 12V on input, which it then simply uses to provide
various voltages for mobo and drives - cheaper PS that way, if you assume
regulation is handled 'upstream', but NOT safe with most car electrical
systems, which in general can fluctuate to 10V or 15V. (or beyond,
especially downward toward or to zero when starting engine) A 'proper' PS
for automotive computing needs to deal with such fluctuations while still
providing clean stable 12V & 5V rails.

j