Subject: Re: Operating a 6V max circuit off of a 12V car battery
Date: Tue, 10 Dec 2002 00:20:46 +0000 (UTC)
NNTP-Posting-Date: Tue, 10 Dec 2002 00:20:46 +0000 (UTC)
User-Agent: tin/1.5.14-20020926 ("Soil") (UNIX) (Linux/2.4.19 (i686))
Da Man wrote:
> wrote in message
>> NleahciM wrote:
>> > wrote in
>> > news:firstname.lastname@example.org:
>> >> It's not, it varies from about 6-80V under various fault conditions.
>> >> What's the current needed from both?
>> > Good lord! Never would have guessed that it would be that bad! Current
>> > should be around 5ma normally. Well, that's going on the assumption that
>> > relays (which will only be operated very occasionally for only a couple
>> > seconds) don't use very much current. Thanks!
>> It's not normally that bad, but that's really what you should design
>> If that's the case, then I'd first consider a battery, to avoid the
>> hastle of connecting to the car.
>> A 7805 will work down to around 7V input.
>> A fairly simple circuit, such as a 1n4007 diode, and a 100 ohm 1W resistor
>> in series to the car supply, a 4700uf filter capacitor going from these
>> (in parallel with a .47uf capacitor) to ground.
> Adding a transorb, low voltage MOV, high power zener diode (also adds
> reverse polarity protection!), etc will significantly reduce the size needed
> for the cap, and will actively limit the spikes. The resistor can be
> replaced with an inductor for more power (almost no voltage drop) (also
> watch out for resonace!).
Very true, but in the case of a one-off, the oversized cap is probably OK,
especially for such a low current.
I don't think anything in a car can kill the above circuit.
> For your needs, a 78L05 will likely be fine (L for low power, comes in small
> packages, such as TO 92, SOT 8, etc)
> If any I/O lines are needed, make sure they are well filtered and buffered!
Shielding is good, though any shielded wires to stuff should have the
shielding grounded only at one end.
http://inquisitor.i.am/ | mailto:email@example.com | Ian Stirling.
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