From: "Da Man"
Subject: Re: Operating a 6V max circuit off of a 12V car battery
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Mon, 09 Dec 2002 18:38:24 GMT
NNTP-Posting-Date: Mon, 09 Dec 2002 14:38:24 AST
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!).
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!
> This will eat all the
> Now, take the input pin of the 7805, connect to the capacitor, and
> on the output hang a .1uf capacitor.
> Then connect the 7805 to the load.
> This should take care of even the worst spikes, as well as limiting the
> current to a few tens of ma under fault conditions.
> http://inquisitor.i.am/ | mailto:email@example.com | Ian
> Q: What do you call a train that doesn't stop at stations?
> A: Thomas the Bastard. --