Date: Thu, 26 Jul 2007 14:40:00 -0700
Reply-To: neil N <musomuso@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: neil N <musomuso@GMAIL.COM>
Subject: Re: Elec.Gurus! DC ground question (fridge fans off shore power)
In-Reply-To: <46A91087.1040200@gmail.com>
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Thanks Mike
I found out from dude at electronics place that modern case/cpu fans
(or DC fans in general) are circuit protected (I think that's the
correct term). And in fact if something were to get jambed in rear
fan, it simply shuts off if it stops turning. Of course a short is a
different beast. Good to know 12V power adaptor (wallwart) may be
fused.
Re inline fuse:
I really should install an inline fuse on 12V power adaptor to this
new circuit.
As for the house supply, since fan is downstream from the 15 Amp fuse
used for fridge etc., shouldn't that suffice?
Neil.
On 7/26/07, Mike Rocket J Squirrel Elliott <camping.elliott@gmail.com> wrote:
> Loren is right that in order to make sure that every bit of current that
> you put into a device gets back to the source without interference from
> other sources+devices you not only provide individual feeds to those
> devices, but individual returns, too. However, for the small currents
> you're talking about, the chassis of the vehicle is a very low impedance
> path and so if your wall wart supply happens to share its return with
> some other devices, there won't be any problem.
>
> Unless I was going up into the 10s, maybe 100s, of amps I would use the
> chassis as the return conductor. The test to see if you're losing
> anything by doing so is to measure the DC voltage between the - side of
> the wall wart power supply and the - input of the device (fans) while
> the fans are running. If the return conductor (chassis) has zero ohms,
> you will read zero volts lost in the chassis. If you see more than zero
> volts then check your connections because those fans probably don't draw
> more than a 1/10th of an amp and if you happen to be reading, say, 1
> volt between the fan - and the wall wart -, then that is Resistance =
> Volts/Current = 1/0.1 = 10 ohms, which says you have some pretty crummy
> connection someplace.
>
> Besides -- fans are hardly sensitive electrical devices. Bang the
> wallwart - to the chassis and the fan - to the chassis and be done with
> it. Fuse the + line if you want to, but wallwarts are usually fused
> internally, I think.
>
> --
>
> Mike "Rocket J Squirrel" Elliott
> 71 Type 2: the Wonderbus
> 84 Westfalia: Mellow Yellow ("The Electrical Banana")
> 74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
> KG6RCR
>
> On 7/26/2007 1:55 PM Loren Busch wrote:
>
> > RE: 12v Grounds for Accessories
> > Some are going to argue that I'm overdoing it and that using chassis ground
> > for accessories will work fine. BUT, having had a really good education of
> > grounds and ground loops back in the '60's (analog computers) I have a
> > simple answer: Run a separate ground for each circuit and ground
> > everything
> > at the same point. I don't tap into any existing circuit if there is any
> > way to avoid it, I run a double wire, hot and ground, to whatever I'm
> > wiring
> > in. All accessories added come out of a fuse block on the cabinet wall
> > behind the drivers seat. That is fed directly (through a 40 amp breaker)
> > from the aux battery. Right now I have a ground buss set up and mounted
> > next to the fuse block, soom to be incorporated into a new, better fuse
> > block that will replace the current setup. In a previous life ('85 Westy)
> > the PO had added two lights in the back, good halogen lights, plenty of
> > illumination. But, they worked sporadically and never when you really
> > needed them. When I got around to troubleshooting them the problem was
> > grounds. The DPO had assumed that he would get a good ground through the
> > mounting screws that went into the body of the Vanagon. Once those grounds
> > were fixed no more problems.
> >
>
--
Neil Nicholson. 1981 Air Cooled Westfalia.
http://web.mac.com/tubaneil
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