Date: Thu, 26 Jul 2007 14:22:15 -0700
Reply-To: "Mike \"Rocket J Squirrel\" Elliott" <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "Mike \"Rocket J Squirrel\" Elliott" <camping.elliott@GMAIL.COM>
Subject: Re: Elec.Gurus! DC ground question (fridge fans off shore power)
In-Reply-To: <86476e250707261355o24b2d3e5g70f4af89defd499@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
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.
>
|