From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: dc to ac power inverters
Message-ID: <9ftu1vccjgnbbroj2aa224mj7gbm9oe00h@4ax.com>
Date: Fri, 10 Jan 2003 20:44:18 -0500

On Fri, 10 Jan 2003 15:41:24 -0800, "BD" <bd_iam@REMOVETHISyahoo.com> wrote:

>uh...how does a 12-14v rectified alternator, wired to a 12 volt battery
>produce spikes to 270 volts?

Exactly the same way the ignition system can make up to 40,000 volts or more -
by quickly interrupting the current flowing through an inductor - such as a
fan, an alternator field, a starter motor, etc.

>I've never had a radio, tape or CD player ever
>go TU from this.

That's because they're designed to work in the environment.  Find you a home
radio that takes 12 volts in from a wall wart and hook it up to your car's
electrical system.  Probably won't last through the first cranking cycle.

>Are you saying it's likely that my 12v accessory outlet
>will one day see over 100 volts, even if only for 400ms? I must have missed
>the previous threads.

All the time.  Hang a scope on the battery terminals and you can see a lower
energy transient each time the ignition coil is fired.  Some tachs work on
that principle - counting the pulses on the incoming 12 volts.  Load dumps are
the mothers of transients in the automotive environment.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: dc to ac power inverters
Message-ID: <450v1vk44qb87fn8gr1iqo9fkckskaa0ad@4ax.com>
Date: Fri, 10 Jan 2003 21:27:00 -0500

On Fri, 10 Jan 2003 17:28:55 -0700, "Tex Houston" <texhouston@pcisys.net666>
wrote:

>I'm a little puzzled about this thread.  Why would I want to run an inverter
>to convert DC to AC to plug in my power supply which feeds DC to my laptop
>when I can take the car/airplane cord which plugs in a DC power outlet into
>which I plug my laptop?

a) because a $29 inverter will do the job (and let you run other stuff) in
place of a $100 car cord.

b) the $29 inverter will take the hit on a severe load dump instead of your
laptop or car cord.

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: dc to ac power inverters
Message-ID: <ipp02vgg830rm6hkjgrt0der4i1i535bbt@4ax.com>
Date: Sat, 11 Jan 2003 13:58:31 -0500

On Fri, 10 Jan 2003 21:23:06 -0800, "BD" <bd_iam@REMOVETHISyahoo.com> wrote:

>Hello again, w_tom
>If you knew me, you'd know I'm neither really terrified, twisted or
>emotionally perverted...well, maybe a little twisted.
>A couple of you folks chimed in with the technical reasons for caution,
>while others have been carrying along pretty much ok with their inverters. I
>believe I'll plug my laptop into the new inverter and pigs will not fly nor
>will the sky fall. If bears fail to shat, that's what insurance is for.
>Thanks for your comments though. I'm sure I'm not the only person interested
>in this topic.

Where you're getting confused is in the role of the inverter.  Presumably the
inverter is designed to live in the environment that exists in a car
electrical system - if it isn't, the warranty claims would be horrendous.  So.
The inverter either survives the load dump transient or it doesn't.  Either
way the transient does not make it through to the AC load on the other side.

I know, because I've been all through the thing figuring out how to run
multiple inverters in parallel, that the Sam's Club Vector is indeed very well
protected from load dumps and other transients.

As w_tom said rather indelicately, load dumps don't happen all that often.
It's kinda like having a storm shelter - a tornado won't hit you very often
but when it does, that storm shelter is mighty nice to have.

John