Fuses ratings and auto system voltage

Fri Jul 27 07:14:31 CDT 2012

14 times 3 is why the proposals were for 42 volts.

for a period it was thought that such a conversion would sell lots
of new parts for all the redesigns that would result.
it appears, however, that all that work by component and subsystem
developers has stopped because it turned out not to be worth the trouble. 

the auto makers concluded that getting everyone to respin everything for a flag day
was just not going to happen, so they'd need a 12v buss anyway.
besides, the systems that would benefit from 42v are already
working at 12v (including electric power steering) and the runs are
short enough to not be a big enough win.

       -mo

Sent from my iPad so please excuse the jammy fingers.

On Jul 26, 2012, at 5:47 PM, Chip Fetrow <tacos at fetrow.org> wrote:

> This kind of system is now being used in come car models.  It isn't to save weight, but to save the cost of copper, and the space in which to run the wires.  Strangely enough, they are still switching grounds, not hot side.  They are currently mostly using it for lights.
> 
> Regarding higher Voltage in cars -- that is on the table too, and we will be seeing that soon.  We will have lights, radios and so on running on a higher Voltage.
> 
> However, that higher Voltage will be no more than 48 Volts, and some are pushing for 36 Volts, but I believe they will lose.  120 Volts is totally out of the question (at least that is what I have read in the IEEE publication).
> 
> The reason for 48 Volts is that it is under 50 Volts, the cut-off Voltage for the requirement of armored or protected cable.  Anything less than 50 is considered low Voltage, and can be run just like todays 12 Volt wiring.  Still, the savings in wire gauge is considerable, and the design of conventional starter motors is greatly simplified.
> 
> The proponents of 36 Volts have a valid point regarding charging Voltage.  Todays 12 Volt batteries are typically supplied with 13.6 Volts for charging.  If the same ratio for charging of 48 Volt batteries holds true, the charging Voltage would be 54.4 Volts, above the cut off for low Voltage wiring.  Of course, the loads could be fed from a regulator, limiting the Voltage and eliminating the need for armored wiring.
> 
> I have no horse in that race, I'm just reporting on what I have read.
> 
> --chip
> 
> On Jul 26, 2012, at 1:00 PM, tacos-request at amrad.org wrote:
> 
>> Message: 2
>> Date: Wed, 25 Jul 2012 13:50:48 -0400
>> From: Mike O'Dell <mo at 131.ccr.org>
>> Subject: Re: Fuses ratings
>> 
>> fast and precise response is an advantage of the
>> "electronic fuses" now being used in "multiplexed
>> power systems".  the name is misleading because
>> it is the *control system* which is multiplexed.
>> the systems are based on the idea of a large DC
>> bus running the length of a boat or RV with taps
>> feeding electronic switching hubs. the drops for
>> switched loads come off those hubs instead of
>> doing the home-run back to a central distribution panel.
>> the system is controlled by a bus system - usually
>> some flavor of RS-485 - which has control panels
>> placed wherever they are wanted.
>> 
>> the power switching hubs can be programmed so that
>> each channel (varies from 2-16 in a hub) listens to
>> one or several switches (for N-way control), and
>> the overcurrent level and response curve for each
>> channel is likewise programmable up to some maximum
>> for the channel (usually enforced with a fuse, breaker,
>> or polyfuse).
>> 
>> some of the hubs have manual over-rides on circuits
>> so they can be switched even if there is a total
>> failure of the control system.
>> 
>> why add all this complexity just for switching
>> DC loads?  the first answer is "flexibility" to
>> move switches around and add additional control
>> stations very easily, but the *real* reason is
>> WEIGHT. a 60 foot bus of a 00 cable pair weights
>> appreciably less than all the home-run tails that
>> would be missing by just running to a nearby hub
>> on the centerline. This is a 12V or 24V system
>> and I-squared-R is *not* your friend and most of
>> those home-runs are (duplex or triplex) #12 or maybe #14
>> (and the wiring standards prohibit anything smaller than #16
>> for anything but very low current control signals).
>> 
>> in my boat, we had to freeze the design before
>> the buss-network switching systems were baked enough
>> to bet on, so we had to do the home-run thing mostly
>> (small sub-panel forward to take care of cabin hotel loads).
>> If we had it to do over and could use a buss-net system,
>> the electrician that did the wiring estimates that
>> we could save at least 3000 pounds of copper,
>> not to mention the *cost* of a ton-and-a-half of copper.
>> 
>> as you might guess, airliners were the first applications
>> of this technology and it has existed for some time in the
>> "why use lead when gold will do" world of commercial aviation.
>> recently with the explosive growth in power semiconductors
>> and the drop in cost of very cheap micro-controllers, it is
>> now easy to put a spring-head in every switch bank and
>> every power switching pod and take advantage of the savings.
>> 
>>     -mo
>> 
>> ------------------------------
>> 
>> Message: 3
>> Date: Wed, 25 Jul 2012 16:28:41 -0400
>> From: "Bob Bruninga" <bruninga at usna.edu>
>> Subject: RE: Fuses ratings
>> 
>> Very good concept.  DC distribution bus with load control devices to spread
>> the power.  I like it....
>> 
>> But one thing that is driving this is the legacy of 12v DC devices
>> throughout these RV and boat systems, so they still have to back convert to
>> 12v DC at each and every load.
>> 
>> A clean-slate approach would not do it that way.  They might do it at 120
>> VAC and still save even more in the copper because even the main bus can be
>> 1/10th the size.  Then all of the load devices can be conventional 120 VAC
>> and can operate equally on shore power or battery power.  With inverters now
>> being 95% efficient, it makes no sense to distribute any power at low
>> voltage DC.  And with CFL lighting that is a big part of the load. Etc.
>> 
>> Shucks, since 330 VDC even saves another factor of 10 in copper, that woiuld
>> be best except for the main reason of broken connections initiating plazma
>> arcs...
>> 
>> Bob, WB4APR
> 
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