Missing the point!: (Was Re: [aprssig] Portable Ops from 12v via 115 VAC}

Norm McMillan njmcmillan at bigpond.com
Wed Dec 29 17:14:31 EST 2004


you're missing the point!

I believe the original point was, that it takes big fat cables to carry 
the POWER any distance at low voltage. (Nothing to do with the POWER 
drawn from the car battery.) I suggest you go back and read the original 
post carefully.

POWER(W) is E(voltage)x I(current) by increasing the E, I can be reduced 
for the same amount of POWER delivered. Since losses over a distribution 
system are I^2 x R. reducing the I in the distribution system (wiring!) 
reduces the losses dramatically!!

Every utility does the same. Raises the voltage for the long lossy runs 
{33 or 66 kV is standard in Australia,) then drops it (230V) at the 
short service connection!

Think about it!!

Richard Amirault wrote:
> Third paty comment....
> Bob, you don't address his main point .. that an inverter (at or near 
> peak load) draws *considerable* amps from the vehicle electrical system.
> And, assuming NO LOSSES (100% efficient) a 12v rig drawing 20 amps will 
> draw 20 amps from the vehicle   (20 amp 12vdc into the inverter, 120vac 
> out to the power supply, 12vdc out to the rig) And of course it wlll be 
> more because of losses.  YES, for *transfering* the power, over a long 
> distance, the higher voltage is better, but his point (I think) was that 
> a vehicle may not be able to sustain this drain for very long.
> Now, having said that, I'm not sure how much current you will need to 
> run all your equipment. This will vary, of course, from person to person 
> but obviously peak power is on transmit, and you won't be transmitting 
> all the time, or even most of the time (unless you were net control, or 
> something).
> Richard Amirault              N1JDU
> Boston, MA, USA          Go Fly A Kite
> ----- Original Message ----- From: "Robert Bruninga"
> Subject: [aprssig] Portable Ops from 12v via 115 VAC
>> For long distance portable operation from the car's 12v
>> system, dont overlook using lots of small wire and an
>> 110 inverter.
>> Run an efficient inverter at the car to 110 then run
>> your portable station on  AC using modern switching
>> supplies.
>> Even if your 12v rig draws 20 amps peak, this should
>> only be about 2 amps over the AC cord and even if the
>> drop over 300 feet of zip cord goes down to 90 volts, almost
>> all switching power supplies these days can still work
>> and deliver their rated output at the load.
> (snip)
>> Bob
>>>>> wa7nwp at jnos.org 12/29/04 2:37:58 AM >>>
>>> Bill, are you suggesting this actually WORKS?
>>> I have a 300 watt inverter (500 surge), and it
>>> Draws OVER 30 AMPS.  No car alternator will put
>>> Out enough current to keep up with the drain at idle.
>>> Are you willing to put a brick on your gas pedal and
>>> Idle at 3,000 RPM to get enough current?
> (snip)
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Norm, VK2XCI,
Voice of The Edge of The Outback.

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