Re: Basic current question

On 14 Jun 2006 22:09:55 -0700, stratus46@xxxxxxxxx wrote:

Jonathan Kirwan wrote:
<snip>
It's especially true in cases, like coax cable runs with RF, that
you
lose at least 50% of your power with each and every
break/connection.
For typical non-RF situations just passing along power, the
connectors
are still important and they will have some resistance. Over time,
their surfaces will oxidize and those surfaces are less conductive
and
therefore will become more of a problem in high current situations.

Losing 50% of your power at every connection? Commercial transmitters
would all be on fire.

My thinking, when saying what I said, came from something along these
lines:

http://www.trendcomms.com/multimedia/training/broadband%20networks/web/main/Copper/Theme/Chapter2/Impedance%20Matching.html

I apologize if I misrepresented that idea.

A very common example you can see in almost any home with a room far
away from the power panel is to hook up a power saw and try to cut a
2x4 piece of wood with the room light on there. You will see the
light dim when the saw is working. But this is because typical home
power circuits are wired with inadequate wire gauges for long runs
(they use the same wire everywhere throughout) and there is enough
voltage drop when pulling 10-15 amps to lose 15-25 volts.

15 volt drop at 10 amps is WAY HIGH. Hope the insurance guy doesn't
find out about it after the house burns down. My house wiring is crappy
and it loses less than 1/3 of that.

I measured roughly a 15V drop, running a power saw in a room at the
far end of a home. The run of wire to that end of the home, including
various loops around the walls and via various connectors, appears to
have been around 100 feet each way. The wiring in the walls was 14
gauge aluminum. Not sure what the actual draw was from the saw, but
it was enough to drop about 15V. The reason I bothered with any
measurements at all was because the lights dimmed quite substantially.
Bugged the hell out of me.

The answer is to use sufficient conductor cross-section for what you
are trying to achieve and/or avoid series connectors along the way
if
that is a problem. Best is to use a separate set of adequate
conductors without any intermediate connectors, going all the way
right back to the main supply for everything (starring.) But that
presents its own problems (cost, time, etc.)

Jon

I disagree about treating the wires as perfect conductors -- even for a
newbie. That is how the dreaded ground loops get started. It can make a
big difference even on signals as 'trivial' as 1V P-P video into a 75
ohm load. Most newbies treat 'ground' as an afterthought while to me,
it is the most important thing to get right from the beginning. Power
regulation/distribution comes next.

I can't imagine trying to understand a schematic by first replacing
every wire with some complex representation of its impedance and then
trying to fathom it. We will just have to disagree with each other on
this point.

By the way, I took courses on the Tektronix campus back in the 1970's
and this is how they also taught me to examine schematics. It's a
thinking method not of my own concoction.

Jon
.

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