Re: Basic current question

On Mon, 12 Jun 2006 20:17:34 GMT,
a?n?g?e?l@xxxxxxxxxxxxxxxxxxxxxxxxxxx (The little lost angel) wrote:

On Mon, 12 Jun 2006 17:44:48 GMT, Jonathan Kirwan
<jkirwan@xxxxxxxxxxxxxx> wrote:

Unless there is a known reason otherwise, the resistance in a wire in
a diagram should be mentally treated as a perfect conductor.

Actually yes, I was trying to figure out if connections matters in a
long chain of components when total current draw is low, say 2A vs 5A.

Ah. Well, "low" or "high" depends on what's going on. For my kinds
of circuits, 2A and 5A are both _very_ high. For a circuit card, that
kind of current will take a little thinking about trace widths and/or
depths.

Sometimes, it helps to redraw things this way.

Yup, that does make it much clearer when I was confusing myself. The
silly thing is, I actually got the thinking right at first, then had a
brain fart when I started putting in numbers :/

I tend to redraw every circuit I'm interested in understanding in
order to remove busing of power around. The wires don't help me
understand them, in fact they make it harder for me, so I get rid of
them. Then I try and divide it into sections such that there is a
minimum of signal lines going between the sections. (You can usually,
without really understanding anything, see what parts can be teased
out leaving only a few lines coming in and out and it is usually the
case that when you do that you will accidentally and in ignorance
luckily also be breaking out the functional groups, as well.)

in high current circuits where the resistance
can matter, in high voltage circuits where some physical distance may
help

How high is high?

Depends on the resistance of the wire (size and run length) you are
working with and how important the quality of the voltage rail is to
the circuit being powered, I think.

I was wondering as said, if putting something
further down the chain could cause it to fail simply due to the
voltage drop. i.e. in a typical PC, you get connectors in a chain of
parallel connections, and usually when people run out, they start
adding splitters. So I figured that if you used too much wire (nothing
huge say just 20cm per extension), things at the end of the chain will
simply not get enough voltage esp since more things = more current =
higher IR losses right?

Somebody told me it's the resistance of the connectors, especially if
the mating is poor, that is probably going to cause problems rather
than wire length. But I couldn't find any information on how to
determine that, searching a site like www.molex.com was just PITA and
spec pages don't appear to tell how much resistance the pins have.

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.

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.

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
.

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