Details about electrons moving in a wire, --interesting Youtube video Options

[Dave_Darling]

Very very tangential relevance to 914s, but if you ever wanted to know details about electrical flow in your electrical system...

This video focuses on what happens when you have one pair of wires connected to another pair in a Y configuration, but it also contains information on how electrons flow through a circuit--


https://www.youtube.com/watch?v=2AXv49dDQJw


It's very neat to watch the visualization of the "wave" of voltage (and similar wave of current flow) that propagates down the wire. The way he set up the measurement is very complicated, but the results are awesome.

--DD

[mskala]

Yes, this is very good at showing high-speed electrical circuit behavior without
a bunch of boring math. He didn't show this, but if there is an equivalent resistance
at the end of the wire matched to the characteristic impedance of the line, then no
reflection back will happen. When building a high-speed circuit board, any stubs
like he has, even very short ones, mess up your signal.

Another thing most people don't think about is like on really old computers, even if
you are running at 1MHz, all this back-and-forth settling is still happening, but it's
settled out before the next clock tick so it doesn't hurt anything.

[Dave_Darling]

... but if there is an equivalent resistance
at the end of the wire matched to the characteristic impedance of the line, then no
reflection back will happen.

That's actually in the next video! (IMG:style_emoticons/default/biggrin.gif)


https://www.youtube.com/watch?v=RkAF3X6cJa4


--DD

[Superhawk996]

Automotive CAN bus principle and why the bus lines have to be terminated properly.

Typical CAN termination resistor is 120 ohms. Only two termination resistors can be used. Measuring across the CAN high and CAN low wire you should see 60 ohms (two 120 ohm resistors in parallel = 60 ohms).

When adding additional CAN modules, if the new module has an unexpected termination resistor, it will screw up the whole CAN bus. Can’t tell you how many times I’ve seen this problem occur on prototype vehicles.

Likewise, if one of the modules that contains the 120 ohm termination resistor dies catastrophically or its termination resistance is removed from the bus there will be communication problems.

Whenever I’m trying to trouble shoot a flaky CAN issue, 1st step is to measure across the high/low lines looking for the proper 60 ohms.


The joys and nightmares of modern vehicles where CAN has become ubiquitous.

[fixer34]

My real job is working on computer systems, seen flaky things with signal termination and reflection for years.

Not so much on static DC voltage connections like our teeners, although the FI/ECU wiring could be affected.

The part that stood out for me in his video was the fact he shorted the end of his 'terminated' wires rather than put an actual load on it. The 9v battery has a rather small amperage capacity that it can generate so after the initial pulse, it would discharge/drain the battery down to nothing in a pretty short time.
I want to see him do this same test with a fully charged 914 battery. The initial current pulse will probably be about the same length of time, but a LOT higher as it lets all the smoke out of the wires...
Closer to reality for some of us.

[Dave_Darling]

You'll note that he was using an O-scope to see all of the stuff. Which I'm pretty sure means that he was turning the circuit on and off quite quickly--I think that he made a small timing circuit to do that for him. If most of the time was spent with the circuit off, the wires were probably safe enough.

Good enough for a demo!

--DD