Talk:Kelvin bridge

Inconsistency

The labeling of the diagram is not consistent with the discussion. Leonard G. (talk) 20:09, 11 May 2008 (UTC)[reply]

Where is Rx? -- ☑ SamuelWantman 03:00, 22 December 2008 (UTC)[reply]

The diagram in the article doesn't match cited one

The diagram on http://www.allaboutcircuits.com/vol_1/chpt_8/10.html implies a true four-terminal resistance measurement (Kelvin method) but the diagram in this article seems to show a (less accurate) three-terminal method. The diagram at allaboutcircuits isn't very illuminating either. The right-hand bridge appears to be floating with no voltage source applied at the top and bottom vertices. All very confusing... Woz2 (talk) 15:25, 4 June 2012 (UTC)[reply]

I don't understand your issues.

Both articles show the KB trying to eliminate the wire resistances. The all about circuit's ("AAC") explanation does a better job, but its diagram is in error by showing tapped connections on R_a and R_x rather than connections at the top and bottom of both resistors. All about circuit indicates an R_wire, but does not explicitly draw it in the diagram. Similarly, the WP article just calls it out as R.

Both articles use 4 terminal sensing. Low impedance drive uses one path; the high Z measurement is taken along another path. One might even say they are using 6 terminal sensing due to the resistive divider bridging R (or R_wire in AAC) adding two more sampling wires.

Everything is floating -- neither article shows a ground. There's an implicit null meter in the WP article measuring U_wy (which is labeled U instead of V because schematic is borrowed from another WP); there's an explicit null meter in the AAC's article (it is drawn as a circle, but it is a meter and not a voltage source).

I don't understand the problem with the diagram not matching that in the cited reference. The diagram need only be consistent within the article.

Glrx (talk) 03:40, 8 June 2012 (UTC)[reply]

The information in either or both article may or may not be correct, and both contain useful clues, but neither were very helpful to someone like me (who claims intelligence and motivation, but who is seeing the concept for first time) come to an understanding. The article contains one or more red herrings. For example:

"Resistance R should be as low as possible (much lower than the measured value) and for that reason is usually made as a short thick rod of solid copper." (Emphasis mine). As I presently understand the KB, that sentence is bogus. The resistance R is unwanted parasitic contact resistance that the bridge tries to engineer around. It is not a component engineered into the bridge as the article seems to imply.
The diagram in the article doesn't explicitly show the four-terminal nature of the measurement like the AAC diagram does. It lacks the parasitic resistances that touch the voltage source.

The floaty nature of the AAC diagram was an artifact of the particular line routing and I now agree the topology is correct at AAC, but the way it is drawn confused me. I like the tapped connections on the AAC diagram because they helped me visualize the parasitics that the KB tries to compensate for. Woz2 (talk) 11:44, 8 June 2012 (UTC)[reply]

The resistance R is designed to be as low as possible. The designers make it as low as possible to minimize the last term.

The diagram in the article doesn't show the resistance in the high level drive, but it does show a four-wire path.

I don't like the AAC diagram because they add confusion. There's an issue about who "owns" the distributed resistance in the connection, but a tapped resistor doesn't point that out well. Compare large connection dots used in article's illustration to delinate measured resistance. See also image at Shunt (electrical)#Use in current measuring where contact resistance is minimized. One could argue that the Kelvin terminals are taps on the resistor, but the zigs are a large part of the schematic symbol.

AAC does the better job explaining rationale of the bridge.

Glrx (talk) 17:54, 10 June 2012 (UTC)[reply]

To me the diagram is confusing because I don't know what the dotted lines mean, why R is darkened, or where the power source is. The explanation and diagram for the Carey Foster bridge, by contrast, is crystal clear.