# Volume I - DC » DC METERING CIRCUITS »

Joined Jul 7, 2009
1,577
I'd suggest adding a paragraph at the end of this section with a practical tidbit based on a Kelvin measurement; something to the effect of:

The Kelvin measurement can be a practical tool for finding poor connections or unexpected resistance in an electrical circuit. Connect a DC power supply to the circuit and adjust the power supply so that it supplies a constant current to the circuit (within the circuit's capabilities, of course). With a digital multimeter set to measure DC voltage, measure the voltage drop across various points in the circuit. If you know the wire size, you can estimate the voltage drop you should see and compare this to the voltage drop you measure. This can be a quick and effective method of finding poor connections in wiring exposed to the elements, such as in the lighting circuits of a trailer. It can also work well for unpowered AC conductors (make sure the AC power cannot be turned on). For example, you can measure the voltage drop across a light switch and determine if the wiring connections to the switch or the switch's contacts are suspect. To be most effective using this technique, you should also measure the same type of circuits after they are newly made so you have a feel for the "correct" values. If you use this technique on new circuits and put the results in a log book, you have valuable information for troubleshooting in the future.

#### Dave

Joined Nov 17, 2003
6,960
Thanks for taking the time to make a suggestion someonesdad.

Just so I am looking at your suggestion in context, are you suggesting this should be put in at the end of the "Kelvin (4-wire)" section: http://www.allaboutcircuits.com/vol_1/chpt_8/9.html

Dave

Joined Jul 7, 2009
1,577
Yes, that's correct Dave. Many of us learn of the Kelvin bridge in our basic physics classes (at least I did, back when dinosaurs roamed the Earth), yet sometimes the practical application of those ideas my not be so obvious. The measurement I'm suggesting isn't a Kelvin bridge, but it uses the Kelvin 4 wire method and is eminently practical. I use it and 4 wire ohms measurements frequently because they're so useful. It could also be presented under a section on Ohm's Law, as it's really nothing more than the defined relationship between current, voltage and resistance. But it seems a bit more "natural" in the section on Kelvin connections/measurements.

#### Dcrunkilton

Joined Jul 31, 2004
418
The suggested passage has been added to the text and an additional image 00502.png . I will post back here one I update the ibiblio copy.

#### Dcrunkilton

Joined Jul 31, 2004
418
I have updated the ibiblio copy for numerous corrections (not just for this thread) over the last few days. The list of affected files and images follows:
DC V1:dcmeter.sml contrib.sml 00502.png
AC V2:basicac.sml contrib.sml
Semi V3: bjt.sml diode.sml contrib.sml theory.sml 03288.png 03304.png 03392.png
Digital V4: boolean.sml contrib.sml