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Author Topic: Extended test leads on Kelvin Bridge  (Read 1451 times)

TJ (Tom) Cornish

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Re: Extended test leads on Kelvin Bridge
« Reply #20 on: April 24, 2018, 04:40:34 pm »

The Kelvin connection for precision testing is known to engineers, maybe not so well known around sound reinforcement forums.

This technique has other applications audio, like using sense leads on speakers to ignore wire losses. Again not common for sound reinforcement.

JR
Not common, but not unheard of.  For a long time Nexo used sense lines from the output of the amp for feedback and limiting.  Now that virtually all vendors have gone to branded amps and proprietary processing, they don't need to see on the other side of the black box via sense lines; they just absorbed the amp into their own black box.  :)
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John Roberts {JR}

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Re: Extended test leads on Kelvin Bridge
« Reply #21 on: April 24, 2018, 04:42:04 pm »

Going around with some folks at work about how close the sense leads have to be to the resistive element.  There's this bright idea that if you partition the board pad for an SMT resistor the current sense is more accurate than if you just run two traces off the one pad.  I tried explaining that the component's internal parasitics from the resistive element down to the board pad would swamp any differences.  Much as the leads on a conventional axial resistor does.  Maybe not as bad, but there are comparatively high resistance metalizations going from the element on top around the sides of the ceramic body to where it's connected to the board.
Easy enough to prove (or not) empirically.... just do it both ways and measure the difference (maybe with another bridge.  ;D

JR
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brian maddox

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Re: Extended test leads on Kelvin Bridge
« Reply #22 on: April 24, 2018, 06:19:35 pm »

Not common, but not unheard of.  For a long time Nexo used sense lines from the output of the amp for feedback and limiting.  Now that virtually all vendors have gone to branded amps and proprietary processing, they don't need to see on the other side of the black box via sense lines; they just absorbed the amp into their own black box.  :)

Our friend John Meyer did the same thing before they decided to just put the amp in the speaker and be done with it.

I still come across UM-1s with the wrong amping/processing and no sense line on occasion.  Needless to say, they don't sound good.  :)
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #23 on: April 25, 2018, 09:22:58 pm »

I just used my extended kelvin bridge today in a swamp and it worked perfectly. And the measurements with the 75 ft extensions match what it measured without them. I may have a winner here, but I need more verification.
« Last Edit: April 27, 2018, 08:48:47 pm by Mike Sokol »
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Chris Hindle

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Re: Extended test leads on Kelvin Bridge
« Reply #24 on: April 26, 2018, 01:36:52 pm »

I just used my extended kelvin bridge today in the swamps of Baton Rouge and it worked perfectly. And the measurements with the 75 ft extensions match what it measured without them. I may have a winners here, but I need more verification.
Always nice when a plan comes together.
Still got all your fingers and toes, i presume?  ::)
Chris.
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #25 on: April 26, 2018, 09:27:45 pm »

Always nice when a plan comes together.
Still got all your fingers and toes, i presume?  ::)
Chris.

Yes, but there was a lot of mud everywhere. There were several other EE's on the site monitoring what I was doing and they all want to buy a Kelvin Bridge now once I explained operation to them. For standard field testing like you need for audio production the accuracy is simply overkill, but for diagnostics of sub-ohm resistances it's really handy. I'll share more later once I'm able to.
« Last Edit: April 30, 2018, 10:16:26 am by Mike Sokol »
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Scott Holtzman

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Re: Extended test leads on Kelvin Bridge
« Reply #26 on: April 26, 2018, 10:17:12 pm »

I went to look up Wheatstone bridges and I am trying to understand why the Kelvin bridge offers better low resistance resolution than the Wheatstone.

Any comments?
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John Roberts {JR}

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Re: Extended test leads on Kelvin Bridge
« Reply #27 on: April 27, 2018, 09:31:35 am »

I went to look up Wheatstone bridges and I am trying to understand why the Kelvin bridge offers better low resistance resolution than the Wheatstone.

Any comments?
The magic about Kelvin measurements is that the reference current injected to develop a voltage drop in the DUT (device under test) to impute resistance, flows through separate leads that are not part of the voltage measurement. In a conventional meter the test lead resistance would introduce errors. With a 4 wire connection, there is no current flowing in the voltage sense leads so no IxR errors.

Obviously this is more of a concern for low impedance measurements where the test lead resistance can be significant.

JR
Quote from: wiki sez
The operation of the Kelvin bridge is very similar to the Wheatstone bridge, but uses two additional resistors.
https://en.wikipedia.org/wiki/Kelvin_bridge
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #28 on: April 29, 2018, 09:29:55 am »

Obviously this is more of a concern for low impedance measurements where the test lead resistance can be significant.

Good explanation and you're quite right about this only really needed for low-resistance measurements. In my case I'm measuring EGC bonding resistance/impedance down to a few tens of milli-ohms, and the two test endpoints can be 100 feet apart or more. Even the short test leads in a standard ohmmeter can introduce a few hundred milli-ohms of series resistance, which contaminates these measurements.

What I like about the Klein tester is that it's a simple handheld meter with a quick calibration procedure and built-in temp gauge that provides the proper correction factor if you want to calculate the actual length of a spool of wire. That's because this unit is being marketed as a way to measure how much wire is left on a spool. So you set if for 10 gauge or 2 gauge wire or whatever, then use the Kelvin clips on both ends of the wire, and will tell you if you've got 100 or 200 or 300 feet of wire on the spool, down to a few percent. I don't care about that since I'm interested in EGC bonding. So Klein is interested in my experiment since they've never considered my application and it might be a new market for them.
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Mike Sokol
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #29 on: April 29, 2018, 09:33:14 am »

Not common, but not unheard of.  For a long time Nexo used sense lines from the output of the amp for feedback and limiting. 

IIRC this was also used to improve damping factor on long cable runs. Pretty simple concept since the sense lines just become part of the negative feedback loop in the power amplifier. In theory you can make a long run of 18 gauge behave like really heavy wire, except for voltage drop and reduced headroom (of course).
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