# joint resistance- readings make sense?



## Cougar Rock Rail (Jan 2, 2008)

Hi guys,

Recently the weather cleared long enough for me to get out and do a little track work on the layout--main goal was to replace a couple of slip-on connectors with clamps that over the years had gradually worked themselves to have too big a gap. Out of curiousity, I measured the resistance (at least that's what I thought I was measuring!) across the joint, before and after the clamps. The track has been down around 15 years now, original LGB, no grease. The reading on my meter was 0.9, Ohms I'm assuming. After I put the clamps on both rails read 0.2 across the joints. I am using a Fluke 175 meter, and my track is DCC so 22V. 
My question is, do these readings make sense and do they still work considering I'm using DCC? I went around the layout and I got readings from 0.2 to 3. I've never noticed a change in speed or performance anywhere on the layout, and I feed my track in two places for about 300'. When I check the voltage I get 22.5 +/- .3 everywhere on the layout.

Thanks,
Keith


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## George Schreyer (Jan 16, 2009)

First, did you measure the resistance of your probes by simply touching the probes together. Some meters compensate for this parasitic, some do not. 

At very low resistances, the only accurate way to measure is at a high test current using a "4 point" probe. You feed a known current to the device under test (2 "probes") and measure the voltage across the device under test with a voltmeter (the other 2 "probes"). This eliminates the impact of the resistance of the probes themselves. Since there is slmost zero current flowing in the voltage probes, any resistance impact that is there is also almost zero. Any resistance in the current probes doesn't matter because it is outside of the "device under test." 

An easy way to do this is to clip a low value resistor (maybe 10 ohms or less) across the track downstream from where you are measuring and then inserting a current meter in series with the rail feeder. Set the current to some known value and then run down the line, joint by joint and measure the voltage across each joint. You don't even have to calculate a resistance if you don't want to, just compare the readings to each other and "fix" the high ones. 

I've measured resistances down in the micro-ohm range (in my former life as "rocket scientist") but it took 100 amps of probe current to validate it. We measured at 100 amps because the joint under test was indended to carry 88 amps. 


If you have two true RMS meters, you can make this measurement with a steady DCC current in the rails as well. If you don't have two true RMS meters, then you can still do it with an AC current meter and an AC voltmeter. You won't get an actual resistance that means much, but the RELATIVE voltage drop across each joint will still tell you where the bad joints are.


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## Semper Vaporo (Jan 2, 2008)

You really should not have power applied to the track when taking resistance measurements, whether it is AC, DC, DCC or any other form. Current flow from the track power will travel through the meter in addition (or opposition) to the power the meter is putting out to measure the current and interpret as resistance. BUT, I don't think the reading are all that out of line with what they could be, but I would not trust them to be valid to begin with.


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## Cougar Rock Rail (Jan 2, 2008)

Hi guys, I don't know what I was thinking...there was no power in the track when I took the readings. George, when I touched the probes together I believe it gave the "infinity" or n/a reading but I can't remember what the exact letters were. It is a true RMS meter. 

Keith


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## SteveC (Jan 2, 2008)

I could easily be mistaken, but the way I understand what George said is that when attempting to measure low resistance values. The only way to do it is to place a known load in the circuit that is capable of handing a high current flow, place a current measuring meter in line with the circuit and set the power supply to a given amp level, then use another meter to measure the voltage drop across any connections. Then calculate the resistance.

But as a matter of practicality, just do what's needed to reduce the voltage drop at each connection to it's lowest level.


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## George Schreyer (Jan 16, 2009)

If you are using and ohm meter, then the track MUST not be powered. When you touch the leads together, the meter should read zero or close to it. Whatever probe resistance that you do read can be subtracted from any measurement that you make.


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## John J (Dec 29, 2007)

I would have been happy to go from a .9 to a .2 

Could one take a voltage reading accross the joint while engines are running. If there is a significant drop accross the joint would it not show up as a voltage drop?

Or is my thinking wrong on this?

I might be in Milli volts.

Or take some aligator clips and clip a 12 volt lamp accross the tracks to create a load. the set a fixed voltage. Then mesure accross the joint? 

JJ


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## George Schreyer (Jan 16, 2009)

You can look across the joint when a loco is running but the result will be ambiguous. 

You will have to be measuring a joint that the loco current is actually running through AND loco currents tend to fluctuate a lot so that the readings won't be steady. You could perhaps locate a really bad joint, but if it was that bad, you'd know about it anyway.


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## Curmudgeon (Jan 11, 2008)

What is track joint resistance? 
Geez, haven't checked it in 18 years, maybe I should.....


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## SteveC (Jan 2, 2008)

Track Joint Resistance = ∞ = TOC Track Power Resistance


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## Curmudgeon (Jan 11, 2008)

Ah, I've got 1500 feet of aluminium rail, if I had to worry about joint resistance, all this stuff would have been on evilbait 17-1/2 years ago.


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## mgilger (Feb 22, 2008)

Just put voltage on the track from your supply, say 20 volts, or what ever your supply will put out. You don't need to be running a train. If you put the DVM in the voltage mode and measure across the joint you are shooting for zero voltage across the joint, which means you have zero or almost zero resistance, *which is good*. If you see voltage, you have resistance across the joint and *that's bad*. Of course if you measure from one rail to the next, you should see your 20 volts +/- depending on the voltage drop through all your bad joints and voltage drop depending on how much track you have. Of course that also will depend on what type of track, Brass, Aluminum, etc. So best test is just do the voltage test across the joint and shoot for zero volts, or a very small voltage, maybe .1 or less. Good luck. It will be another month ore more before I even see my track through the snow and ice. 


Regards,
Mark
http://mmg-garden-rr.webs.com


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## Greg Elmassian (Jan 3, 2008)

Nope, you need to read under load... the resistance is too hard to read for exactly the reasons outlined by George above. 

I have a load that draws 8 amps that can clip on the track anywhere, and I use just relative measurements just as George said. 

I doubt very few people on this forum have the equipment to measure very low resistance accurately. 

You must know Ohms law: V=IR.... well with no load, I = 0, thus zero times R will equal zero, so you will not see any voltage drop across the joint, even if it does have resistance.

I hope you see that logic.


Greg


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## Cougar Rock Rail (Jan 2, 2008)

Ah, I've got 1500 feet of aluminium rail, if I had to worry about joint resistance 

Ha ha, Yah...I should have seen that one coming! Besides, you've probably got about that much wiring in your old Shaguar to keep you troubleshooting for years to come!  

You forget I was never worried about resistance, nor was I ever having a problem with electrons making a leap of faith across the joiners. I just took the track apart to fix a expansion/contraction issue and when I switched to the clamps I wanted to see how they compared out of curiousity more than anything else. I can see now how curiousity killed the cat! 

Keith


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## Greg Elmassian (Jan 3, 2008)

Yep, TOC did not miss an opportunity! 

I heard he is converting to all DCC 1 April... 

Greg


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## Curmudgeon (Jan 11, 2008)

Yup, boosters and all. 
02APR we give up and tear up the railroad......


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## mgilger (Feb 22, 2008)

Greg,
Your correct about the load. Without the potential difference across the two rails, no voltage drop across the joint resistance. My mistake. 

Mark


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## Greg Elmassian (Jan 3, 2008)

Yeah, not wanting to get on your case, but this is a constant source of confusion for newcomers. 

When I tell someone that the voltage drop is directly related to the current draw, it usually goes in one ear and out the other... the typical response I get is "OK, enough with the theories Greg!" 

Then I say, "put a zero in for current in Volts = Current times Resistance... so that makes volts zero right?" So you will get no reading across the joint with no current. 

The next question is usually about how much current the meter draws... when I tell them it's effectively zero, then the "Ah Hah" finally hits. 

If I have someone over at my house, then I can do a physical demonstration with a voltmeter across the joint and then with and without a load... that normally does it right there. 

I'm always searching for the "perfect" explanation... I've come to the conclusion that I need to make more youtube videos ha ha! 

Regards, Greg


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## TrotFox (Feb 15, 2008)

Posted By Cougar Rock Rail on 10 Feb 2011 10:32 PM 

I can see now how curiousity killed the cat! 

Keith 
How, with an online forum? 

Trot, the 4-point-measureing, fox... (cryogenic temperature sensors, donchaknow?)


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