Resistance Problem

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Bryan Cox
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Location: Owensville, Mo

Resistance Problem

#1 Post by Bryan Cox »

Hey gents. Having an electrical issue at work and wanted to see if someone here had any insight.

1. I have 4 separate tamper film circuits (DC) that each measure 60-90 ohms resistance. Each measures at ~75 ohms.
2. Circuits are combined in-series via a junction box with 1 cable out.
3. When I test the combined (series) resistance of these circuits I get ~160 ohms. My manual says I should receive 240-360 ohms.

Now the question...

If each circuit tests at approximately 75 ohms separately (within range) why would their combined resistance be only ~160 ohms? What could cause a drop in series resistance like that?

I was thinking two of the circuit paths might be shorted to each other providing a a quicker return path, lessening the measured resistance. But I just don't know.
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Bill Fitzmaurice
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Re: Resistance Problem

#2 Post by Bill Fitzmaurice »

They should measure 240-360 ohms, so something is wrong. What I can't say, not knowing what kind of devices they are.

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Bryan Cox
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Location: Owensville, Mo

Re: Resistance Problem

#3 Post by Bryan Cox »

I could post a pic of the line drawing tomorrow. But it's just a flat tamper film: flat copper lines, closely lain to cover a large area. Therese are there to detect if someone drills through an assembly. Imagine rear window defrost lines, but copper and almost no space between the lines. No other components at all.

What I think happened is there's a short between two of the 4 circuits changing the path from series to series/parallel.

I'll get a pic of the line drawing with more detail when I go back to work tomorrow.
Otop 12 x 4 (Delta Pro 12-450a) 2x melded, 2x straight
Titan 39 x 4 (3012LF) 20" wide

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Tom Smit
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Re: Resistance Problem

#4 Post by Tom Smit »

Are you able to start measuring just one, then further down the trace measure both sides of the next one? It should double. Continue on to the third, and then the fourth one making sure that the impedance rises each time. Somewhere along the line, you should be able to find where it is shorting out.
TomS

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Bill Fitzmaurice
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Re: Resistance Problem

#5 Post by Bill Fitzmaurice »

Bryan Cox wrote: Wed Dec 09, 2020 11:49 am What I think happened is there's a short between two of the 4 circuits changing the path from series to series/parallel.
The path would still be series, but with lower resistance with any shorting between the conductors. Think of resistance being linear with respect to the length of the pathway. A short circuit that literally shortens the pathway reduces resistance.

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Bryan Cox
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Re: Resistance Problem

#6 Post by Bryan Cox »

Tom Smit wrote: Wed Dec 09, 2020 11:39 pm Are you able to start measuring just one, then further down the trace measure both sides of the next one? It should double. Continue on to the third, and then the fourth one making sure that the impedance rises each time. Somewhere along the line, you should be able to find where it is shorting out.
Did some more testing today and found that two pins that should not have continuity have resistance of only 30 ohms. Think I found the issue. Eventually I plan to make a shorting plug so I can do just what you're saying above. Thanks!
Otop 12 x 4 (Delta Pro 12-450a) 2x melded, 2x straight
Titan 39 x 4 (3012LF) 20" wide

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Bryan Cox
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Location: Owensville, Mo

Re: Resistance Problem

#7 Post by Bryan Cox »

Bill Fitzmaurice wrote: Thu Dec 10, 2020 7:56 am
Bryan Cox wrote: Wed Dec 09, 2020 11:49 am What I think happened is there's a short between two of the 4 circuits changing the path from series to series/parallel.
The path would still be series, but with lower resistance with any shorting between the conductors. Think of resistance being linear with respect to the length of the pathway. A short circuit that literally shortens the pathway reduces resistance.
Thank you, Bill. This helped me understand it much better.
Otop 12 x 4 (Delta Pro 12-450a) 2x melded, 2x straight
Titan 39 x 4 (3012LF) 20" wide

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