Sensor lead puzzle

eaglewood

Supporting Member
Join Date
Jan 2022
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Reno, Nevada
Posts
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I have 15 wires coming from a black box sensor. I can't open it to look within. I know that two of those fifteen wires will comprise the feedback loop for my PLC. But I'm not sure how to determine which two wires are the ones I need.

Any ideas?
 
externally powered... brown and blue provide power in......

choose the black and white for loop..... then send it!










also.... you need to give more info than that, narrow it down somehow.. or pictures.... something.
 
Yeah, I was purposely keeping it black box to see if I could learn some bigger lesson -- tricks for detecting a sensor given several leads. That is my situation, but I also have the manual. I'll attach photos. vacuum research transducer.jpg

vacuum research pins.PNG


Thanks, guys
 
Yeah, I was purposely keeping it black box to see if I could learn some bigger lesson -- tricks for detecting a sensor given several leads.

Here is the lesson: no such trick exists. For some simple sensors, the convention "brown - blue - black - white" may be surmised, at your own risk. Otherwise, it is wide, wide open.
 
So there are two different sensors, each with two leads. Would the leads that carry the 4-20mA circuit be both leads labeled "Signal" on the attached table? Or would it be one "Signal" and one "Neg. Power."

vacuum research pins.PNG
 
Yeah, I did give them a call. Tech support there said it's not possible. Which I doubt. I mean don't pretty much all sensors work with a two-wire feedback loop?
 
The unit may require to much power to be a loop powered sensor. If it needs more that the bottom 4mA to run the sensor then you need an additional two wires for power. Did the tech say it the two signal wires were voltage or current?
 
Tech didn't say. Wasn't getting a lot of help, so I thought I could figure it out on my own. (Guess I figured wrong, so far anyway.) The sensor is receiving the power it needs, as it's hooked up to the Vaccuum Research Manometer it works with.

The thing is, I want to read pressure on my PLC for other reasons. I used a DB15 Y splitter to grab the wires. I used a multimeter to check for voltage changes corresponding to pressure changes. Didn't see any. So now I'm thinking it's 4-20mA.

On the above "pins" table, I would think it's either both of the "signal" wires, or maybe one of them with the negative power that makes the loop. Unless I'm completely wrong. I've never come across this setup before.

db15 splitter.PNG
 
The left side sensor could be a strain gage type sensor. The power wires could be for excitation voltage for a whetstone bridge and the output would be the signal wires. https://www.omega.com/en-us/resources/strain-gages

The right side looks similar but not sure what the "gain" wires would refer to. There is a strain bridge setup with 6 wires but the 2 extra wires are usually called "sense" wires. They are beneficial where there are long wire lengths. The controller or amplifier uses those to sense the actual excitation voltage at the load cell and compensate the excitation voltage to account for line losses. Maybe "gain" is a bad translation of "sense"?
 

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