Negative resistance between two pins on a DB9 connector with nothing else attached

Thread Starter

sjzimm23

Joined Jun 22, 2017
5
I've been using a solder-on DB9 connector to connect four resistive measurement devices to 4 female BNC connectors en route to a Wheatstone bridge circuit. For the second time now I've damaged one of my resistive devices, and upon further investigation, have found that the DB9 connector (even after it's been completely removed from the circuit) has a negative resistance and between 100-300mV between two of the pins. There should normally only be about 500mV going through the circuit when it's powered on.

What could possibly cause this issue? Could a spike in voltage damage the connector and leave it with a capacitance or something?

For the record, I'm a Mech Eng by training, so I have only a general understanding of the circuitry.

Thanks in advance for your help.
 

#12

Joined Nov 30, 2010
18,210
Resistance is not measured in millivolts.
(If you really have created a negative resistance in an ordinary material, our Department of Defense want to know.:D)

Try using the "ohms" function to measure ohms. Then come back and we will discuss it.
 

Thread Starter

sjzimm23

Joined Jun 22, 2017
5
Resistance is not measured in millivolts.
(If you really have created a negative resistance in an ordinary material, our Department of Defense want to know.:D)

Try using the "ohms" function to measure ohms. Then come back and we will discuss it.
Thanks mate, I'm aware of that. The resistance was negative 90 kOhms, and there was a DC voltage of 100-300mV. I assumed the first response would ask me to measure the voltage as well if I didn't report it in the original post. If I reversed the leads I got a positive resistance with >10 MOhms, which I think makes sense given the DC voltage. I removed all of the connections from this piece and it still had a voltage. My question is how I damaged the connector, with the result being that it seems to have become a capacitor.
 

#12

Joined Nov 30, 2010
18,210
I'm sure it is a measuring problem
even after it's been completely removed from the circuit
because an ordinary part standing alone can not generate a voltage.

A small DC voltage will make an ohm meter lie exactly as you said this one did.
If I reversed the leads I got a positive resistance with >10 MOhms,
Then:
I removed all of the connections from this piece and it still had a voltage.
NFL (Not Frankly Likely). The capacitance between pins on a DB-9 connector is so small that the ohm meter should overpower it, immediately. I'm thinking you have a broken meter or you just think you removed all the connections.
 

Thread Starter

sjzimm23

Joined Jun 22, 2017
5
I'm sure it is a measuring problem

because an ordinary part standing alone can not generate a voltage.

A small DC voltage will make an ohm meter lie exactly as you said this one did.

Then:

NFL (Not Frankly Likely). The capacitance between pins on a DB-9 connector is so small that the ohm meter should overpower it, immediately. I'm thinking you have a broken meter or you just think you removed all the connections.
Look, I understand your skepticism, but I can assure you that I had removed all solder connections from the DB9 connectors (males in both cases) in question and they were sitting alone on a wooden table. I'm also using a very expensive Fluke multimeter that repeatably reports resistances to the nearest hundredth of an Ohm, and continues to do so on the undamaged devices.

I'm telling you, after experiencing problems with my measurement device, I stepped along the circuit until I found the origin of the problem, and proceeded to remove and replace the piece. After it had been replaced, I checked the resistance across the pins using a spare female DB9, and sure enough saw a negative resistance as well as a non-zero DC voltage. I'm now experiencing the same problem.

I would greatly appreciate it if you could offer an explanation other than that I've made a mistake with my measurement. My best guess at this point is that I exceeded the voltage rating for the connector and have somehow stored a potential difference between two pins. I'm sure this is temporary (I'm not under the impression that I've created a free-energy device), but it is certainly associated with my other problems. As such, I'd like to know exactly how this happened so that I can avoid it in the future. In particular I'd like to know if I can expect to avoid the problem by cutting out the DB9's and just going directly to the BNC connectors.
 

MrChips

Joined Oct 2, 2009
21,618
We are experts in all things electrical and electronic. We are not mystics.

It would appear that you have the intelligence to analyze the situation and phenomenon just as well as we can and even better because we are not there to witness first hand this amazing discovery.

Hence we cannot offer any explanations to your observations.
 
OK guys. Give us a diagram of your set-up including links to manuals. Maybe even a picture. I set up systems to measure very high resistances > 10 G ohms and low resistances.

The system can get very problematic especially with shields involved. Initially, I will SUSPECT a ground loop or a current path that you don't see.

I don't know if your system uses guard or ground or what the range of resistances you intend to measure.

So, I can immediately think of a scenario that could cause issues. Let's assume a 4-wire bridge, 4 BNC's and a whole bunch of unintended ground loops.

I would EXPECT that the BNC's that do the voltage measuring to NOT have their shields connected at the DUT (Device Under Test) end. Those connections could actually be guarded, not shielded. They would have a potential on them. This measurement should be done differentially.

In SOME cases you have to use two meters and look at the difference because of where ground is positioned.

The current source would be grounded and likely attached to earth ground somewhere. These leads would be surrounded by ground, but would only be connected at the ONE END.

Hall effect is very similar to a wheatstone bridge. It's really tricky to set up. We never automated our setup. It was done manually with a 30 kilo-Gauss variable magnet, I set up systems to do thin films as well as bulk, but without the proper equipment to do it easily.

Two electrometers were used to measure voltage and the values subtracted. The reference was ground, not just across the points you want to measure. In some cases, that doesn;t work.

Designing the thin-film fixture was the tough part having to deal with both ground and guard.

When doing bulk samples of low resistivity, I did a 5 point type of measurement, where one side had two contacts rather than one, Electrically, with a potentiometer, the point could be nulled by a potentiometer.

You can use an expensive meter wrong too. One meter had a 100 ohm resistor internally that caused me never ending grief in another system. I was able to "help" it out bit, by using the sense leads of the voltage source to electronically remove the 100 ohm resistor. It was a gotcha, after the $5,000 USD meter was delivered.
 

Thread Starter

sjzimm23

Joined Jun 22, 2017
5
Thanks for the reply. I've attached 7 pictures of my observations. Note that the tan box contains only 16 copper wires that split 2 DB9 connectors to 8 BNC connectors, and no other components whatsoever. Neither side of the box is connected to anything--it's just sitting on the wooden table. In one picture, you'll see the DB9 connector I described yesterday, with a resistance from pin-pin of 183 kOhms, which is far less than the resistance between any other two pins (usually >50 MOhms if I get a reading at all). In a pair of images, you can see that I'm reading ~50mV between those two pins, and 0mV when the leads are disconnected. In 4 other pictures, you can see the voltage readings across the female BNC connector that had been connected to the two subject DB9 pins, taken with two different multimeters. Both show ~170mV across this connector, with 0mV across adjacent connectors. These readings all fluctuate wildly when I give the connectors a wiggle. There seems to be a very weak electrical connection through the insulation and a charge built up on at least one component. I'm wondering if this connection was established by a voltage spike, perhaps some flux that seeped into the connector and formed a path? Interested in your thoughts now that we've (hopefully) established that I know the difference between a Volt and an Ohm.IMG_20170623_144951.jpg IMG_20170623_145035.jpg IMG_20170623_135220.jpg IMG_20170623_135210.jpg IMG_20170623_134851.jpg IMG_20170623_134937.jpg IMG_20170623_111542.jpg
 

Attachments

This seems to really be a case of phantom voltages, caused by long cables. Although usually a prblem with AC measurments. See: http://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/electrical/strayvoltage and https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiJm6vX19TUAhWCGz4KHYPeAX0QFggmMAA&url=http://support.fluke.com/find-sales/Download/Asset/2105317_A_w.pdf&usg=AFQjCNHOsslKlLs6xAtOJ-2BNU3O3cHXpQ

There is something called the triboelectric (motion generates voltages) effect and the piezoelectric effect (compression generates voltages) but I doubt these are your issues.

You PROBABLY have coupling between he 60 Hz that all around you and the meter.

BNC's probably should not be used.

I need an idea of the magnitude of stuff your trying to measure - the bridge values and the meter model #,

I'll SUGGEST that you measure V using twisted-pair shielded cable to a dual banana plug. Take a pigtail off of it and connect to the shield. Earth ground the shield AT ONE END ONLY. So, your cable has a bit of heat shrink at each end.

Best practices is to GROUND at the sourcing end, but it's not always possible.

If you put a low resistance at the end, 10K even, the problem should go away, but that introduces another one.

FWIW:
twisting helps remove EMI. basically 50/60 Hz interference
Sheilding helps remove RFI (interference from a close by radio station)

it's possible that under the right conditions and a close AM transmitter, speaker wires can receive a radio station. Teeth and fillings can too.

The line frequency (50/60 Hz) is everywhere and it's strong. A single uninsulated wire attached to a scope will see a large voltage. A shielded one - not so much.

A high impedance wire 10 M ohms) has to be short or shielded. What's at the other end decides if it will affect your measurement.
 

Thread Starter

sjzimm23

Joined Jun 22, 2017
5
Thanks for the response. The bridge voltage is around 500mV I believe. The resistive measurement device is fixed at around 14 Ohms and I measure the voltage necessary to enforce that condition (i^2*R heating controls the resistance of the device). BNC's are a must, since they're the connectors to my bridge, my analog filters, and my DAQ board.

If the problem was EMI, shouldn't I see this voltage across all 8 of my connectors? One of my 8 sensors was fried and I found upon investigation that both the DB9 and BNC connectors in that circuit seem to have damaged insulation (R way too low) and this weird voltage. The other 7 are still A-OK. Also, like I said, the R and V values for the damaged connectors fluctuate wildly when I tug on them/squeeze them. The other 7 do not respond at all to tugging/squeezing.

UPDATE: I think I may have an explanation:
https://www.cirris.com/learning-center/testing-guidelines/diagnosing-failures/86-flux-can-cause-i-r-failures-or-shorts

This would explain why cleaning the outside of the connectors didn't help.
 

Alec_t

Joined Sep 17, 2013
11,501
One possible explanation could be that one or more wires or pins of the connector has surface oxidation. Copper oxide can act to form a rectifier. Your wiring may therefore be rectifying/demodulating local radio stations! You've built a crystal set :).
 
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One possible explanation could be that one or more wires or pins of the connector has surface oxidation. Copper oxide can act to form a rectifier. Your wiring may therefore rectifying/demodulating local radio stations! You've built a crystal set :).
With acidic flux and possibly other chemicals together with dissimilar metals, it might be more likely that he has built a battery :D. Try cleaning the connector.
 
I was going to mention flux.

Vibrations in coax cables can induce voltages. I HAD to worry about this. Cables made with a graphite lube is used to minimize this, Take a look here: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0ahUKEwiz6fSNlNfUAhXIFz4KHdQDA1sQFghHMAg&url=https://www.axon-cable.com/publications/Coaxial-cables.pdf&usg=AFQjCNH9-OgNDQJ46iqrvfVlpBQFnKWyWA&cad=rja

I was using Triax cables. These have two shields and an inner conductor. The outer shield is ground and the inner shield is maintained at the potential of the inner conductor, so that leakage cannot occur or it doesn't influence the measurement.
 
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