Question for fellow EEs: negative resistance

[JohnCU]

I am testing code I wrote for the automation of a sourcemeter to plot IV curves for detectors. The resistor I had burned out so I'm using graphite from a lead pencil I took apart. The current is a linear function of voltage (sourcing voltage, measuring current) up until about 300 volts, then the resistance increases, then apparently becomes negative, because from about 300 to 500 volts, the voltage increases, but the current decreases, linearly. After that, it starts smoking and open circuits...

I tried looking up some information and it says scientists have seen this in carbon nanotubes or something and on Wikipedia it says this is seen in some materials but I don't know, it's weird. I remember in class they said you couldn't have a negative resistance because it'd be producing energy (I think) and that couldn't happen but in our calculations you sometimes get a negative resistor when dealing with dependent sources.


Picture of a graph (bad one because this one isn't as smooth as the others): it's I over V, so it's conductance, but you can still see what I'm talking about

 

[LordSnailz]

hmm, interesting ...

 

[Yossarian]

negative resistance?!?!?! incontheivable!

 

[MetalMat]

No such thing

 

[JohnCU]

I know there is no such thing, but there appears to be a property that resembles it.

 

[tikwanleap]

Maybe the graphite was breaking down at the higher voltages and reducing the current that can go through. Maybe it was transforming it's physical properties because of the heat and/or the electrical connections was breaking apart inside.

One thing to try is to see if you can start at about 320V go to 380V and go back to 320V. If the current allowed at the first 320V is the same as the second 320V then you have some cool sh!t there... lol

 

[JohnCU]

good read

there is indeed a negative resistance region.

 

[JohnCU]

Originally posted by: tikwanleap
Maybe the graphite was breaking down at the higher voltages and reducing the current that can go through. Maybe it was transforming it's physical properties because of the heat and/or the electrical connections was breaking apart inside.

One thing to try is to see if you can start at about 320V go to 380V and go back to 320V. If the current allowed at the first 320V is the same as the second 320V then you have some cool sh!t there... lol

let me try that, be back in a few.

 

[MetalMat]

Originally posted by: JohnCU
I know there is no such thing, but there appears to be a property that resembles it.

Well Im a little rusty, but the resistane would increase cause the temp would increase correct? looking at V = I x R, The resistance may just be increasing at a much higher rate than the current is decreasing due to the nature of the graphite becoming non-linear after 300v.

Course I could just be talking out my @$$

 

[91TTZ]

Originally posted by: JohnCU
The graphite (I assume that's what it is) is a constant, linear function of voltage (sourcing voltage, measuring current) up until about 300 volts, then the resistance increases, then apparently becomes negative, because from about 300 to 500 volts, the voltage increases, but the current decreases, linearly.

Huh?

Do you mean the current is a linear function of voltage?

 

[Eli]

Originally posted by: JohnCU
good read

there is indeed a negative resistance region.

I wish I understood half that.

 

[JohnCU]

Originally posted by: 91TTZ

Originally posted by: JohnCU
The graphite (I assume that's what it is) is a constant, linear function of voltage (sourcing voltage, measuring current) up until about 300 volts, then the resistance increases, then apparently becomes negative, because from about 300 to 500 volts, the voltage increases, but the current decreases, linearly.

Huh?

Do you mean the current is a linear function of voltage?

oops, yeah, sorry.

 

[91TTZ]

I don't see how that's "negative resistance". It just seems like increasing resistance with heat to me.

 

[RaynorWolfcastle]

No such thing as negative resistance unless you use active elements. What you're describing is a change in conductivity, probably caused by a change in the material properties becayse of heating.

 

[JohnCU]

Originally posted by: JohnCU

Originally posted by: tikwanleap
Maybe the graphite was breaking down at the higher voltages and reducing the current that can go through. Maybe it was transforming it's physical properties because of the heat and/or the electrical connections was breaking apart inside.

One thing to try is to see if you can start at about 320V go to 380V and go back to 320V. If the current allowed at the first 320V is the same as the second 320V then you have some cool sh!t there... lol

let me try that, be back in a few.

Results here

 

[JohnCU]

Originally posted by: RaynorWolfcastle
No such thing as negative resistance unless you use active elements. What you're describing is a change in conductivity, probably caused by a change in the material properties becayse of heating.

After the conductivity changes, though, in that negative resistance region (differential resistance, dV/dI), the slope of that line is a negative constant, so it does appear to be a negative resistance.

 

[Eli]

Originally posted by: JohnCU

Originally posted by: JohnCU

Originally posted by: tikwanleap
Maybe the graphite was breaking down at the higher voltages and reducing the current that can go through. Maybe it was transforming it's physical properties because of the heat and/or the electrical connections was breaking apart inside.

One thing to try is to see if you can start at about 320V go to 380V and go back to 320V. If the current allowed at the first 320V is the same as the second 320V then you have some cool sh!t there... lol

let me try that, be back in a few.

Results here

That's pretty cool.

Hmm...

Did you let the graphite fully cool before measuring the second drive to 380V?

 

[JohnCU]

Originally posted by: Eli
That's pretty cool.

Hmm...

Did you let the graphite fully cool before measuring the second drive to 380V?

well, I set it to a 3 second delay between each point, and measured it from 300 to 380 in 10 step increments, then decreased it to 300 in 10 increments with the same delay so I'm not sure how much it cooled in those 3 seconds between each measurement.

 

[RaynorWolfcastle]

Originally posted by: JohnCU

Originally posted by: RaynorWolfcastle
No such thing as negative resistance unless you use active elements. What you're describing is a change in conductivity, probably caused by a change in the material properties becayse of heating.

After the conductivity changes, though, in that negative resistance region (differential resistance, dV/dI), the slope of that line is a negative constant, so it does appear to be a negative resistance.

No, negative resistance would be like putting a 50 ohm resistor in series with a black box, then measuring the resistance across both and finding it's 25 ohms. There are circuits designed to do just this over some range of voltages, usually using opamps. They are usually used to reduce the effects of parasitics.

What you're describing is simply a lower resistivity of the material. It probably has to do with applying a large voltage across the material, could be changing the electronic properties of the material, or maybe due to heating (though usually heating causes non-linear increases in resistance).

 

[Eli]

Originally posted by: JohnCU

Originally posted by: Eli
That's pretty cool.

Hmm...

Did you let the graphite fully cool before measuring the second drive to 380V?

well, I set it to a 3 second delay between each point, and measured it from 300 to 380 in 10 step increments, then decreased it to 300 in 10 increments with the same delay so I'm not sure how much it cooled in those 3 seconds between each measurement.

Ahh, I see.

Interesting. Lots of variables.. It probably did have some effect.

But, your graph would be an indication of the physical/chemical change, right? Since the current it can support on the 2nd pass is lower than on the 1st...

I was playing with a pencil and my DC power supply the other day...

I seriously need more coffee, but IIRC.. I was able to touch the two seperate pieces of graphite together, raise the voltage.. and then slowly seperate them...

An arc would form, and the current would jump.. I thought that was pretty interesting.

Difficult to keep the arc struck since it consumes the graphite, thereby changing the distance between the two "electrodes".

It looks like you're working with much higher voltages than I though.. Might be more fun to play.

The arc is very, very bright.

 

[JohnCU]

Originally posted by: RaynorWolfcastle

Originally posted by: JohnCU

Originally posted by: RaynorWolfcastle
No such thing as negative resistance unless you use active elements. What you're describing is a change in conductivity, probably caused by a change in the material properties becayse of heating.

After the conductivity changes, though, in that negative resistance region (differential resistance, dV/dI), the slope of that line is a negative constant, so it does appear to be a negative resistance.

No, negative resistance would be like putting a 50 ohm resistor in series with a black box, then measuring the resistance across both and finding it's 25 ohms. There are circuits designed to do just this over some range of voltages, usually using opamps. They are usually used to reduce the effects of parasitics.

What you're describing is simply a lower resistivity of the material. It probably has to do with applying a large voltage across the material, could be changing the electronic properties of the material, or maybe due to heating (though usually heating causes non-linear increases in resistance).

well from what i've read, this is a non-ohmic material so that makes it weird. you are right that the static resistance at any point, V/I is positive, but the confusion is because there is also dynamic resistance, dV/dI and it is negative over part of the graph.

 

[LordSnailz]

Can you start again with some new graphite but as Eli said, start at 300 volt ...

 

[JohnCU]

Originally posted by: Eli

Originally posted by: JohnCU

Originally posted by: Eli
That's pretty cool.

Hmm...

Did you let the graphite fully cool before measuring the second drive to 380V?

well, I set it to a 3 second delay between each point, and measured it from 300 to 380 in 10 step increments, then decreased it to 300 in 10 increments with the same delay so I'm not sure how much it cooled in those 3 seconds between each measurement.

Ahh, I see.

Interesting. Lots of variables.. It probably did have some effect.

But, your graph would be an indication of the physical/chemical change, right? Since the current it can support on the 2nd pass is lower than on the 1st...

I was playing with a pencil and my DC power supply the other day...

I seriously need more coffee, but IIRC.. I was able to touch the two seperate pieces of graphite together, raise the voltage.. and then slowly seperate them...

An arc would form, and the current would jump.. I thought that was pretty interesting.

Difficult to keep the arc struck since it consumes the graphite, thereby changing the distance between the two "electrodes".

It looks like you're working with much higher voltages though.. Might be more fun to play.

yeah, I did the same thing this morning at 1100 volts, the graphite disintegrated because I had a short piece and it just couldn't handle it, big spark. It's pretty fun though to see the arcs form like you're doing.

and yes you are correct about the physical/chemical composition changing I believe. Haven't gotten all that deep into the physical properties of conductors yet.

 

[JohnCU]

Here is a new piece of graphite, from 300 to 400 back to 300 in 10 V increments. The temperature effect on the 2nd one I posted earlier was probaly greater since I set the delay longer, it was holding the graphite at higher voltages longer to get a more stable reading. this time I set the delay to 500 ms for each reading so it'd go quicker and maybe not heat up as much.

#3

Hmm wait, the meter cut off at 320 V, let me try again.

 

[Eli]

Originally posted by: JohnCU
Here is a new piece of graphite, from 300 to 400 back to 300 in 10 V increments. The temperature effect on the 2nd one I posted earlier was probaly greater since I set the delay longer, it was holding the graphite at higher voltages longer to get a more stable reading. this time I set the delay to 500 ms for each reading so it'd go quicker and maybe not heat up as much.

#3

Hmm wait, the meter cut off at 320 V, let me try again.

hehe, was just going to say.... it's a little screwy on the last half... 390, 380, 370, 350, 360, 350, 340, 330, 320...