Ohmic and Non-Ohmic Resistances

[MrAl]

Hello again,

I think Eric's example of the incandescent bulb is a good one to look at.

The true relationship between E and I is exponential like:
r=A*v^B {note v is a variable here}

yes we still call it 'ohmic' sometimes because it dissipates power as heat.
But it doesnt follow Ohm's Law until we force it to by linearizing the curve at one particular voltage point for example:
Rv=A*B*I^(B-1) {note I is a constant here}

and now we assume it obeys Ohm's Law over a small operating range:
E=I*Rv

But in this case we forced it to obey Ohm's Law and realize that this will only hold over a short range of I.

However, the device dissipates power over the entire range so sometimes we call it ohmic anyway because resistance is an energy dissipator. Perhaps a better terminology would be to simply call it 'resistive'.

 

[Ratchit]

Hello again,

I think Eric's example of the incandescent bulb is a good one to look at.

The true relationship between E and I is exponential like:
r=A*v^B {note v is a variable here}

yes we still call it 'ohmic' sometimes because it dissipates power as heat.
But it doesnt follow Ohm's Law until we force it to by linearizing the curve at one particular voltage point for example:
Rv=A*B*I^(B-1) {note I is a constant here}

and now we assume it obeys Ohm's Law over a small operating range:
E=I*Rv

But in this case we forced it to obey Ohm's Law and realize that this will only hold over a short range of I.

However, the device dissipates power over the entire range so sometimes we call it ohmic anyway because resistance is an energy dissipator. Perhaps a better terminology would be to simply call it 'resistive'.

Since Ohm's law is a property law of resistive linearity, it cannot be forced to be something else. Disregarding the small error over a small resistance range is not the same as saying it obeys Ohm's law within that range. All this arguing about definitions like Ohm's law, current flow, or charging a battery or capacitor, is about using technical slang instead of precise terms that accurately describe technical activities. Even science oriented organizations like NASA use silly space slang like "space walking".

I like your precise definition of a material as being "resistive" instead of "ohmic" when you mean the material possesses the property of electrical resistance.

Ratch

 

[MikeMl]

The lamp is really an interesting example of a "linear resistor" that changes its resistance over time...

Think about how it can be used to stabilize a Wein-Bridge oscillator. On a cycle-by-cycle basis at frequencies > ~20Hz, it acts like a linear resistor whose resistance increases with the temperature of the filament.

Over many cycles, the RMS voltage across it heats its filament. The filament has a thermal mass, and a thermal resistance to ambient. The temperature of the filament has a thermal time constant long enough to average many cycles of the oscillator.

If you plotted the I vs V curve of the lamp such that you dwell on each V for many seconds before reading I, you get a non-linear plot. If you sweep V at a rate exceeding (one over) the thermal time constant of the filament, you get a linear current vs voltage display whose slope is a function of the voltage sweep amplitude. (I have done this test on my Textronics Curve Tracer).

 

[KeepItSimpleStupid]

Put the worms back in the can please.

The incandescent lamp is a "resistive element'.

If you fix temperature and some other variables and R=V/I holds true for the specified region of interest, it's ohmic. Passing current will likely raise it's temperature and R=V/I will fail. At some point, there will be voltage breakdown.

Yea, so it's k= V(temperature, radiation, magnetic field, V< breakdown, Johnston noise = k, Cosmic ray density =0, C negligible, negligible etc. ) divided by I (temperature, radiation, magnetic field, V< breakdown, Johnston noise = k, Cosmic ray density =0, Capacitance negligible, Inductance negligible etc. ) it's "ohmic"

The incandescent lamp can be thought of as a "current dependent resistor".

The NTC and PTC thermister is a temperature dependent resistor.

If I fix temperature what good are they?

"Ohmic" is an adjective e.g. "Ohmic contact"

The key here is the "region of interest". I expect an "Ohmic contact" to be a linear relationship between V and I under the specified conditions. I would not expect it to look like a diode, because that is a "rectifying contact" and I don't expect it to act as a voltage source.

When we join dissimilar metals we call that a "junction" or even 1/2 junctions. We have terms like "contact resistance", incandescent lamps, NTC thermisters and PTC thermisters.

An ohmic contact is a non-rectifying junction: an electrical junction between two conductors that has a linear current–voltage (I-V) curve as with Ohm's law.

Note "linear current–voltage (I-V) curve"

If you want to put your NTC or PTC thermister in a temperature controlled oven, I'll let you call it ohmic, otherwise forget it.

 

[Ratchit]

Put the worms back in the can please.

What worms? The concepts I enumerated are fairly straight forward.

The incandescent lamp is a "resistive element'.

Indeed it is.

If you fix temperature and some other variables and R=V/I holds true for the specified region of interest, it's ohmic. Passing current will likely raise it's temperature and R=V/I will fail. At some point, there will be voltage breakdown.

Voltage breakdown is irrelevant to the definition of ohmic. R=V/I is always true no matter whether it is ohmic or not. Ohmic is defined according to the physics text book I quoted as having a linear V-I curve or constant resistance over the range of interest.

Yea, so it's k= V(temperature, radiation, magnetic field, V< breakdown, Johnston noise = k, Cosmic ray density =0, C negligible, negligible etc. ) divided by I (temperature, radiation, magnetic field, V< breakdown, Johnston noise = k, Cosmic ray density =0, Capacitance negligible, Inductance negligible etc. ) it's "ohmic"

What thing is V(blah, blah...) divided by I(blah, blah...)? What thing is ohmic?

The incandescent lamp can be thought of as a "current dependent resistor".

Then a incandescent lamp by definition of the above referenced text book is not ohmic, is it?

The NTC and PTC thermister is a temperature dependent resistor.

They are nonohmic by design.

If I fix temperature what good are they?

They tell you what the fixed value of the temperature is.

"Ohmic" is an adjective e.g. "Ohmic contact"

Yes, it is a slang adjective. The adjective "resistive" as suggested by MrAl is best.

The key here is the "region of interest". I expect an "Ohmic contact" to be a linear relationship between V and I under the specified conditions. I would not expect it to look like a diode, because that is a "rectifying contact" and I don't expect it to act as a voltage source.

Why would you expect a resistive contact to be any of the above?

When we join dissimilar metals we call that a "junction" or even 1/2 junctions. We have terms like "contact resistance", incandescent lamps, NTC thermisters and PTC thermisters.

Yes, and the point is?

Ratch



Note "linear current–voltage (I-V) curve"

If you want to put your NTC or PTC thermister in a temperature controlled oven, I'll let you call it ohmic, otherwise forget it.

A thermistor is nonohmic by design.

 

[KeepItSimpleStupid]

We agree. Nuff said.

 

[MrAl]

Voltage breakdown is irrelevant to the definition of ohmic. R=V/I is always true no matter whether it is ohmic or not. Ohmic is defined according to the physics text book I quoted as having a linear V-I curve or constant resistance over the range of interest.

Hi again,

Well we have to be careful with this R=V/I.

In the universe we dont always find resistance just because we find voltage and current. If something is non resistive it of course can still have a V-I curve. That makes it non ohmic, or at least that's how it is sometimes referred to (non resistive is probably the better terminology again). In this case it does not dissipate heat. I am sure you are aware of this i just wanted to point it out for clarification in the thread. So R=V/I isnt always true.

 

[Ratchit]

Hi again,

Well we have to be careful with this R=V/I.

In the universe we dont always find resistance just because we find voltage and current. If something is non resistive it of course can still have a V-I curve. That makes it non ohmic, or at least that's how it is sometimes referred to (non resistive is probably the better terminology again). In this case it does not dissipate heat. I am sure you are aware of this i just wanted to point it out for clarification in the thread. So R=V/I isnt always true.

The only instance here on Earth that I can imagine for that to happen is a V-I curve for an ideal inductance or capacitance, which has no resistive component. In that case, the formula changes to Z=V/I. This implies that the V-I phase has to be taken into consideration.

Ratch

 

[MrAl]

Hi again,

Yes, but you only need one contrary instance to show that R=V*I isnt always true, which was my main point, which you just proved so well

Yes, storage elements are not ohmic so they wont follow R=V*I in general because there is no R. I was pointing that out partly because it's not true for some elements in the universe and partly just to show something that was really non ohmic, or as we all seem to prefer, non resistive.

 

[Ratchit]

Hi again,

Yes, but you only need one contrary instance to show that R=V*I isnt always true, which was my main point, which you just proved so well

Yes, storage elements are not ohmic so they wont follow R=V*I in general because there is no R. I was pointing that out partly because it's not true for some elements in the universe and partly just to show something that was really non ohmic, or as we all seem to prefer, non resistive.

Before something can be nonohmic, it has to have resistance. In other words, before electrical nonlinearity can be exhibited, there has to be a resistance. Non ohmic does not mean no resistance.

Ratch

 

[MrAl]

Hi,

Yes but this is a tangent to the main idea. The main idea does not require a discussion on parasitics.

 

[Ratchit]

Hi,

Yes but this is a tangent to the main idea. The main idea does not require a discussion on parasitics.

I don't understand your statement about parasitics. I thought the main idea was a definition of ohmic and nonohmic.

Ratch

 

[Tony Stewart]

All insulators are dielectric and all dielectrics have an effective series resistance and parallel leakage. Air in sparkplugs that ionizes and detonate have a low incremental negative resistance in series with a larger positive ESR of the current discharge path. So it generates heat but it could be used as a noisy amplifier from the negative resistance or an oscillator.

Reactive elements if they were ideal ( which does not exist ) would not have a series resistance, but they every one has a conductor with loss and the dielectric and a conductor such as film so they all have ESR.

Basic everything has resistance except perhaps a vacuum with no contamination, until sufficient voltage is applied to cause breakdown.

Basically everything that exists in nature and man-made has resistance such that with sufficient voltage or current, thus heat is generated.

Obviously ideal parts are a goal to reach that often comes at a compromise in cost, size, or some other attribute.

Many materials have a strong temperature coefficent such as some Metal OXides used for PTC fuses or ICL (NTC) inrush current limiters, Tungsten when hot is about 10x the resistance of cold.

All diode and semi's when saturated have a bulk series resistance or ESR that may be called RdsOn or Rce in BJT's that is fairly constant within a wide tolerance. That includes Zerners and LEDs

All batteries have ESR and leakage R.

Parasitics refer to the reactive elements (L,C)
WHich is a whole discussion on geometry of the conductors and the core.

 

[Ratchit]

All insulators are dielectric and all dielectrics have an effective series resistance and parallel leakage. Air in sparkplugs that ionizes a and detonate have a negative resistance in series with the positive ESR.

Reactive elements if they were ideal ( which does not exist ) would not have a series resistance, but they every one has a conductor with loss and the dielectric and a conductor such as film so they all have ESR.

Basic everything has resistance except perhaps a vacuum with no contamination, until sufficient voltage is applied to cause breakdown.

Basically everything that exists in nature and man-made has resistance such that with sufficient voltage or current, thus heat is generated.

Obviously ideal parts are a goal to reach that often comes at a compromise in cost, size, or some other attribute.

Many material have a strong temperature coefficent such as some Metal OXides used for PTC fuses or ICL (NTC) inrush current limiters, Tungsten when hot is about 10x the resistance of cold.

All diode and semi's when saturated have a bulk series resistance or ESR that may be called RdsOn or Rce in BJT's that is fairly constant within a wide tolerance.

How does the above eclectic series of facts pertain to the idea under discussion?

Ratch

 

[MrAl]

Hi,

Ratch:
Perhaps you were talking about something else. It sounded like you were talking about the series resistance that almost always accompanies inductance or capacitance. I apologize if it misinterpreted your intent.
You said before something can be non ohmic, it has to have resistance. So it sounds like you are saying that everything has resistance, but resistance is considered a parasitic in cases of storage devices, where the main part of the behavior is the storage of energy not the dissipation of energy. Feel free to clarify your meaning though.

Tony:
Everything real has resistance in most cases, but when we talk theory we separate the various components so that we can understand the pieces better. Only after we study the pieces do we put it back together again and then start making observations about the whole. In the case here, the inductance or capacitance stores energy and does not dissipate energy, and this is even true in real life if we are just talking about the energy storage part and neglecting the parasitics. So we can say that the capacitive part is non ohmic.

 

[Ratchit]

Hi,

Ratch:
Perhaps you were talking about something else. It sounded like you were talking about the series resistance that almost always accompanies inductance or capacitance. I apologize if it misinterpreted your intent.
You said before something can be non ohmic, it has to have resistance. So it sounds like you are saying that everything has resistance, but resistance is considered a parasitic in cases of storage devices, where the main part of the behavior is the storage of energy not the dissipation of energy. Feel free to clarify your meaning though.

Tony:
Everything real has resistance in most cases, but when we talk theory we separate the various components so that we can understand the pieces better. Only after we study the pieces do we put it back together again and then start making observations about the whole. In the case here, the inductance or capacitance stores energy and does not dissipate energy, and this is even true in real life if we are just talking about the energy storage part and neglecting the parasitics. So we can say that the capacitive part is non ohmic.

MrAl,
I was talking about ohmic and nonohmic. I am going by the definition of ohmic/nonohmic given by the physics text quote in post #18 of this thread. I will iterate what I said before. Ohmic means resistive linearity, nonohmic means resistive nonlinearity. It is a property of a material. The question is mute if no significant resistance is present. Some folks think that ohmic means that a material has resistance, but that is a slang definition that is not really true. Therefore, I am not stating that every material has significant resistance when I say nonohmic. I am saying that before nonohmic or resistive nonlinearity can be declared, a resistance must exist.

Ratch

 

[Tony Stewart]

How does the above eclectic series of facts pertain to the idea under discussion?

Ratch

Ratch which part did you not understand? Conductance is a property of every material, without exception. Leakage is a weak parallel conductance specified for dielectrics but all dielectrics which are insulators have a rated series leakage current and thus effective series resistance.

Materials are either conductors, insulators or semiconductors. All have ohmic losses.

One non-Ohmic loss is associated with magnetic eddy currents which are not ohmic in nature but rather due to the resistance of rotating magnetic domains. THese are the only "non-ohmic" losses I can think of but these are magnetic losses not conduction losses.

Acadamia uses the term "non-ohmic" to mean OHm's Law does not apply. I reject this definition, because companies that make MOSFETS and BJT's specifiy an OHM's LAW equivalent series resistance so I can use OHM's LAW all the time for any saturated diode. Diodes Inc have more than 50 patents on their devices with all have Rce specified which satisfies Ohm's Law.

If you have followed any of my forum answers you would already know this as I used Ohm's Law all the time for Semiconductors including LEDs. Thermal characteristics affect all devices so Ohm's Law strictly applies for a fixed material temperature with a coefficent.... which is why all semi's are rated at 25'C

 

[Ratchit]

Ratch which part did you not understand?

I did not understand how that barrage of facts related to what ohmic and nonohmic mean.

Conductance is a property of every material, without exception. Leakage is a weak parallel conductance specified for dielectrics but all dielectrics which are insulators have a rated series leakage current and thus effective series resistance.

And resistance can be considered a property of a material, but often times it is considered insignificant. But, what does that factoid have to do with the definition of ohmic and nonohmic?

Materials are either conductors, insulators or semiconductors. All have ohmic losses.

Another factoid, same question.

One non-Ohmic loss is associated with magnetic eddy currents which are not ohmic in nature but rather due to the resistance of rotating magnetic domains. THese are the only "non-ohmic" losses I can think of but these are magnetic losses not conduction losses.

Currents caused by alternating magnetic fields produce heat. Therefore they are resistive in nature. If the resistance is constant when the currents vary, then the material is ohmic. Otherwise the material is nonohmic. Ohmic and nonohmic refer to a property of a material.

Acadamia uses the term "non-ohmic" to mean OHm's Law does not apply. I reject this definition, because companies that make MOSFETS and BJT's specifiy an OHM's LAW equivalent series resistance so I can use OHM's LAW all the time for any saturated diode. Diodes Inc have more than 50 patents on their devices with all have Rce specified which satisfies Ohm's Law.

Before I can answer the above paragraph, I need to know what you mean by Ohm's law. Is it V=IR, or whether a material has a linear IV curve?

If you have followed any of my forum answers you would already know this as I used Ohm's Law all the time for Semiconductors including LEDs. Thermal characteristics affect all devices so Ohm's Law strictly applies for a fixed material temperature with a coefficent.... which is why all semi's are rated at 25'C

Same answer as before.

Ratch

 

[MrAl]

Hi again,


Tony:
Well, i think i see what you are getting at, but you are telling us one thing and i honestly believe you are thinking another. I can say this because you are in fact able to acknowledge the issue, and to acknowledge the issue you must understand it completely, which i can see you surely do.
So put another way, you are *using* the fact that the resistance can be *deemed* constant, but in the back of your mind you still know that it can change, and you are prepared to deal with that change. Thus you are dealing with the entire issue, not just the part where we have the element biased at some particular operating point.
I hope i made this clear.

MrAl,
I am saying that before nonohmic or resistive nonlinearity can be declared, a resistance must exist.
Ratch

Yes that doesnt make any sense to me, unless i still dont understand the point you are trying to make. That's like saying in order to call something a liquid or non liquid it "has to be at least a little wet". It still sounds like you are saying that "everything has resistance" which although holds in real life in most cases, in theory we dont always have to deal with that resistance. So theory really tells us that even if everything does have resistance, it can still be separated from the other part(s) and that part or parts can be discussed independently.
To put this another way, if something doesnt have any resistance then it has to be non ohmic. But if something is non ohmic, that doesnt mean that it has to have no resistance at all.

 

[JimB]

It is the thought of threads like this one which bring me out in a cold sweat whenever I type the words "Ohms Law" in reply to some simple question here on ETO.

JimB