Difference between ohmic and non ohmic resistors

In summary: Today, there are still devices where the resistance is not a linear function of voltage, but that is a story for another time.
  • #1
netsky
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I was reading about ohmic and non-ohmicresistors, but I'm unsure as to what thedifference between them is. I understand that ohmic devices have a constantresistance, meaning the voltage-current graph produced for it will show a straight line and that non-ohmic devices have a changing resistance, leading to a curved voltage-current graph.

What I don't understand is why this happens. Non-ohmic resistors have changingresistance because as the voltage increases, the electrons transfer more energy to the atoms of the conductor, meaning there are greater vibrations leading to an increase in the temperature and resistance. But why is this not the case with ohmic devices? Is there not any increase in the vibrations of atoms in ohmic devices? Everywhere I've read, I've found that ohmic devices have a "limited temperature range", but that doesn't make sense to me. Surely any increase in voltage will lead to an increase in temperature, so a limit on the temperature is not possible.

Thanks in advance.
 
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  • #2
netsky said:
Everywhere I've read, I've found that ohmic devices have a "limited temperature range", but that doesn't make sense to me. Surely any increase in voltage will lead to an increase in temperature, so a limit on the temperature is not possible.
You are misinterpreting that. A "limited temperature range" means that the resistor is ohmic only within a certain temperature range. For a high enough temperature, any resistor will start behaving in a non-ohmic way.
 
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  • #3
DrClaude said:
You are misinterpreting that. A "limited temperature range" means that the resistor is ohmic only within a certain temperature range. For a high enough temperature, any resistor will start behaving in a non-ohmic way.
Ohhh, so basically every resistor is non-ohmic, but can be ohmic for a certain temperature range in which any change in temperature caused by a change in voltage, will not result in a change in resistance?
 
  • #4
netsky said:
but can be ohmic for a certain temperature range in which any change in temperature caused by a change in voltage, will not result in a change in resistance?

will not result in a SIGNIFICANT change in resistance

keep in mind as well that it is the current flowing through the resistor, not the voltage across it, that is the primary cause of the change in temperature
( yes, it is a cause and effect situation, but it is the current that induces the heating)Dave
 
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  • #5
davenn said:
will not result in a SIGNIFICANT change in resistance

keep in mind as well that it is the current flowing through the resistor, not the voltage across it, that is the primary cause of the change in temperature
( yes, it is a cause and effect situation, but it is the current that induces the heating)Dave
Alright, thank you both very much for the clarification!
 
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  • #6
netsky said:
Alright, thank you both very much for the clarification!

you are welcome :smile:

Remember resistors have a tolerance value, eg 1%, 5%, 10%, being the 3 common ones
This takes into account manufacturing process errors, material the resistor is made of (metal film are usually always 1% where as carbon can be 5% or 10%)
and I suspect, if I dig into resistor spec's deep enough, I would find that a temperature range is also taken into accountDave
 
  • #7
Netsky: You always should be aware that in electronics NOTHING is ideal.
* Each resistor has a temperature dependence and parasitic properties (capacitive, inductive)
* Something similar applies to other parts: Capacitors and inductors have parasitic resistive properties.
* More than that, no formula (gain, input/output resistance,...) is correct by 100%. Each formula contains simplifications and is neglecting some minor influences.

Knowing this, it is one of the primary tasks of a good engineer to decide for which applications he can use simplified expressions as well as idealized parts properties.
This decision is to be made taking into account parts tolerances and application-specific accuracy requirements. In this context, the operating frtequency range plays a major role.
Always remember: Each design should not as good/exact as possible but "only" as good/exact as necessary.
 
  • #8
Why don't people actually read Ohm's Law? It only applies for a metal at constant temperature. Even a lamp filament will 'obey Ohm's Law' if you can manage to keep the filament at a constant temperature but, of course, its resistance varies over a 10:1 [Edit 1:10] range as it warms up from room temperature to 2500K.
It is sloppy to use the term "Ohmic" in all cases because people get the wrong idea. The resistivity of some metals does not vary a lot over a practical range of temperatures so that's what to choose when you need to rely on the resistance, marked on the component.
As stated earlier - no electronic component is idea. We are lucky that the manufacturers have managed to produced devices that are usually 'near enough' to treat them as ideal in most circuits. Early devices were nothing like 'near enough'.
 
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Related to Difference between ohmic and non ohmic resistors

1. What is the main difference between ohmic and non-ohmic resistors?

Ohmic resistors have a linear relationship between the applied voltage and the resulting current, while non-ohmic resistors do not have a linear relationship and their resistance changes with the applied voltage.

2. How do ohmic and non-ohmic resistors behave under different voltage levels?

Ohmic resistors maintain a constant resistance regardless of the voltage applied, while non-ohmic resistors can experience a change in resistance when the voltage changes.

3. Can you give an example of an ohmic resistor and a non-ohmic resistor?

An example of an ohmic resistor is a simple metal wire, while a semiconductor diode is an example of a non-ohmic resistor.

4. What causes the difference in behavior between ohmic and non-ohmic resistors?

The difference is caused by the material properties of the resistor. Ohmic resistors have a constant resistivity, while non-ohmic resistors have a varying resistivity based on the material and its properties.

5. How do ohmic and non-ohmic resistors affect circuit performance?

Ohmic resistors are typically used to limit current in a circuit, while non-ohmic resistors can be used for more specialized purposes such as voltage regulation or temperature compensation.

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