why practical resistors does not strictly follow ohms law?

 

In what way?

They do follow ohms law, however temperature can change their resistance. Even then, they still follow ohms law, just with a new value of resistance.

 

Practical resistors never have the exact resistance you want, and the accuracy is usually denoted by the tolerance of the resistor. However, all resistors are close enough to use for your application. Practical resistors do follow ohms law!

 

Tolerance spec is primarily a manufacturing process variance.

Carbon comp resistors is a chunk of carbon between two wire connections. The manufacturing tolerance of making a specific diameter and length to the carbon block is reason for its tolerance.

Metal film resistors is carbon film coated around an insulator, typically a ceramic cylinder. They can be trimmed with laser cuts on the carbon film to achieve their desired resistance. The laser cut is typically a screw like 'barber pole' pattern. Tigher tolerance just requires a little more care in the trimming (which translates as time on the expensive laser trimmer)

Chip resistors on surface mount component is just like above but resistive ink is printed on one side of ceramic block. Termination must be solderable and metal in termination must not migrate under humidity exposure. This was a big problem in early surface mounted parts where palladium silver was used for terminating ends.

All material used to make a resistor has some temperature and aging variance.

 

Resistors are not perfectly linear. They have a voltage coefficient of resistance which varies with the type of material they are made from.

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Ron

 

Ohm's law only technically works under very tightly controlled laboratory conditions, and even then with todays modern measuring equipment the equations don't actually work in any real world situation because there is so much more going on that we can now measure. It's a very functional rule of thumb though.

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