Are Carbon Film Resistors inductive?

[pentoad]

According to the RS website they are not:

https://uk.rs-online.com/web/general...esistors-guide

[ms660]

"Firstly, the film is largely non-inductive"

Lawrence.

[tritone]

+1 ^ Lawrence.

Any length of conductor has inductance (about 1nH per inch), the helical cut on most film resistors will add to this, not a lot though. The sideways cut on surface mount resistors is much better.

[pentoad]

So they are not spiral cut film then

[broadgage]

The lower ohm values are virtually non inductive as they consist of a solid rod or stick of carbon composition, or sometimes of a film of carbon on a ceramic cylinder.

Different values are made by altering the carbon composition or the dimensions.

Higher values of carbon resistor may be slightly inductive as they often consist of a carbon film with a spiral groove cut into it.
This approximates to an air cored coil of a few turns.

[Ed_Dinning]

As has been said, not very inductive, BUT, what is the application and will it matter

Ed

[kalee20]

There's two types of carbon resistor - carbon composition, and carbon film.

Carbon composition is ground-up graphite, mixed with a binder and finely-milled insulating clay, and cast into rods. The composition varies, but I've seen from 10 ohms to 10 meg ohms. It's about as non-inductive as you can get. It also has very good surge capabilities. But it produces more noise than the fundamental Johnson noise, and carbon composition resistors have a reputation for drifting in value. (In fairness this is due to the precise mix chemistry: high-stability mixes were available, they were just rarely used).

Carbon film is produced by forming a film of carbon on an insulating rod. Often it is done in an atmosphere of hydrocarbon vapour, heated to several hundred degrees C, whereupon the hydrocarbon molecules start to break down and dump excess carbon onto available surfaces. Hence, this type of resistor is sometimes referred to as 'cracked carbon'. The final value is arrived at by cutting a shallow spiral groove, as has been described. So, there is more inductance. And the ability to withstand surges is inferior. But, they are lower noise, and are far more stable. Iskra of Yugoslavia produced these from 10 ohms to 1 megohm (the UPM range) and history to date shows these are usually above suspicion when seen in equipment.

[Radio Wrangler]

Philips was another big producer of carbon film resistors.

David

[joebog1]

So was Roedestein

Joe

[Ed_Dinning]

If low inductance is vital and the resistance required is not too high, it is possible to get wire wound resistors that are wound with 2, wires and the current passes in opposite directions in each wire, thereby reducing the inductance to a low value. Expensive though

Ed

[pentoad]

Originally Posted by Ed_Dinning

As has been said, not very inductive, BUT, what is the application and will it matter

Ed

The application would be for an oscillator in a valve signal generator

[thermionic]

Carbon composition resistors are usually able to work at higher voltages without breakdown due to their construction, which is a plus point where low noise is not a concern.

SimonT

[Ed_Dinning]

The inductance in this application should not matter as it will fall out when you set up and calibrate

Ed