How linear are 1/4W carbon film resistors? - Page 1
Forum section: electronics / general

author: dusan
author_rank: Regular Contributor
timestamp: April 09, 2023, 12:30:45 pm
content: Assuming constant temperature, how linear are normal 1/4W carbon film resistors? Can I use carbon film resistors to calibrate linearity of ADC?

author: alm
author_rank: Super Contributor
timestamp: April 09, 2023, 01:31:30 pm
content: How do you define linearity of a resistor? The voltage / power coefficient? Maybe the voltage / power coefficient of a resistive divider made out of two resistors? What kind of ADC is this about?I have not done any measurements, but I'd prepare to be disappointed. My experience with carbon film (and cheap metal film) resistors and high resolution measurements is that they drift all over the place when you even look at them. Their temperature coefficient will ruin your day, even if the ambient temperature is stable. Any change in current through the resistor will change the temperature, and the resistance.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 09, 2023, 03:08:37 pm
content: I wouldn't use a carbon film, though I don't have any data that says they're bad. Quality metal films are very good, but with any resistor you have to watch for heating due to power dissipation. Is this DC or AC? Can't you get a wire-wound or metal-foil type?

author: dusan
author_rank: Regular Contributor
timestamp: April 09, 2023, 03:13:42 pm
content: It will be very low frequency measurement, 10s of Hz.

author: TimFox
author_rank: Super Contributor
timestamp: April 09, 2023, 03:22:16 pm
content: I have measured the non-linearity (i.e., voltage co-efficient of resistance) on high-resistance thick film resistors, and it is noticeable.I never considered carbon for this application.Metal film leaded resistors should be OK.In general, the actual non-linearity in a physical resistor is a function of the voltage gradient along the resistive element.Therefore, physically shorter (therefore lower power) resistors will be worse than longer units for critical applications.If you want to measure non-linearity, you can construct a Wheatstone bridge with known good resistors, using low resistances on two legs, and vary the excitation voltage.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 09, 2023, 04:36:25 pm
content: Carbon film resistors usually have quite some temperature coefficient. This makes them not that desirable for a precision application. Modern thin film resistors are not that expensive and definitely the preferred. There is no real need to use a more poor grade carbon resistor. Even of it would work, there is always the possibilty that the resistor may be nonlinear. If possible one wants part where one has not much doubt in the linearity or other error contribution.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 09, 2023, 05:55:13 pm
content: Quote from: TimFox on April 09, 2023, 03:22:16 pmYes, forgot about that. Long ago I built something where linearity was important and quickly learned that thick film resistors were to be avoided for low noise work and low THD work. I suspect some recipes are better than others. MOX resistors were also quite bad in HV circuits and caused problems in feedback loops, transferring their non-linearity to the amplifier output.