Using carbon film resistors for a car project

Projects General Guidance
1

I'm working on a project to put in a car.

I have gotten for free a huge set of old 5% carbon film resistors (it's about 10kg of resistors!). And I have some old knowledge that tells these kind of resistors don't stay stable in humidity, maybe they degrade over time, but I am not an EE so I feel I should ask.

One of the parts of the project involves a 3-point resistor ladder connected to the key input of the car stereo which appears to be a current sensing ADC pushing about 3 volts through it. I will optoisolate the ladder and this ADC from the Arduino.

The ADC in the radio records the value of the pressed button and it does not appear to have a function to use a range of values so no tolerance for drift.

I am unsure if it is wise to use these old stock resistors for the task - if they change too much the ladder may malfunction. I live in place near the sea and it is humid here often and of course my preference is not to have to fix it later if it does malfunction.

Has anyone used these in a car project where high humidity caused a problem or am I being overly concerned?

Also what other kind of fun things can we do with 10kg of free resistors?

2

Use your Ohmmeter to got today's value! Yes, they change with age. I was given a box of old resistors because they had changed values, but still usable if you measure.

3

They should be stable. Usually resistors only "go bad" when they burn-up, which normally happens after a transistor or MOSFET burns-out and shorts-out fist, causing too much voltage across, and current through, the resistor. (Or of course, it could be a poor design with a resistor not rated for the power it dissipates.)

Carbon resistors might be a little more noisy in critical applications like microphone preamps or instrument amplifiers, but I'm not sure.

4

OK, I marked this as the solution as it is the most practical thing to do, thank you all. The device won't be too hard to access if it does misbehave.

Not that I'm going to measure all 10 kilograms of 1/4 watt resistors, mind you. :upside_down_face:

5

I hope they are properly marked and not all the same value!

6

Of course you do, and then you correct the markings if there's any drift.

7

@Paul_KD7HB, they are as new old stock should be - yellowing old paper and the box they were in had been moved around so many times it was falling apart.

But they are pristine, bagged, unopened, and have a big label affixed to each bag.

I actually forgot I had them until I stumbled upon them.

1 Like
8

Resistor ladders often need accurate resistors (1% accuracy) (depending on your needs for the amount of bits of your reading).
If you have 100 resistors (or more) of the target value, you should be able to find 10 or 20 that are within the 1% range...