Do carbon resistors not follow tolerance as they age?

Question

I'm trying to figure out the problem with my LCD and resistors. The micro I'm using is AT89S52 and the LCD is a standard display based on the HD44780 chipset.

Part of the code in my micro allows for the output lines P1.0 and P1.1 to be configured in any combination so I tested all combinations and noticed only two things. One contrast setting, and completely blank display.

The completely blank display is normal with P1.0 and P1.1 being high. But it seems all other combinations return the exact same brightness intensity even though the resistor values are different.

The only thing I could possibly contribute the problem to is the age of some resistors. At a local shop, the resistors are probably 40+ years old because sometimes when I look at them the leads are not shiny silver. They're dull-ish silver and the markings are not vivid and bending them is not super easy. The resistors I order online seem brand new: shiny leads, vivid markings, nicely coloured, etc. In all cases, the resistors are carbon film with 5% tolerance according to the markings.

Does age make the resistor value out of spec?

Also, My tests were done with a regulated 5.5V power (brand new 9V battery passing through a LM2940 regulator).

The reason why I use this setup is because I want to enable the dark setting (P1.0 and P1.1 = low) when the battery is almost dying (circuit voltage under 4.2V) and when the battery is brand new (circuit voltage at 5.5V), I want to enable only the light setting (P1.1 = low and P1.0 = high) and when the battery is sort-of new then I want the medium setting (P1.0 = low and P1.1 = high)

So does resistor age cause my problem, or are the values of my resistors not optimal? and is 5% carbon OK? or must I use 1% metal film?

Answer 1

So you are using 5% resistors and are worried about stability over time.

Your 1kΩ, 5% resistor can range from 950Ω to 1050Ω. There is no substitute for getting a good quality DMM and making appropriate measurements.

From Resistor01:

Shelf Stability/Drift. In addition to all the environmental and electrical factors listed above, resistors experience a long-term drift that may be as high as 2% per year for carbon composition resistors. Metal, oxide, metal film, and wire-wound resistor are typically 10 times more stable.

So carbon resistors can change ±2,000 ppm/year, which would equate to ±2Ω for a 1kΩ (0.2%, which is not 2%).

From Carbon composition resistor:

Fifty years ago, carbon composition resistors were widely used in consumer electronics. Because of the low stability of the resistance value, this type of resistor is not suitable for any modern high precision application. For example, the resistance value can change up to 5% over a shelf life of one year. With heavy use the value changes even more: up to 15% for a 2000h test at full rating with 70°C. Soldering can cause a 2% change. The reason for this instability is inherent to the design of the resistor. The carbon composition contains materials with different heat expansion properties. When the conducting carbon particles and the nonconducting binder heat up or cool down, stresses arise in the resistor body. The mechanical contact between the conducting particles will change, and this leads to a change in resistance value. Also noise properties are not good due to the mixture of different materials. The noise level increases when current flows. Resistors of 0.25W and 0.5W, have a maximum voltage of respectively 150V and 500V. Insulation resistance is poor with approximately 10^9 Ω (order of magnitude worse than other types). One more reason for the decrease in the use of this type of resistor, is the high temperature coefficient of around 1200 ppm/°C. The operating temperature range is between around -40 to 150°C. However, the resistor derate above 70°C.

Proving you get what you pay for! If it is critical maybe a better quality resistor is required.

If something is that critical, you have to get into the datasheets and measurements.