What resistance change can be expected over time using 5% metal film
If I select a couple 5% metal film resistors to use in series to get a close 1% value, how stable will the resistor value be over a couple year period?
Are 1% resistors more stable in terms of resistance change over time than 5% metal film resistors?
Thanks,
-Bill
While I can't answer your question in specifics, I worked in a CRG (customer returned goods) dept. for Collins Radio, where we had a lot of older equipment come back for repair. The one trend I noticed is carbon film resistors tended upward over time (and with internal heating), sometimes quite a bit over their rated value.
If you're the same gentleman that runs the hobby circuits website, thank you. Lot of good stuff in there.
While I can't answer your question in specifics, I worked in a CRG (customer returned goods) dept. for Collins Radio, where we had a lot of older equipment come back for repair. The one trend I noticed is carbon film resistors tended upward over time (and with internal heating), sometimes quite a bit over their rated value.
If you're the same gentleman that runs the hobby circuits website, thank you. Lot of good stuff in there.
resistance change over time is generally not a concern unless the resistor is stressed by overheating. Some data sheets do give informaion on this. Generally of much more significant concern is resistance change with temperture. If have circuit that requires a stable resistance but is used in an environment that has large temperature changes the changes due to temperature far exceed the change due to time. Again data sheets define temperature stability. High stability costs more money.
The cost between 5% and 1% metal film these days is negligable for most applications Why not just by 1% instead of going to the trouble of selecting
5%. Are you sure the measuring instrument you use to measure the resistors is accurate to 1%?Dan
I serviced some Rockwell/Collins C-band equipment that ran so hot that the fiberglass turned black over a large part of the PC board because there wasn't enough airflow. The equipment was used in a 65 degree room, yet the cases were warm to the touch. The didn't use any fans to circulate air inside the aluminum box, and it caused lots of damage.
It's possiblle that the 5% resisters are made while the guy is adjusting the machine to the 1% accuracy needed to make 1% resistors :)
OTOH the metallic material in the 1% units may have a better temperature coefficient.
if thermal stability is not an issue the 5% kludge may be sufficient, but otherwise use 1% parts.
Get the "Resistors - General" PDF document near the bottom of the page. I looked at the document examples briefly and they did not indicate that 1% or
5% tolerance had any effect on long term stability. Temperature over time apparently has the greatest effect regardless of initial tolerance.Disclaimer: I only read part of it.
Cheers, John
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