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06-07-2023, 04:53 PM
Hi Champ81
That was not quite what I said either, except that it is true that thermal drift of the resistor value will be higher with higher dissipated power and/or ambient heat.
One of the parameter columns is 'temperature coefficient' with values that look like 25ppm/C, as an example. Most inexpensive power resistors will be 100ppm/C or higher, wire-wounds being mostly 300ppm/C. The small resistors used in most parts of the circuit are usually selected (by me or you) to be 50ppm/C, but also range up into the 100s of ppm/C.
For example, if you have a 100k resistor, 1ppm of the value is only 0.1 ohm, or 100mR. 50ppm is then 5R and 100ppm is 10R. If this is a resistor with 5% tolerance, its actual value can be off by 5k, and even as a 1% the value can be off by 1k. So, you can see that the temperature coefficient of the value stability represents a very small variation in practical numbers, but it still reflects something about the integrity of manufacture.
For a power resistor, maybe you have a 1k 5% wire-wound. Its tempco might be 300ppm/C, which is 300mR. If the resistor heats up to 5W its temperature might rise by 170C, which could change the value by 51R, or 5% of the nominal resistor value. We have not even counted the external heat sources.
Meanwhile a 1k resistor used as a cathode bias resistor may only dissipate 4mW and heat is minuscule, so the value drift will be also negligible even with the same high tempco as above. It is hard to find a metal film resistor with that high a tempco in the lower power ratings.
I choose components for reliability and circuits for tone. There are limits as to how far you have to go with these selections.
That was not quite what I said either, except that it is true that thermal drift of the resistor value will be higher with higher dissipated power and/or ambient heat.
One of the parameter columns is 'temperature coefficient' with values that look like 25ppm/C, as an example. Most inexpensive power resistors will be 100ppm/C or higher, wire-wounds being mostly 300ppm/C. The small resistors used in most parts of the circuit are usually selected (by me or you) to be 50ppm/C, but also range up into the 100s of ppm/C.
For example, if you have a 100k resistor, 1ppm of the value is only 0.1 ohm, or 100mR. 50ppm is then 5R and 100ppm is 10R. If this is a resistor with 5% tolerance, its actual value can be off by 5k, and even as a 1% the value can be off by 1k. So, you can see that the temperature coefficient of the value stability represents a very small variation in practical numbers, but it still reflects something about the integrity of manufacture.
For a power resistor, maybe you have a 1k 5% wire-wound. Its tempco might be 300ppm/C, which is 300mR. If the resistor heats up to 5W its temperature might rise by 170C, which could change the value by 51R, or 5% of the nominal resistor value. We have not even counted the external heat sources.
Meanwhile a 1k resistor used as a cathode bias resistor may only dissipate 4mW and heat is minuscule, so the value drift will be also negligible even with the same high tempco as above. It is hard to find a metal film resistor with that high a tempco in the lower power ratings.
I choose components for reliability and circuits for tone. There are limits as to how far you have to go with these selections.
