What is the meaning of the tolerance of a resistor?
- Common are 5% and 1%.
- Sometimes they are marked with an extra color band indicating tolerance.
- The tolerance is the distance between this resistance and the marked value.
- Say you have a resistance of 100 ohms, 5%. It should be between 95 and 105 ohms.
- If you need more precision, a 1% resistance would be between 99 and 101 ohms.
Of course, with a higher tolerance, the cost is higher. It is therefore nice to design circuits that can support 5% of resistances. Use the 1% if necessary, but really try, really avoid needing a trimming potentiometer – to buy, expensive real estate, expensive to calibrate during assembly.
Index – two resistors of the same band, even if 5%, often have an almost identical value.
They can be 96.7 ohms, but they will both be 96.7. This is useful in some amplifier designs (instrumentation amplifiers) where you need suitable resistors.
If you are looking at capacitors, you may be shocked by their tolerances. Ive saw + 80 / -50% on the front capacitors .
The resistors are made of a different resistive material and the resistor is the product of the resistivity of this material times the length of the material divided by the width and thickness of the material.
So there can be four physical variations, the resistivity and the three dimensions. Therefore imperfect, the resistance will vary.
The tolerance depends on the variation allowed.
Tougher tolerances involve more workable, more expensive materials, quality control and perhaps even some selections.
The usual tolerances are available: +/- 20%, +/- 10%, 5%, 1%, 0.5% and more restrictive. For cheap products made in bulk, 10 or 20% of parts are used.
I never use less than 1% for the less critical functions of my designs, which concern industrial equipment insensitive to cost differences.
In some cases, I use 0.1% resistors for precision measuring bridges.
The cost of these is a bit higher, but you should know that if you buy a resistor whose initial value is tight, you also want it to vary very little with time or temperature variations, properties that you obtain in association with tight tolerance parts.
As if you buy cars with large engines and sports suspensions, you will automatically switch to high-speed tires.
Is the amount that a resistor can initially differ from its stated rated resistance.
This is at room temperature and when it is new.
For example a resistance with a tolerance of 10%, take the example of a resistance of 100 ohms.
The nominal value is 100 ohms, but the initial value at room temperature can be as low as 90 or as high as 110 ohms, which corresponds to plus and minus 10%.
In aging, resistance can change in value, this is called aging. As the temperature changes, it can also change due to the temperature coefficient.
If you overheat it by dissipating more power or simply heating it too hard, you can change its value permanently.
So that all these factors (and others) can make the value of a resistance quite far from its declared value. Depending on the critical and delicate design, this can be a bad thing or not.
High precision resistors minimize tolerance, temperature changes and aging but cost relatively more.
The cheapest resistors are sold with a tolerance of 20%. aging and the effects of temperature can be several percent more.
high precision resistors with very low temperature coefficients can be obtained with a tolerance of 0.1% and are not uncommon in precise industrial instrumentation; more precision is available.
Resistances are sold in standard values. When you buy resistors at 10% tolerance, the available values are separated by 20%.
If you buy tolerance resistances of 5%, the standard values will be separated by 10%; for 1% resistances, the standard values are separated by only 2%.
