Why do resistors have a tolerance

When you think you want precision, think carefully: sometimes you really just want matching. You can do decent matching with a bag of resistors by hand: pick resistors, measure all of them, and then pick the closest two. You can do very good matching by adding a potentiometer in series with one of the resistors and adjusting it until the two legs match in resistance. Standard resistor values, packages, matching, and accuracy versus cost tradeoffs.

Edit - Simulate. Lewis holds a Bachelor of Science, and occasionally teaches classes on how to program for the Internet. What Is Tolerance in Resistors? Related Articles How to Lower Amperage. What Is J-Standard Soldering? How to Test a Blower Resistor. Raw Materials Used in the Manufacture of Electronic How to Test Diodes in Circuit. To get around this, the third band indicates that a certain number of zeros should be added after the first two digits to make up the full resistor value.

In the example above, the third stripe is brown, indicating that a single zero should be added to the right of the first two digits. If you want to go deeper into the math, this third band is officially referred to as a multiplier. The color of the band determines the power of 10 you need to multiply the first two resistor digits by. The fourth color band indicates the resistor's tolerance. If not, why is tolerance a percent and not a fixed amount of Ohms?

Why are tolerance relative and not absolute? If you imagine making a resistor just by cutting pieces of a material, let's say a special metallic film;. You want your resistor to fit in a usable box, else it's pointless, so you cannot make super long strips or incredibly short ones.

So you use film that's different thickness of the same metal. Now, say that you have a bunch of thicknesses, each thickness is ten times less resistive than the one that's one step thinner.

And they all have to be 10mm long to fit your box, so that you can only cut away from a standard strip width, let's say 5mm. If you want to make 10 Mohm, you take the thinnest one, and you have to remove half of its width. So you have to remove 2. If the material works linearly, which we'll assume for the ease of it, that means you "cut away" 10 Mohm in 2. To remove 10 Ohm more or less, that would mean cutting with an accuracy of brackets for clarity of order, not because they are needed :.

Written in meters that is 0. If you wanted to get the same 10 Ohm error on a Ohm resistor, you'd take the foil that's five steps up, which if it's still linear would get you about 50 Ohm 2 bits of Ohm in parallel , so you'd have to cut off 2. But this time, you can cut away only accurate to:. And that's when your resistor's box is allowed to be 10mm x 5mm, which is around 10 times the size of the most commonly used types these days.

Now, obviously resistors aren't made in an elf workshop full of reels of metal film We've gotten much better at making more different thicknesses of different materials, so it's gotten better. But, it does illustrate the point, even if you would use laser-trimming on everything, trimming to one part per million, which is 10 Ohm on 10 Mohm, is going to be a very difficult process to get consistent and it will even then still create a lot of parts that are over or under trimmed.

Only when you need a very accurate reference, which is uncommon if your name isn't Fluke, Keysight, Keithley or any of those others, will you want someone to give you a resistor that's better than 0. Though the 0.