All system parameters are subject to random variation, and the designer must deal with it. Tolerance is often, but not always, expressed as the percentage of variation permitted from a specified value. Background Definitions Tolerance In engineering, tolerance is the permissible limit or limits of variation in some parameter of a system or component ( Source). For a given series, it does not mean that you can find a specific resistor value within the tolerance range of a standard resistance value. The tolerance on a resistor value simply means that the manufacturer guarantees that a resistor value is within the tolerance % of that specific value. I was just surprised that the E series standard allowed these gaps. This is not a big deal because I can just specify a 1% or 0.5% resistor to get closer to value that I need. My objective here is to demonstrate the issue and provide a couple of ways of dealing with it.

For the 2% (E48), 1% (E96), and 0.5% (E192) resistors, you will NOT always be able to find a standard resistor value within 2%, 1%, or 0.5%, respectively, of the value you want.

For 20% (E6), 10% (E12), and 5% (E24) resistors, you can always find a standard resistor value within 20%, 10%, or 5%, respectively, of the value you want.

When can you find an x% resistor within x% of a specific value? The answer is 'it depends …' It was an interesting side trip that I thought was worth discussing here.

I was so surprised that I stopped what I was doing and learned how the E-series of standard resistor values were determined. I was a bit surprised that I could not find an E48 value (☒% tolerance) within 2% of my design value. This means that you can always find an E12 resistor value within 10% of your required value.

☑0%) values – notice that each tolerance range overlaps the adjacent ranges. For example, Figure 1 shows the E12 series (i.e. Until last week, I believed that the values of the E-series standard resistor values were selected to ensure that if I needed a resistor within x% of a specific value, I simply needed to choose a resistor from the x% tolerance set.

While I have always thought of resistors as simple devices, I recently discovered that I have been laboring under a misconception about the standard resistor values. I have been designing circuits with resistors since I was a kid working on science fair projects – I still remember building my first Radio Shack photocell project.

Common resistor values