Precision Resistors

A lot of hobbyists want to make their own circuits but they face a problem: precision resistors.

But what is the difference between a precision resistor and an ordinary resistor? Answer: none.

By the way, the term precision is unappropriated because the tolerance of the component doesn't show us the precision of the resistor but how much its real value can differ from the nominal value.

- What you mean by none?

I will explain. By no means the manufacturer can assure that during a manufacturing process of resistors all of them will have the same value.

So, the manufacturer measures the resistance of each component and then it groups them by ranges of values. Each group has the same tolerance or (in)precision.

But what does tolerance means? It means that manufacturer assures that the real value of the resistor will not be either greater nor lower than nominal value plus or minus a percentage that is indicated by the tolerance band.

Let's see a sample.

Say a manufacturer is producing resistors with resistances of 100 ohms (nominal value) and that he sells them with 1%, 2%, 5%, 10% and 20% of tolerance.

Manufacturer will measure each resistance of 100 ohms and he will group them as follows:

All resistors with resistance varying between 99 and 101 ohms will be put together in the group of components with 1% of tolerance.

All resistors with resistance varying between 98 and 99 ohms or between 101 and 102 ohms will be put together in the group of components with 2% of tolerance (you should remember that all components with resistances varying between 99 and 101 ohms are in the group of resistors with 1% of tolerance).

All resistors with resistance varying between 95 and 98 ohms or between 102 and 105 ohms will be put together in the group of components with 5% of tolerance (you should remember that all components with resistances greater than 98 and less than 102 ohms were put in the groups of resistors with 1% and 2 % of tolerance).

This process continues until all components are classified in their tolerance band.

So I say again: There is no difference between a precision resistor and an ordinary resistor.

There are many projects where the ratio between resistances is more important than the actual values. In these case we can do the same thing as the manufacturer. We can measure our resistors to select the most appropriated.

Let me show an example. Suppose we have an inverter amplifier as shown in figure 1.


Gain is determined by equation

We can see that the circuit's gain is determined by the ratio between Rf and Ri.

Suppose we want a gain of X.

If Rf = 10,000 ohms and Ri = 5,000 ohms the gain is -2.

If Rf = 20,000 ohms and Ri = 10,000 ohms the gain is -2 too.

If we use resistors with tolerance (precision) of 5% the gain could vary between -1.81and -2.21 depending on real values of resistors Rf and Ri.

If we use resistors with tolerance (precision) of 2% the gain could vary between -1.92 e -2.08 what is much better.

What if we don't have precision resistors? What can we do?

We can use our ordinary resistors (tolerance of 5%). Take a bunch of them and measure each one until you find two resistors with a resistance ratio as near as possible of the value we wish (It is 2 in our sample).

Let me illustrate with an example. Say we take a bunch of resistors of 20K and 10K ohms (nominal values) to measure.

Suppose we found some resistors of 20,900 and 10,400 ohms. So we could do Rf = 20,900 ohms and Ri = 10,400. With these values, the gain would be -2.01 that is a better result than estimated with 2% tolerance resistors.

If we found Rf = 21,000 and Ri = 10,500 than the gain would be exactly 2.

The method, presented in this post, to use ordinary resistors instead of precision resistors is suitable only in prototype development. In commercial scale it is not feasible and the use of precision resistors is mandatory.

Syndicated 2008-03-17 02:38:00 from Marcio Andrey Oliveira