# What is resistance tolerance?

1. Feb 26, 2010

### hover

Say I have a 20 ohm resistor with a tolerance of 5%. What does it mean to have a tolerance of 5%??

Thanks

2. Feb 26, 2010

### Staff: Mentor

The value is guaranteed to be between 0.95 * R and 1.05 * R.

3. Feb 26, 2010

### Staff: Mentor

Another way to say it is +/- 5%

4. Feb 26, 2010

### hover

So would it make perfect sense to say that for this example that the value of this resistor is guaranteed to be between (.95)(20) and (1.05)(20) which is 19 ohms and 21 ohms? Is that the correct idea?

thanks

5. Feb 26, 2010

### Staff: Mentor

Exactly. 5% of 20 Ohms is 1 Ohm.

6. Feb 26, 2010

### hover

Just one more question
Say you put 2 of these resistors in series. The resistance will jump to 40 ohms but what happens to the tolerance? Will it double or stay the same?

7. Feb 26, 2010

### Staff: Mentor

You can figure that one out for yourself, using the equation for the total resistance.

You know that the smallest you can get is 19+19 Ohms, and the largest is 21+21 Ohms. What tolerance does that indicate.

Now do it for the parallel resistor case.... Do you get a similar or different answer...?

8. Feb 26, 2010

### hover

Oh ok I see. Just do 2(19)/40 and 2(21)/40 and you get the same tolerance of 5% I believe. In other words, whether in series or parallel, the tolerance doesn't change.

Thanks!