"Andy Dingley" wrote in message
...
On 9 Sep, 16:14, "Fredxx" wrote:

The tolerance of a resistor is the maximum extreme of measured
resistance.

AIUI, it isn't - although this depends on the resistor technology.

Cheap resistors (carbon rod) were made by little more than the "bake &
sort" approach, so were individually measured and sorted. Tolerance
(which was pretty broad then) was an absolute limit, but the
distribution was sufficiently broad that you would frequently
encounter resistors close to the limits of this band.

High quality resistors are also measured and so have some hard cut-off
for tolerance.

For most modern resistors though (i.e. 1% & 2% films) production
process quality is such that they're now made "to spec" and the
resistors are made in separate batches for each value without needing
to be tested or sorted afterwards. Tolerance is however now based on a
Gaussian distribution (or close to it). It's also possible that a
resistor from the batch could be out of spec, but it's unlikely to be
so (some accepted large proportion of the batch will be). The 2%
figure is set at some number of standard deviations away from the
mean, such that 9*.*% of the resistors will be within that band.

So if you combine 2% resistors, where in series or in parallel, the
maximum
deviation of actual resistance either singly or combined is still only
2%.

That only holds if the tolerance is an absolute. If it's a Gaussian,
it doesn't hold (but is still predictable, with a bit more maths)

This is a definition of tolerance as applied to resistors from the Vishay
website


Tolerance: The tolerance on delivery is the range within
which the resistor can deviate percentually from the value at
the time of delivery.

Electrical and electronic design rely upon absolute tollerances. For any
component where there is a gaussian tolerance, the datasheet would include
the standard deviations so the user could determine the probability that
99.9999% of resistors were within tolerance when they left the factory. Can
you cite any manufacturers datasheet, where they don't specify tolleance in
an absolute percentage form, but in a gaussian form?