"Andy Dingley" wrote in message
...
On 8 Sep, 16:34, pete wrote:

Specifically, I'm trying to calculate the best way to get
close to 5250 Ohm, using E12 preferred values. Power consumption
is not an issue and I'd like the value to be +/- 2% as that's
the resistor tolerance.

Is this one-off or production? Can you afford the time to measure
individuals.

You've already quoted a value to 1% precision, so this is either
excessive for the accuracy you need or else you can accept a rather
wider range of target values than simply 5250 alone. If you really
must have that, then you're looking at hand-selecting resistors from a
batch to get that close. Unless you really are going that close and
testing individual examples, there's just no point in serial
connection of 4700 and 47 resistors together: one's lost in the
tolerance of the other.

AFAIR, tolerances of cheap resistors are also non-Gaussian (owing to
sampling and sorting artefacts during manufacture), particularly so
for E12s and the higher tolerance bands. Otherwise it's a reasonable
assumption that tolerances add according to the classic Einsteinian
drunkard's walk rule of sqrt(n), i.e. two 2% resistors should be
treated as a tolerance of 1.414 x 2% or about 3%


The tolerance of a resistor is the maximum extreme of measured resistance.
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%.
As you suggest, the way resistors are selected means that the statistical
shape of the error is likely to be non-gaussian.