On Sep 9, 4:50*pm, The Natural Philosopher
wrote:
Man at B&Q wrote:
On Sep 8, 6:46 pm, Gordon Henderson wrote:
In article , Dave Baker wrote:

"pete" wrote in message
...
Does anyone know of an online utility that can calculate
what combination of series and parallel resistors are needed
to get a particular value?
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.
I'm not looking for the answer, I'm looking for the way to
find the answer. There are lots of websites where you can
tap in resistor values and have it calculate the result, but
that gets long winded. I've got a combination that gives
5253R with 4 resistors, but I'd like to do better
I'm puzzled. Knowing nothing about electrickery I've Googled resistors,
found out what E12 means, got a table of what values are possible which
appear to be 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and so on in
further powers of ten.
So what's wrong with the following four in series, 4700 + 470 + 68 +12 =
5250 or am I missing something obvious?
Missing the "Countdown" music in the background ;-)

I think there's more than that missing here.

It's the way I'd do it - you subtract the biggest and so on, however
the tolerances are cumulative - so if you used 1% resistors, you might
be fine, but lower tolerance resistors and it might be out by too much....

Eh? 2% is 2%, whether it's (A + B) + 2% or ((A + 2%) + (B + 2%)).

MBQ

Actually, if the tolerances are randomly distributed , ten 10k 10%
resistors in parallel is actually a 1k 1% resistor. See monte carlo
analysis.

Maybe to a mathematician, but you can't rely on that kind of analysis
in the real world of engineering.

If they're from the same production lot then the actual values are
unlikely to be distributed randomly. ten 10k - 10% resistors still
make a 1k - 10% resistor.

MBQ