Man at B&Q wrote:
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.


Oh dear. You had better tell Boeing, NASA, IMB, INTEL and everyone else
who uses it all the time.


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.


worst case perhaps, bit in practice its likely to be a lot closer.

The chances of getting 10 resistors ALL out by the same maximum
tolerance is the tenth power of the chances of geting one out that far,
given *uniform* distribution.

Given Gaussian, its *even smaller*.

In real life. engineering design consists in reducing the probability of
failure to below the probability of failure of the leasts reducible
other issue, and provided that is acceptable, building it. The
improbable failures are tested for, and if tests are passed, then the
subassembly is fit for purpose.

At least one piece of Acorn hardware is known to have a bug that will
cause it to crash every ten years or so of continuous use, on average.

However, that is insignificant compared with the software that would run
on it, or in fact its power supply and hard drives. Or indeed, its
estimated service life.



MBQ