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

"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

...he..he - showing my age, but in the era of solid carbon resistors it
was very simple. Choose a value slightly lower than you want, then
with the Avo across it on Ohms file a nick in the carbon until it was
spot on. Excellent for making meter shunts and the like.

AWEM

..he..he - showing my age, but in the era of solid carbon resistors it
was very simple. Choose a value slightly lower than you want, then
with the Avo across it on Ohms file a nick in the carbon until it was
spot on. Excellent for making meter shunts and the like.

Phew! It's not just me then ;-)

Al.

On 8 Sep, 17:00, "Andrew Mawson"
wrote:

..he..he - showing my age, but in the era of solid carbon resistors it
was very simple. Choose a value slightly lower than you want, then
with the Avo across it on Ohms file a nick in the carbon until it was
spot on.

Works for film resistors too, you just need a triangular needle file
and work along the spiral groove (one slip and it's open circuit
though!).

"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?
--
Dave Baker

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 ;-)

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...

Have to say, it's been many years since I tinkered in electronics, but it
was rare to actually require something that precise (test system exepted)
- so a 4k7 or 5k6 would be tried...

Gordon

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?

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 ;-)

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...

Surely if you used, say, 2% resistors the maximum you could be out in
total is 2% (high if they're all high or low if they're all low). In all
probability some will be high and some low so the total error will be
less than 2%.

Which brings me to the method where you find a nominal value of resistor
(or a couple combined) close to what you want and measure a few until
you find one where the actual value is near enough spot on.

--
Dave

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 ;-)

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...

Have to say, it's been many years since I tinkered in electronics, but it
was rare to actually require something that precise (test system exepted)
- so a 4k7 or 5k6 would be tried...

Gordon
theers a 5.1k resistor in E20 range.

Add a 150 and that's 5250
Add a 3 ohm and that's 5253

Gordon Henderson coughed up some electrons that declared:

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 ;-)

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...

Have to say, it's been many years since I tinkered in electronics, but it
was rare to actually require something that precise (test system exepted)
- so a 4k7 or 5k6 would be tried...

Gordon

I thought the E12 scale was specifically developed so you could get close
enough (TM) to almost anything you practically want with just two
resistors - series or parallel.

Or is that another factoid in my myth database?

Tim S wrote:

I thought the E12 scale was specifically developed so you could get close
enough (TM) to almost anything you practically want with just two
resistors - series or parallel.

Or is that another factoid in my myth database?

Each one is 20% more (roughly) than the previous one.

Andy

Andy Champ wrote:
Tim S wrote:

I thought the E12 scale was specifically developed so you could get close
enough (TM) to almost anything you practically want with just two
resistors - series or parallel.

Or is that another factoid in my myth database?

Each one is 20% more (roughly) than the previous one.

that being the sort of tolerance on them anyway, when first introduced.

Its a log scale.

Andy

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

In article ,
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%)).

You're right. Brain not in gear.

Gordon

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.

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

In message , Dave Baker
writes

"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?

Well, 2% of 4700 is 94, which puts both 68 and 12 'within the noise' of
the 4700 resistor. The preferred way to do it used to be to use a number
of similar, but different, values of resistor to make up the value,
hoping that the tolerances of the different batches would even out in
opposite directions. I remember writing a program for the BBC computer
to do this, that gave recommended serial and parallel combinations.

--
bof at bof dot me dot uk

In article ,
pete wrote:
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

Have you Googled for one? 'resistor calculator prog' seems to give plenty
hits.

The one I use - on this Acorn - gives 82k and 5k6 in parallel at a 0.15%
error. With just two in series the best it can achieve is 0.95% (1k3 and
3k9)

--
*Why do they put Braille on the drive-through bank machines?

Dave Plowman London SW
To e-mail, change noise into sound.

In article , Dave Plowman (News)
writes
In article ,
pete wrote:
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

Have you Googled for one? 'resistor calculator prog' seems to give plenty
hits.

The one I use - on this Acorn - gives 82k and 5k6 in parallel at a 0.15%
error. With just two in series the best it can achieve is 0.95% (1k3 and
3k9)

The general rule for parallel friggery being:
Start with the standard value above the desired value and add a parallel
resistor from around a decade (x10) up, 56000 gives 5091, 68000 gives
5174 and as DaveP says, 82000 gives the closest at 5242.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs

On Tue, 08 Sep 2009 18:54:11 +0100, Dave Plowman (News) wrote:
In article ,
pete wrote:
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

Have you Googled for one? 'resistor calculator prog' seems to give plenty
hits.

The one I use - on this Acorn - gives 82k and 5k6 in parallel at a 0.15%
error. With just two in series the best it can achieve is 0.95% (1k3 and
3k9)

Yes, there are lots of downloadable ones (that I presume do what I want).
However I'm looking for an online utility - one where I can go to a web
page that asks:
what value do you want to find?
what family of values (E12, E24 ...) to use?
how close do you want to be

and then it tells you that for a value of X, you need an A and a B in
parallel and a C in series - or whatever the answer might be. The ones
that google spits back merely calculate the result from values you
type in.
I've (easily) got this solution, but I'm looking for a more general
solution in the future. Although, in practice the answer is to
choose the next lowest value and add a preset (multi-turn for extra
accuracy) and tune for maximum smoke.

In article ,
pete wrote:
Have you Googled for one? 'resistor calculator prog' seems to give
plenty hits.

The one I use - on this Acorn - gives 82k and 5k6 in parallel at a
0.15% error. With just two in series the best it can achieve is 0.95%
(1k3 and 3k9)

Yes, there are lots of downloadable ones (that I presume do what I
want). However I'm looking for an online utility - one where I can go to
a web page that asks: what value do you want to find? what family of
values (E12, E24 ...) to use? how close do you want to be

Right - sort of misread the question.

--
*Why do we say something is out of whack? What is a whack?

Dave Plowman London SW
To e-mail, change noise into sound.

On 08 Sep 2009 15:34:45 GMT, pete wrote:

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

Use a spreadsheet.

Put the E12 values in a row at the top, and again in a column. At each
intersection of row/column put the formula for the combination you want
to try, i.e. parallel Rp = 1/(1/Rr + 1/Rc), or series Rs = Rr + Rc.

Pick the best result by eye.

OR

Take any 4k7 out the box. Measure it. Add approriate series resistor/s.
In the trade, this series resistor is labelled SOT (Select On Test).

Phil

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.

Not an answer to your question, but sticking a 4k7 resitor in series with a
1k pot would be my solution.

Oh, and yes, programs to do exactly what you want have been around for
years. I wrote one. In Fortran. In 1979. I'm pretty sure I have the punched
tape somewhere around here ... ;-)

These days I'd just muck about in Excel and use goal seeking or whatever to
get the answer.

Al.

pete wrote:
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

There's lots of R calculators on the web but most just give one
answer, when what most people want, is a list of toleranced result
options, to pick and choose from. This prog is OK but has not a
specific "within 2%" sort.
(the best pair comes to 0.1% anyway!)
http://chris.gillings.com/resist.php

On Sep 8, 4:34*pm, pete wrote:
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


People have suggested a main resistor plus a parallel tweaker
resistor, which is a fine way to go. A close main R plus a relatively
high value parallel tweaker can often get you there with just 2 Rs.

But one error to avoid is to calculate your tweaker r value using the
nominal value of the main R. Instead use the real value of the main R
to work out what tweaker R you need, and the result will be far more
precise.

For some apps you can use basic 5% Rs to get 1% accuracy, but for some
you cant. Wider tolerance Rs are so for 2 reasons. First is simply
selection, its cheaper to stick them in the E12 range than E24. The
other one is tempco, Rs do change value with temp, so if you keep your
R temp steady you can use cheap Rs for quite good accuracy.

PS one of the key ideas of the E ranges is that every real value of R
produced can be placed in a nominal value band and be sold.


NT

NT wrote:

PS one of the key ideas of the E ranges is that every real value of R
produced can be placed in a nominal value band and be sold.

I have to say that when I first discovered this fact, I was
struck by the sheer cunning of the arrangement.

Chris
--
Chris J Dixon Nottingham UK


Have dancing shoes, will ceilidh.

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%

"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.

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)

"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?

Fredxx wrote:
"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 tolerances.

They do not. I could cite you a hundred examples of how and why nearly
all digital electronics is actually made to a monte carlo statistical
model of tolerances. The aim is that, given Gaussian distribution of
(mostly time delays through the kit) 99.9% of the units will work within
the specified temperature range, and the 0.5% that do not are thrown
away, or sold off to cowboy board makers.

If cumulative worst case delays were used it would result in about 10
times more expensive kit. Or about 1/4 the current clocking speeds.
Whatever. Statistical analysis is THE way most large designs are done.
Most small designs do NO analysis for tolerance at all, until a batch of
semiconductors 'doesn't work'

The ONLY time I was required to do worst case analysis was in military
and avionic equipment, and even there, only for the most critical
elements. For the rest, it was simply tested over the temperature range,
and if it failed, it was fixed till it did not, by replacing parts.


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.

They would not, and they do not. I know. I spent a day measuring 1500
phototransistors. The spread was beautifully gaussian., with the top and
the bottom tails chopped off. Except for two, which had either slipped
through the manufacturers selection, or had in fact been thrown in to
'make up the numbers' since the manufacturer did specify 'no more than
tow parts per thousand out of spec' Hmm.

I did this because I needed to establish whether or not a particular
circuit could be produced without recourse to setting up
potentiometers, and whether or not any in spec transistor would work.,
Fortunately the answer was yes to both.

That's semiconductors, where you get pretty much perfect gaussian
distribution.

Resisrorors are a different kettle of fish. Currently resistors are made
on machines that actually cut a spiral groove in a carbon film on a
ceramic substrate. You set the desired resistance on the machine and it
simply makes them up to as near an exact figure as the machine is
capable of. In general that's better than 1%, so although you do get a
gaussian distribution, its a very narrow one. It seldom exctends over
the full range allowed by the tolerance. In fact ion a givenm batch of
say 1000 resistors, its likely that e.g. a 1k will all be 1.03k or
something, plus minus a shade, that being the way the machine spat them
out. With the occasional odd one out, that clearly slipped into the bin
during manufacture from somewhere else ;-)

None of this is mentioned on any data sheet, because to do so would pin
the manufacturer down to a tighter spec than is needed in most cases.

With resitosrs, apart from a few instances, they can vary enormously
without affecting the circuits final performance. Only in a few cases do
you need precision, and those few case are catered for by specially
selected precision resistors, or setting up with a trim pot.




Can
you cite any manufacturers datasheet, where they don't specify tolleance in
an absolute percentage form, but in a gaussian form?



Of course not. But that means nothing. Beyond the fact that they have
selected examples OUTSIDE tolerance and called them something else.

"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'd use 5.6k and a 82k in parallel. It's 8 ohms out!

1/5600 + 1/8200 = 1/5242

You should be able to get 1% resistors as well. Even with 1% the tolerance
would be 53 ohms.

QED