| Author |
Message |
Ian Iveson
Guest
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 Posted:
Fri Oct 07, 2005 11:44 pm Post subject:
Obtaining an accurate resistor |
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I need two accurate resistors.
The first, (R1) is for a tuned analogue circuit in which all the
other components are already fixed. Correct tuning of the circuit
requires an accurate resistor as close as possible to its nominal
value.
The second (R2) is for a matched pair. Neither needs to be close to
the nominal value, but they must be matched as accurately as
possible.
The kind of resistors I want are only available at 1% tolerance. I
am only building one example of the circuit. I don't care about
anything except accuracy in both cases.
For each resistor, R1 and R2, am I better off:
a) Buying one resistor of the right nominal value.
b) Buying X resistors each of 1/X th of the nominal value and
connecting them all in series.
c) Buying Y resistors of the correct nominal value and selecting the
best one.
d) Buying X * Y resistors, each of 1/X th of the nominal value, and
selecting the best series combination of X resistors.
If I decide to produce the circuit in quantity, should that make a
difference to which I choose?
Thanks, Ian
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Don Pearce
Guest
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| Posted:
Sat Oct 08, 2005 12:01 am Post subject:
Re: Obtaining an accurate resistor |
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On Fri, 07 Oct 2005 18:44:17 GMT, Ian Iveson wrote:
| Quote: | I need two accurate resistors.
The first, (R1) is for a tuned analogue circuit in which all the
other components are already fixed. Correct tuning of the circuit
requires an accurate resistor as close as possible to its nominal
value.
The second (R2) is for a matched pair. Neither needs to be close to
the nominal value, but they must be matched as accurately as
possible.
The kind of resistors I want are only available at 1% tolerance. I
am only building one example of the circuit. I don't care about
anything except accuracy in both cases.
For each resistor, R1 and R2, am I better off:
a) Buying one resistor of the right nominal value.
b) Buying X resistors each of 1/X th of the nominal value and
connecting them all in series.
c) Buying Y resistors of the correct nominal value and selecting the
best one.
d) Buying X * Y resistors, each of 1/X th of the nominal value, and
selecting the best series combination of X resistors.
If I decide to produce the circuit in quantity, should that make a
difference to which I choose?
Thanks, Ian
|
If you want a really accurate single resistor (I'm presuming you have the
means to measure accurately), but the next lower value, measure it and then
pad it up to the correct value with a smaller resistor or resistors. For
the equal value resistor pair, just pad up the smaller of the two with a
suitable small resistor. Some resistance wire will allow you to get as
close as you need to an accurate match.
d |
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Ian Iveson
Guest
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| Posted:
Sat Oct 08, 2005 2:37 am Post subject:
Re: Obtaining an accurate resistor |
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"Don Pearce" <donald@pearce.uk.com> wrote
| Quote: | If you want a really accurate single resistor (I'm presuming you
have the
means to measure accurately), but the next lower value, measure it
and then
pad it up to the correct value with a smaller resistor or
resistors. For
the equal value resistor pair, just pad up the smaller of the two
with a
suitable small resistor. Some resistance wire will allow you to
get as
close as you need to an accurate match.
Thanks, Don. The possibility of using strings of resistors of |
different values adds another dimension to the plot! Although it
would clearly provide a simpler solution, "Trim on test" is not
allowed here. Home-made resistors are definitely not an option in
this case. Only selection from like-value resistors is allowed.
If I were stuck with the options I have outlined, which would be
best?
cheers, Ian
| Quote: | On Fri, 07 Oct 2005 18:44:17 GMT, Ian Iveson wrote:
I need two accurate resistors.
The first, (R1) is for a tuned analogue circuit in which all the
other components are already fixed. Correct tuning of the circuit
requires an accurate resistor as close as possible to its nominal
value.
The second (R2) is for a matched pair. Neither needs to be close
to
the nominal value, but they must be matched as accurately as
possible.
The kind of resistors I want are only available at 1% tolerance.
I
am only building one example of the circuit. I don't care about
anything except accuracy in both cases.
For each resistor, R1 and R2, am I better off:
a) Buying one resistor of the right nominal value.
b) Buying X resistors each of 1/X th of the nominal value and
connecting them all in series.
c) Buying Y resistors of the correct nominal value and selecting
the
best one.
d) Buying X * Y resistors, each of 1/X th of the nominal value,
and
selecting the best series combination of X resistors.
If I decide to produce the circuit in quantity, should that make
a
difference to which I choose? |
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Ruud Broens
Guest
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| Posted:
Sat Oct 08, 2005 3:08 am Post subject:
Re: Obtaining an accurate resistor |
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"Ian Iveson" <IanIveson.home@blueyonder.co.uk> wrote in message
news:5mz1f.78527$iW5.2846@fe3.news.blueyonder.co.uk...
: I need two accurate resistors.
:
: The first, (R1) is for a tuned analogue circuit in which all the
: other components are already fixed. Correct tuning of the circuit
: requires an accurate resistor as close as possible to its nominal
: value.
:
: The second (R2) is for a matched pair. Neither needs to be close to
: the nominal value, but they must be matched as accurately as
: possible.
:
: The kind of resistors I want are only available at 1% tolerance. I
: am only building one example of the circuit. I don't care about
: anything except accuracy in both cases.
:
: For each resistor, R1 and R2, am I better off:
:
: a) Buying one resistor of the right nominal value.
:
: b) Buying X resistors each of 1/X th of the nominal value and
: connecting them all in series.
:
: c) Buying Y resistors of the correct nominal value and selecting the
: best one.
:
: d) Buying X * Y resistors, each of 1/X th of the nominal value, and
: selecting the best series combination of X resistors.
:
: If I decide to produce the circuit in quantity, should that make a
: difference to which I choose?
:
: Thanks, Ian
See my measurements post - if you're interested, could send you
the data, comma delimited or so,
for this particular batch and brand (BC 0.6W 50 ppm 1% mf)
distribution is rapidly falling off towards the extremes, so series
connecting does sharpen the resulting peak, so increases the
change that the compound value is closer to the nominal series
value. For tube circuits, the voltage rating is much improved with
series connecting, eg. a 0.6 W resistor can handle some 300 V
without adverse effects, smaller resistors are usually 200 V tops.
50 ppm/K, 2ppm/Vr resistors only cost about 1 eurocent, in bulk,
so easy to just find out by gettin' some .
Cheers,
Rudy
nb increasing the change of tighter tolerance is not the same
as equals tighter tolerance, so multiple series connecting
seems overkill to no avail. |
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vinylbigot
Guest
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| Posted:
Sat Oct 08, 2005 4:05 am Post subject:
Re: Obtaining an accurate resistor |
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You state that you need them in volume which changes the methods available.
What kind of volumes are you looking at and what values?
For high volumes, wirewound could give tight accuracy--and you could get
them made and sorted in China.
Thus would require tooling and set up costs.
For really low volumes--you could wind or sort by hand.
THe problem is for medium volumes (1,000 to 100,000). Tecktronics has
solved that problem somehow because I see surplus components from them with
..01% tolerance. Maybe give them a call and find out where their stock comes
from?
"Ian Iveson" <IanIveson.home@blueyonder.co.uk> wrote in message
news:5mz1f.78527$iW5.2846@fe3.news.blueyonder.co.uk...
| Quote: | I need two accurate resistors.
The first, (R1) is for a tuned analogue circuit in which all the other
components are already fixed. Correct tuning of the circuit requires an
accurate resistor as close as possible to its nominal value.
The second (R2) is for a matched pair. Neither needs to be close to the
nominal value, but they must be matched as accurately as possible.
The kind of resistors I want are only available at 1% tolerance. I am only
building one example of the circuit. I don't care about anything except
accuracy in both cases.
For each resistor, R1 and R2, am I better off:
a) Buying one resistor of the right nominal value.
b) Buying X resistors each of 1/X th of the nominal value and connecting
them all in series.
c) Buying Y resistors of the correct nominal value and selecting the best
one.
d) Buying X * Y resistors, each of 1/X th of the nominal value, and
selecting the best series combination of X resistors.
If I decide to produce the circuit in quantity, should that make a
difference to which I choose?
Thanks, Ian
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Chris Hornbeck
Guest
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| Posted:
Sat Oct 08, 2005 4:22 am Post subject:
Re: Obtaining an accurate resistor |
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On Fri, 07 Oct 2005 18:44:17 GMT, "Ian Iveson"
<IanIveson.home@blueyonder.co.uk> wrote:
| Quote: | I need two accurate resistors.
The first, (R1) is for a tuned analogue circuit in which all the
other components are already fixed. Correct tuning of the circuit
requires an accurate resistor as close as possible to its nominal
value.
The second (R2) is for a matched pair. Neither needs to be close to
the nominal value, but they must be matched as accurately as
possible.
The kind of resistors I want are only available at 1% tolerance. I
am only building one example of the circuit. I don't care about
anything except accuracy in both cases.
For each resistor, R1 and R2, am I better off:
a) Buying one resistor of the right nominal value.
b) Buying X resistors each of 1/X th of the nominal value and
connecting them all in series.
c) Buying Y resistors of the correct nominal value and selecting the
best one.
d) Buying X * Y resistors, each of 1/X th of the nominal value, and
selecting the best series combination of X resistors.
If I decide to produce the circuit in quantity, should that make a
difference to which I choose?
|
Without any cost constraints, all answers are equal. Just
buy a billion and sort in-house.
You're obviously after something more significant, so
maybe the question should be framed more tightly.
Good fortune,
Chris Hornbeck |
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flipper
Guest
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| Posted:
Sat Oct 08, 2005 4:43 am Post subject:
Re: Obtaining an accurate resistor |
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On Fri, 07 Oct 2005 23:22:36 GMT, Chris Hornbeck
<chrishornbeckremovethis@att.net> wrote:
| Quote: | On Fri, 07 Oct 2005 18:44:17 GMT, "Ian Iveson"
IanIveson.home@blueyonder.co.uk> wrote:
I need two accurate resistors.
The first, (R1) is for a tuned analogue circuit in which all the
other components are already fixed. Correct tuning of the circuit
requires an accurate resistor as close as possible to its nominal
value.
The second (R2) is for a matched pair. Neither needs to be close to
the nominal value, but they must be matched as accurately as
possible.
The kind of resistors I want are only available at 1% tolerance. I
am only building one example of the circuit. I don't care about
anything except accuracy in both cases.
For each resistor, R1 and R2, am I better off:
a) Buying one resistor of the right nominal value.
b) Buying X resistors each of 1/X th of the nominal value and
connecting them all in series.
c) Buying Y resistors of the correct nominal value and selecting the
best one.
d) Buying X * Y resistors, each of 1/X th of the nominal value, and
selecting the best series combination of X resistors.
If I decide to produce the circuit in quantity, should that make a
difference to which I choose?
Without any cost constraints, all answers are equal. Just
buy a billion and sort in-house.
You're obviously after something more significant, so
maybe the question should be framed more tightly.
Good fortune,
Chris Hornbeck
|
He's not really seeking advice or suggestions, he's trying to
demonstrate, using a 'real world example', his statement in the "Where
to get 1watt 1% resistors" thread. Namely that "Using resistors in
series or parallel IMPROVES the % accuracy of the final
value."
Problem is, it's the selecting of values that achieves it and not the
series or parallel connection itself although allowing multiples
increases the odds he'll find more suitable pairs than seeking singles
only.
But the resultant value of any two random 1% resistors in series or
parallel can still be 1% off nominal and that's what one must design
to, unless you do selection. |
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Chris Hornbeck
Guest
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| Posted:
Sat Oct 08, 2005 4:43 am Post subject:
Re: Obtaining an accurate resistor |
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On Fri, 07 Oct 2005 22:04:59 -0500, flipper <flipper@fish.net> wrote:
| Quote: | He's not really seeking advice or suggestions, he's trying to
demonstrate, using a 'real world example', his statement in the "Where
to get 1watt 1% resistors" thread. Namely that "Using resistors in
series or parallel IMPROVES the % accuracy of the final
value."
Problem is, it's the selecting of values that achieves it and not the
series or parallel connection itself although allowing multiples
increases the odds he'll find more suitable pairs than seeking singles
only.
But the resultant value of any two random 1% resistors in series or
parallel can still be 1% off nominal and that's what one must design
to, unless you do selection.
|
Beautifully framed; thanks very much.
The gods designed us by incorporating (nudge-nudge) the
selection process into our universe. Should we do less?
Arf!
Chris Hornbeck |
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Joseph Meditz
Guest
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| Posted:
Sat Oct 08, 2005 4:43 am Post subject:
Re: Obtaining an accurate resistor |
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| Quote: | So, if I use series strings rather than individual selected
resistors, would I be likely to get an equally accurate result from
less than a billion resistors?
|
Hi Ian,
Suppose you bought a bag of 100 1k resistors and 200 500R resistors.
And, without measuring them, soldered together pairs of 500R resistors.
You would then have two bags of 1k resistors both of which will have
the same variation. Neither set will will be of tighter tolerance than
the other.
However, there are 19,900 different pairs that can be created from the
set of 200 500Rs. This sample space is equivalent to a bag of 19,900
1k resistors. Given enough resistors you are likely to find one that
is very very close to 1k as well as one that is very very close to 1k
+/- 1%.
If both the 500R and 1k resistors were rated at 1/2W, then, in service,
the resistors in the series combination will be stressed less and so
should be more accurate in service.
Lastly, if you string together resistors you will add reactance to your
circuit which, depending on what you are doing, can affect performance.
Joe |
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Chris Hornbeck
Guest
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| Posted:
Sat Oct 08, 2005 4:43 am Post subject:
Re: Obtaining an accurate resistor |
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On Sat, 08 Oct 2005 01:50:03 GMT, "Ian Iveson"
<IanIveson.home@blueyonder.co.uk> wrote:
| Quote: | I reckon getting the question right is always the hardest part.
|
Yeah, and it seems that it's the part gotten wrongest
in our modern world-a-go-go.
| Quote: | I can't put a price on quality, but where strategies give the same
quality, I want to choose the cheapest.
|
Then two new factors enter: cost of sorting (in-house)
and absolute quantity. The second is because the number
of rejects falls for your second case (matching) with
larger absolute values.
Without defining those values, though, I don't think
that there's any real answer to your question.
| Quote: | So, if I use series strings rather than individual selected
resistors, would I be likely to get an equally accurate result from
less than a billion resistors?
And is the answer the same for the prototype as it would be for a
production run?
|
A million years ago, when we were young, pretty accurate
military accessments of reliability for electronic
equipment were taken as the reciprocal of the number
of solder joints. With modern flow soldering, etc, this
is just a passing fancy, but.... FWIW.
Thanks, as always,
Chris Hornbeck |
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Ian Iveson
Guest
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| Posted:
Sat Oct 08, 2005 4:43 am Post subject:
Re: Obtaining an accurate resistor |
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"Chris Hornbeck" <chrishornbeckremovethis@att.net> wrote
| Quote: | Without any cost constraints, all answers are equal. Just
buy a billion and sort in-house.
You're obviously after something more significant, so
maybe the question should be framed more tightly.
|
You have chosen to select rather than connect all billion in series,
which says something...hopefully more than a recognition of the
problems of finding billionth-ohm resistors, soldering them all, and
fitting them in the chassis.
I reckon getting the question right is always the hardest part.
I can't put a price on quality, but where strategies give the same
quality, I want to choose the cheapest.
So, if I use series strings rather than individual selected
resistors, would I be likely to get an equally accurate result from
less than a billion resistors?
And is the answer the same for the prototype as it would be for a
production run?
cheers, Ian
in message news:1h0ek1h8q4uv58ejf18rup7bvskt2bd1u8@4ax.com...
| Quote: | On Fri, 07 Oct 2005 18:44:17 GMT, "Ian Iveson"
IanIveson.home@blueyonder.co.uk> wrote:
I need two accurate resistors.
The first, (R1) is for a tuned analogue circuit in which all the
other components are already fixed. Correct tuning of the circuit
requires an accurate resistor as close as possible to its nominal
value.
The second (R2) is for a matched pair. Neither needs to be close
to
the nominal value, but they must be matched as accurately as
possible.
The kind of resistors I want are only available at 1% tolerance. I
am only building one example of the circuit. I don't care about
anything except accuracy in both cases.
For each resistor, R1 and R2, am I better off:
a) Buying one resistor of the right nominal value.
b) Buying X resistors each of 1/X th of the nominal value and
connecting them all in series.
c) Buying Y resistors of the correct nominal value and selecting
the
best one.
d) Buying X * Y resistors, each of 1/X th of the nominal value,
and
selecting the best series combination of X resistors.
If I decide to produce the circuit in quantity, should that make a
difference to which I choose?
Good fortune,
Chris Hornbeck |
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flipper
Guest
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| Posted:
Sat Oct 08, 2005 10:09 am Post subject:
Re: Obtaining an accurate resistor |
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On Sat, 08 Oct 2005 04:13:05 GMT, Chris Hornbeck
<chrishornbeckremovethis@att.net> wrote:
| Quote: | On Fri, 07 Oct 2005 22:04:59 -0500, flipper <flipper@fish.net> wrote:
He's not really seeking advice or suggestions, he's trying to
demonstrate, using a 'real world example', his statement in the "Where
to get 1watt 1% resistors" thread. Namely that "Using resistors in
series or parallel IMPROVES the % accuracy of the final
value."
Problem is, it's the selecting of values that achieves it and not the
series or parallel connection itself although allowing multiples
increases the odds he'll find more suitable pairs than seeking singles
only.
But the resultant value of any two random 1% resistors in series or
parallel can still be 1% off nominal and that's what one must design
to, unless you do selection.
Beautifully framed; thanks very much.
The gods designed us by incorporating (nudge-nudge) the
selection process into our universe. Should we do less?
|
You mean let the resistors select themselves? ;)
| Quote: |
Arf!
Chris Hornbeck |
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Ian Iveson
Guest
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| Posted:
Sat Oct 08, 2005 4:42 pm Post subject:
Re: Obtaining an accurate resistor |
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"flipper" <flipper@fish.net> wrote
| Quote: | He's not really seeking advice or suggestions, he's trying to
demonstrate, using a 'real world example', his statement in the
"Where
to get 1watt 1% resistors" thread. Namely that "Using resistors in
series or parallel IMPROVES the % accuracy of the final
value."
|
Sneer all you like, I'm just an amateur trying to learn. This is a
different thread, specifically about accuracy. I have tried to frame
it in practical terms, and narrow its scope, precisely to focus on
what I am wondering about.
Is that supposed to be a quote from me, BTW?
| Quote: | Problem is, it's the selecting of values that achieves it and not
the
series or parallel connection itself although allowing multiples
increases the odds he'll find more suitable pairs than seeking
singles
only.
|
Thank you. That's a very complicated sentence though. Could you
explain please?
| Quote: | But the resultant value of any two random 1% resistors in series
or
parallel can still be 1% off nominal and that's what one must
design
to, unless you do selection.
|
That is a different issue, but thanks for the thought anyway.
in message news:dpbek1lctvhhff54o4s33a6ipam41b2ksg@4ax.com...
| Quote: | On Fri, 07 Oct 2005 23:22:36 GMT, Chris Hornbeck
chrishornbeckremovethis@att.net> wrote:
On Fri, 07 Oct 2005 18:44:17 GMT, "Ian Iveson"
IanIveson.home@blueyonder.co.uk> wrote:
I need two accurate resistors.
The first, (R1) is for a tuned analogue circuit in which all the
other components are already fixed. Correct tuning of the circuit
requires an accurate resistor as close as possible to its nominal
value.
The second (R2) is for a matched pair. Neither needs to be close
to
the nominal value, but they must be matched as accurately as
possible.
The kind of resistors I want are only available at 1% tolerance.
I
am only building one example of the circuit. I don't care about
anything except accuracy in both cases.
For each resistor, R1 and R2, am I better off:
a) Buying one resistor of the right nominal value.
b) Buying X resistors each of 1/X th of the nominal value and
connecting them all in series.
c) Buying Y resistors of the correct nominal value and selecting
the
best one.
d) Buying X * Y resistors, each of 1/X th of the nominal value,
and
selecting the best series combination of X resistors.
If I decide to produce the circuit in quantity, should that make
a
difference to which I choose?
Without any cost constraints, all answers are equal. Just
buy a billion and sort in-house.
You're obviously after something more significant, so
maybe the question should be framed more tightly.
Good fortune,
Chris Hornbeck
He's not really seeking advice or suggestions, he's trying to
demonstrate, using a 'real world example', his statement in the
"Where
to get 1watt 1% resistors" thread. Namely that "Using resistors in
series or parallel IMPROVES the % accuracy of the final
value."
Problem is, it's the selecting of values that achieves it and not
the
series or parallel connection itself although allowing multiples
increases the odds he'll find more suitable pairs than seeking
singles
only.
But the resultant value of any two random 1% resistors in series
or
parallel can still be 1% off nominal and that's what one must
design
to, unless you do selection.
|
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flipper
Guest
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| Posted:
Sat Oct 08, 2005 6:39 pm Post subject:
Re: Obtaining an accurate resistor |
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On Sat, 08 Oct 2005 11:42:22 GMT, "Ian Iveson"
<IanIveson.home@blueyonder.co.uk> wrote:
| Quote: | "flipper" <flipper@fish.net> wrote
He's not really seeking advice or suggestions, he's trying to
demonstrate, using a 'real world example', his statement in the
"Where
to get 1watt 1% resistors" thread. Namely that "Using resistors in
series or parallel IMPROVES the % accuracy of the final
value."
Sneer all you like, I'm just an amateur trying to learn. This is a
different thread, specifically about accuracy. I have tried to frame
it in practical terms, and narrow its scope, precisely to focus on
what I am wondering about.
|
It wasn't a sneer. You were just getting all kinds of clever advice on
how to solve the problem so I thought I'd clarify it a bit.
| Quote: | Is that supposed to be a quote from me, BTW?
|
No, that was Phil. My mistake on the attribution.
| Quote: | Problem is, it's the selecting of values that achieves it and not
the
series or parallel connection itself although allowing multiples
increases the odds he'll find more suitable pairs than seeking
singles
only.
Thank you. That's a very complicated sentence though. Could you
explain please?
|
Combining resistors gives you more choices than looking only for good
singles.
| Quote: | But the resultant value of any two random 1% resistors in series
or
parallel can still be 1% off nominal and that's what one must
design
to, unless you do selection.
That is a different issue, but thanks for the thought anyway.
|
Not really because it explains why your option 'B' won't work 100% of
the time. No matter how many you randomly string together there always
remains a non-zero chance of the combination being off 1%.
At least mathematically. One could argue it becomes so rare you're
willing to live with odds that good if it weren't for the cost.
| Quote: | in message news:dpbek1lctvhhff54o4s33a6ipam41b2ksg@4ax.com...
On Fri, 07 Oct 2005 23:22:36 GMT, Chris Hornbeck
chrishornbeckremovethis@att.net> wrote:
On Fri, 07 Oct 2005 18:44:17 GMT, "Ian Iveson"
IanIveson.home@blueyonder.co.uk> wrote:
I need two accurate resistors.
The first, (R1) is for a tuned analogue circuit in which all the
other components are already fixed. Correct tuning of the circuit
requires an accurate resistor as close as possible to its nominal
value.
The second (R2) is for a matched pair. Neither needs to be close
to
the nominal value, but they must be matched as accurately as
possible.
The kind of resistors I want are only available at 1% tolerance.
I
am only building one example of the circuit. I don't care about
anything except accuracy in both cases.
For each resistor, R1 and R2, am I better off:
a) Buying one resistor of the right nominal value.
b) Buying X resistors each of 1/X th of the nominal value and
connecting them all in series.
c) Buying Y resistors of the correct nominal value and selecting
the
best one.
d) Buying X * Y resistors, each of 1/X th of the nominal value,
and
selecting the best series combination of X resistors.
If I decide to produce the circuit in quantity, should that make
a
difference to which I choose?
|
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Simon G Best
Guest
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| Posted:
Sun Oct 09, 2005 11:04 am Post subject:
Re: Obtaining an accurate resistor |
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Hello!
I've read through the threads, and thought I'd respond :-)
I'll state now, up-front, that I'm taking 'tolerance' to mean 'certainly
no more than [whatever the tolerance is] away from the nominal value'
(and that any resistors which fall outside their stated tolerances are
therefore duff, and to be regarded as defective).
Ian Iveson wrote:
| Quote: | I need two accurate resistors.
[...]
The kind of resistors I want are only available at 1% tolerance. I
am only building one example of the circuit. I don't care about
anything except accuracy in both cases.
For each resistor, R1 and R2, am I better off:
a) Buying one resistor of the right nominal value.
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For R1, it could be as much as 1% above or below the nominal value
(obviously). For R2, you could end up with one of the pair being as
much as 2.020202...% above the other (if one's 1% up, and the other's 1%
down).
| Quote: | b) Buying X resistors each of 1/X th of the nominal value and
connecting them all in series.
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That'll tend to 'home in' on the mean value of those resistors. The
bigger X is, the better (but you'll still occasionally be unlucky).
However, the mean value of a bunch of resistors is not the same as the
nominal value. Indeed, the mean doesn't even have to be particularly
close to the nominal for the resistors to meet the tolerance
requirements (the smaller the range of actual values, the further from
nominal the mean can be).
So, for R1, this isn't much good. But, for the pair of R2s, this will
tend to be good, as long as each R2 in a pair is taken from the same,
well-mixed bunch of resistors. But it's still possible to be unlucky,
and get R2s in a pair that differ by up to about 2% (from each other).
| Quote: | c) Buying Y resistors of the correct nominal value and selecting the
best one.
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That's much better than (b) for R1, but there's still no guarantee of a
close match. All Y resistors might be, say, 0.3% to 0.7% below nominal.
| Quote: | d) Buying X * Y resistors, each of 1/X th of the nominal value, and
selecting the best series combination of X resistors.
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That won't work for R1, as you'll just end up with Y networks tending to
be of the mean, rather than nominal, value. You might, just possibly,
be (very) lucky, but you're much more likely to find a better match with
(c).
For the R2s, assuming the X*Y resistors are in a well-mixed batch, this
is the best method, yet. It's like (b), but with lots of networks to
choose from. With Y such networks, you'd have Y*(Y - 1)/2 possible
pairs :-)
| Quote: | If I decide to produce the circuit in quantity, should that make a
difference to which I choose?
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Well, for a single R1, (c) is the best of the options, even though it's
not guaranteed to work very well in practice (but would still work
better than the other options). Options (b) and (d) are just lousy.
So, for producing the circuit in quantity, you could either settle for
1% plain and simple with option (a), or you could, perhaps, automate the
measuring and selecting process for option (c).
For pairs of R2s, it's either option (b) or (d), depending on whether or
not you want to do the measuring. With (d) you have to measure and
select, but with (b) you can just get on with it (as long as the batches
of resistors are well-mixed, of course). With (b), though, there might
be quality control issues, as it's always possible for bad matches to
occasionally occur by chance. But, for both options, have a large X.
Personally, though, I don't much like these kinds of approaches. I'd
much rather make the need for calibration a feature and a selling point,
complete with exciting panel meters and the like :-)
Simon |
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