Re: Obtaining an accurate resistor
- From: "Ian Iveson" <IanIveson.home@xxxxxxxxxxxxxxxx>
- Date: Tue, 11 Oct 2005 03:21:20 GMT
"Simon G Best" <s.g.best@xxxxxxxxxxxxxxx> wrote in message
news:diatfi$d9q$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 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).
Fair enough. Still some cloud around Phil's contention that service
life is included in the quoted tolerance, which of course would mean
there would be tails on the final distribution regardless of
rejection at the factory. Does seem unlikely though so I will assume
as you have done.
>
> Ian Iveson wrote:
>> 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.
>
> 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).
OK. same as for singles.
>
>> b) Buying X resistors each of 1/X th of the nominal value and
>> connecting them all in series.
>
> 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).
Yes. From the few we have measured. For example, Rudy's set of 159
56k 1% can be seen here
http://www.ivesonaudio.pwp.blueyonder.co.uk/rudysdata.gif
It just about conforms to a worst case scenario: it is offset in one
direction, and skewed in the other. Hence the mode will move away
from the nominal value. The mean will of course stay the same, and
the distribution variance will decrease, resulting in a set of more
accurately inaccurate strings.
> 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).
Yes. So accurate matching may be more likely, but if for a one-off
circuit that needs to be right, there is no substitute for selection
and trimming.
>
>> c) Buying Y resistors of the correct nominal value and selecting
>> the best one.
>
> 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.
OK
>
>> d) Buying X * Y resistors, each of 1/X th of the nominal value,
>> and selecting the best series combination of X resistors.
>
> 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).
Yes
> 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 :-)
Woohoo!
>> If I decide to produce the circuit in quantity, should that make
>> a difference to which I choose?
>
> 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).
Yes. I'm getting worried about this agreeing thing.
> 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.
Yes. sigh...
But what about if I get many smallish batches from different makers
and mix them. Then they should be gaussian and with a mean of the
nominal value, considering the nominal value is the only target
value.
I have a bridge with a parallel port. Always wondered how to use it.
Might be interesting to find out. But my mind is already wandering
to valve matching, sepp, pse, and randomly selected massively
parallel mixed pentode and triode. RSMPMPT. Catchy eh?
> 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
> :-)
And a bluetooth-enabled microprocessor-controlled "total precision
management" calibration system with optional manual override and a
"reset defaults" option.
Thanks Simon
cheers, Ian
.
- References:
- Obtaining an accurate resistor
- From: Ian Iveson
- Re: Obtaining an accurate resistor
- From: Simon G Best
- Obtaining an accurate resistor
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