[sdiy] Hand selecting capacitors

Miles Stevens milesstevens89 at gmail.com
Fri Apr 9 15:10:29 CEST 2021


I've sat down and matched 1000+ caps of the same value (in
factory packaging, so I'm assuming the same batch) more times than I'd like
to remember. Alway, I always got picture perfect gaussian distribution but
its has always been skewed off centre from the nominal value.

My assumption was that inside a single batch the tolerance is typically
quite tight, and the overall 5% / 10% / etc tolerance is to
accommodate batch to batch variations.

On Fri, 9 Apr 2021 at 19:04, Florian Teply <usenet at teply.info> wrote:

> Am Thu, 8 Apr 2021 23:45:42 -0700
> schrieb David G Dixon via Synth-diy <synth-diy at synth-diy.org>:
>
> > Hey Team,
> >
> Hey Dave,
>
> > So, I've now got a ton of orders for my little two-Dome-filter
> > frequency shifter, which I've figured out how to build in euro format
> > (12HP, if you're curious).  I hand select all the film capacitors for
> > the Dome filters so that they all have the same (or nearly so)
> > mantissas -- I try to get them either all within 0.2%, or 0.4% at
> > worst.  My phase displacement simulation tells me that this gives
> > very nearly perfect results.
> > Anyway, I ran low on capacitors to the point where I couldn't put
> > together decent collections for the 18-stage filter (I need 4 102s, 4
> > 103s, 4 104s, and 6 105s for it).  I bought 250 each of the smaller
> > values and 200 of the 105s.  Tonight I measured all of the 105s,
> > thinking that they would ideally form a Gaussian distribution around
> > 1.000uF.  Well, they were nearly all clustered between about 1.030uF
> > and 1.050uF, and in fact, about 20% of them were off spec (higher
> > than 1.050uF, and these are 5% caps).  Not a single one of them was
> > less than 1.020uF.  They are Kemet caps.  So now I've got to hope
> > that the other values also tend on the high side of their tolerance
> > ranges, or else I'm going to have to buy even more caps.
> > I think I've given myself a fool's errand here.
> >
> You're not the first one to observe the rather skewed distribution of
> tolerance in passives: Even though manufacturing yields aproximately a
> gaussian distribution of values, what ends up in distribution pretty
> commonly has the central part of the distribution missing. The
> reasoning behind is pretty simple: those parts, that qualify for a
> tighter tolerance are sold as such, as they can charge more for tighter
> tolernce groups. And you also get economies of scale by not running
> separate lines or manufacturing batches for different tolerance groups.
> On the other hand, if you're buying larger quantities (quantities way
> beyond what one would need for DIY, that is), you sometimes end up with
> a batch of tighter tolerance parts thrown in depending on what they had
> in stock.
>
> So, when one needs very tight matching between parts, there are (at
> least) two solutions: a) buy excess quantity and select parts (excess
> here meaning usually upwards of three times the number you want to end
> up with), or b) divide critical parts into parallel and/or series
> combinations of at least two devices. This could be either an
> approximate 50/50 split or 90-95 % plus one for fine-tuning. For a
> 50/50 split you'd need either some luck with your purchase (as you
> mention nearly all your parts are on the upper end of the tolerance
> range) or again sufficient excess, while for the fine-tune approach
> (95/5 split) you'd need a decent stock of smaller value parts to meet
> the final value. Matching to specific target values becomes easier when
> you have more devices to work with (either in form of stock or in form
> of several devices connected together to form the target value), but of
> course you quickly reach a point of diminishing returns as sorting and
> selection is both money- and time-intensive such that in most cases
> it's moe economical to buy tighter tolerance parts in the first place.
>
> Doesn't help much in the situation you're facing right now, but worth
> considering in the future nonetheless...
>
> Best,
> Florian
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