Re: Obtaining an accurate resistor


"Ian Iveson" <IanIveson.home@xxxxxxxxxxxxxxxx> wrote in message
news:gZ62f.84949$iW5.43451@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
: "Ruud Broens" wrote
:
: > See my measurements post - if you're interested, could send you
: > the data, comma delimited or so,
:
: Yes please Rudy!

consider it done :-)

: Even though I record all the values of resistors I use, I have used
: less than 100 in my life, and I have no more than 9 of any value in
: stock.

shame on you ;-)
:
: Even so, my experience selecting from sets of 10 is that they seem
: to be in tight clusters centred off the nominal value. That would be
: consistent with a production process using blanks with a constant or
: slowly changing variation from the one used to set up the trimming
: machine. Or just a hysterical trimming machine.

heh. emotions in power tools - no thanks, dangerous enuf already :-)
:
: > 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,
:
: My contention is that this is true regardless of the original
: distribution, simply because averaging always reduces variance.
:
yep, the watering down effect

: > so increases the
: > chance that the compound value is closer to the nominal series
: > value.
:
: Doesn't quite necessarily follow. Certainly increases the chance
: that it is closer to the mean of the original distribution, but our
: measurements suggest that is not the same as the nominal value. Good
: for matching, but there is still an outside chance in my mind that
: it may not improve the probability of getting closer to nominal. It
: is possible to contrive a distribution from which more series pairs
: would be further from nominal than the singles, and contrive an
: argument to suggest this would worsen the chance of getting closer
: to the nominal value.

well, i'd expect a feedback process to target the nominal value
probably set up more accurately than my DVM - it has a basic
0.15 % accuracy, that i assume is a systematic error, repeatability
is much better, so could be all measurements were 0.15 % too low
:
: Consider a distribution that is symmetrical and bell-shaped, but
: shifted to one side of the nominal value, with tails still fitting
: entirely within the tolerance limits. Averaging by series connection
: will reduce variance, and so emphasise the bell shape. The mean
: variation from nominal will not change, so on average the series
: connection will not result in being closer to nominal. Fewer will be
: very close to nominal, but fewer will be very far. In other words,
: they will be more consistently inaccurate.


yes, i had that inprecisely formulated, thanks

: Now consider the same distribution, skewed in the opposite direction
: to the shift. Averaging will in this case move the *mode*
: (distribution peak) away from the nominal value. You could then say
: that the most common series value is less accurate than the most
: common single value. Variance would still be reduced, and the mean
: variation from nominal would still be the same I think (not sure
: about this though).

ok, needin' some coffee ... report back l8er maybe ;-)
:
: > 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,
:
: Right. I must admit that I don't understand resistor voltage
: ratings. The only way to get 1W through 1Mohm for example, is to put
: 1kV across it. So what does a 1W, 1Mohm, 500V spec mean? A resistor
: that never gets very hot, I suppose.
:
: > so easy to just find out by gettin' some .
:
: Next opportunity, I'll pick up a whole roll of surplus resistors,
: just for the hell of it.
:
: > nb increasing the change of tighter tolerance is not the same
: > as equals tighter tolerance, so multiple series connecting
: > seems overkill to no avail.
:
: To most of the world, overkill is what we seem to be here for!
:
: thanks
:
: cheers, Ian
:
:


.

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