Author Topic: Resistor sub tolerance  (Read 998 times)

EBK

Re: Resistor sub tolerance
« Reply #20 on: January 11, 2018, 06:02:23 PM »
Damn! All these responses and really no answer to my question. Maybe I phrased it wrong so let me put it this way.

If I need an 82K resistor and I only have a 75K. Can I use it? It is within 10%
To answer a different way, do you happen to have a 150K and a 180K resistor?  If so, put them in parallel, and you'll have 82K.

100K in parallel with 470K would work too.
« Last Edit: January 11, 2018, 06:05:04 PM by EBK »
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ElectricDruid

Re: Resistor sub tolerance
« Reply #21 on: January 11, 2018, 06:16:02 PM »
The circuit dictates the resistor and its tolerance, not the other way round.

+1 absolutely to that. Sometimes it matters very little, other times, it'll screw stuff up. As ever "it depends..."

That said, one of my definitions of a "well designed" circuit is one where tight tolerances *aren't* required. A dead give away of a poorly designed circuit in my view is one that has ten trimmers so you can adjust everything "just right". Ideally, you *should* be able to bang +/-10% parts in there and still get it to work better than acceptably. Sometimes that isn't realistic, but it's not a bad goal to aim at.

T.

R O Tiree

Re: Resistor sub tolerance
« Reply #22 on: January 12, 2018, 01:43:25 PM »
As has been said, there are relatively few occasions where a small(ish) change in value is going to give you a problem.  One that immediately springs to mind is something like a multi-band Eq pedal.  There's a long thread on here from about 5-6 years ago about how to "tune" the bands.  Eventually, someone found an online calculator for an op-amp based filter and it was able to work out precise values.  So, the OP had to measure each of his caps (10% tolerance) as accurately as possible, then plug their values in turn into the calculator and it would spit out the precise resistor values to give the required centre-frequency and Q for each band.  That's when 1% resistors in the E196 series really mattered.  Even for most filter applications, you won't hear the difference, though.
...you fritter and waste the hours in an off-hand way...