Unless your confusion has infected me as well, the Bourns chart looks like it uses different terminology than I've seen from other mfrs; and Omega's C is what I would call reverse log. I'm used to thinking of A as log, B as linear, C as reverse log, and D as a steeper variant on A.
Panasonic makes some reverse-log pots, but in the Digikey catalog I don't see dual ones; just combined A/C, to be used as a stereo balance control. Noble sell them, but I imagine you'd have to order 10,000.
Speaking of audio taper pots...
....does anyone have an equation for what the relative resistance of an audio taper pot is supposed to be vs. position? Empirically I came up with R(pos)=Rmax*(1.4^pos-1)/(1.4^10-1), where "pos" ranges from 0-10, but I imagine somewhere this must be defined.
Hi Martin,
I meant resistance; I'm using one to tune some op-amp parameters rather than as a volume control.
Yeah, I was hoping for the theoretical curve. For my purposes, if a 1k pot is still 1 ohm at the far end, that's close enough to zero for mean even if it only is 60dB attenuation.
---Joel
duh, one of those days. just looking for some rev log pots for a S+K HP filter, I can never remember which curve is which. But does law C look wrong
martin
After the first death, there is no other. (Dylan Thomas)
Do you mean relative resistance or attenuation, as in Volume Control?
Its difficult to define because of the bad cut off at the bottom of a typical pot. Do you consider the 40dB,60dB or80dB atteuation point as the other measuring point, compared with 0dB att.
Thats what I like about having a linear pot driving a log VCA, you can so easily define the end points! I cant honestly see any point in having any attenuation, below -60, it makes the scale too cramped. A 40dB range would be better
but i'm sure J.Woodgate will know
martin
After the first death, there is no other. (Dylan Thomas)
I read in sci.electronics.design that martin griffith wrote (in ) about 'Pot tapers, always confused', on Wed, 20 Apr 2005:
That's very flattering, but WHAT will I know? The relevant IEC standard is IEC 60393-5, but I don't know whether it defines non-linear resistance 'laws'.
Note that the 'log' laws are actually exponential!
-- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
have a look at one of my defunct websites...
martin
After the first death, there is no other. (Dylan Thomas)
In practice, the usual inexpensive pots are made by screening 3 layers of resistive ink, so merely approximating to a log / whatever curve. See the manufacturers' curves, the change of slope is very obvious.
There are better ways to get good log accuracy but they cost more.
Graham
Depends on the exact law. JIS audio tapers A ( volume control ) and C ( 'reverse log' ) have 15% of total track ressitance at the central position. In practice anything between about 10% and 20% actually.
That 1 ohm is normally called the 'hop on' resistance.
Other laws are available - usually desired for the 'reverse log' taper. e.g 10C - means 10% total R @ centre. Manufacturers haven't defined these curves well and they may vary from one manufacturer to another. The 'RD' curve from Alps is perhaps best knows - I think it's similar to the 10C. I use a 'Y' taper from Taiwan Alpha - or 05C pot in one of my mic pre-amps. If you're sweeping frequency with a pot used as a series element - the ideal Centre value depends on the range of control. E.g. I use a 20C type part for a swept mid frequency EQ circuit.
You'll only get these parts when making at least modest quantites though.
Graham
Dude,
That kicks ass. Thanks.
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