POTENTIOMETERS     LINEAR   AND   LOG'




POTENTIOMETER TAPERS

The taper (also called "law") of a pot is important.
We need not worry with trimpots, since they are almost always linear, and I do not know of a supplier of anything other than linear trimpots.
For all panel pots, we must be aware of the use the pot will have, and select the correct type accordingly.
The most common use of a pot in audio is as a volume control.
Since our hearing has a logarithmic response to sound pressure, it is important that the volume control should provide a smooth variation from soft to loud, such that a given change in position of the pot causes the same sensation of volume change at all levels.


POTENTIOMETER TAPERS

First, the term "taper" needs some explanation.
Potentiometers have different resistance tapers.
A linear taper potentiometer has the same resistance change for given angle rotation of the shaft regardless of whether the wiper is at the lower, middle or top end of its travel.
When starting from zero signal and increasing the control slightly, the audio signal will rapidly increase at first.
Above the mid point there seems to be relativley little increase in volume.
To overcome this undesirable feature, potentiometers are available with logarithmic tapers.
In the early days, when an audio taper (logarithmic, or just log) was needed, the resistance element was indeed tapered,
so that it provided a different resistivity at different settings.
By changing the physical taper, it was possible to make a pot provide the exact gradient of resistance needed.
By definition, a linear pot has no taper as such (the resistance element is parallel sided), but the term has stuck, so we might as well get used to it.

The violet curve in the diagram shows an antilog or reverse audio taper pot.
These are uncommon, but used for balance controls using a log/antilog dual section (commonly called dual gang) pot.
It is shown on the graph mainly for its interest value, but they are generally an historical component now.

All this tapering proved a rather expensive exercise, so manufacturers economised ("they" won't notice the difference!), and worked out a method of using two resistance elements of differing resistivity, and joining them to create what I referred to as the "Commercial log" taper.
In short, it doesn't work (not properly, anyway),
and the discontinuity where the two sections join is almost always audible with cheap "log" pots.
I suggest that you get an old pot and dismantle it
so that you can see exactly what is inside.
I could show you some photos, but there is nothing like doing it yourself to really get to know the subject.

Pot Markings
Now, this should be dead easy - a simple code to indicate the resistance and law of a pot should cause no grief to anyone,
right? Wrong! It wouldn't have been so bad if someone hadn't decided to change it,
and even then, it wouldn't have been so bad if there was no overlap between the "old" and "new" "standards" ... I think you can see where this is headed by now.

Taper Old Code New Code Alternate
Linear A B LIN
Log (Audio) C A LOG
Antilog F N/A N/A


Wasn't that a nice thing to do?
It is obviously important to check before you make assumptions, or you can easily get the wrong type - especially if working on older equipment.
At least the resistance marking is usually sensible, so a 100k pot will be marked as 100K - but not always.
The coding system used for capacitors is sometimes used as well (especially on small trimpots), so a 100k pot could also be marked as 104 (10 followed by 4 zeros) or 100,000 (100k) ohms.
Because they are variable, there is a much smaller range of potentiometer values, almost always in a 1, 2.5, 5 sequence.
Common values for panel pots are 1k, 5k, 10k, 25k, 50k, 100k, 500k and 1M
- 2.5k and 250k went missing along the way, and these are not stocked by very many distributors.
25k pots are becoming harder to get as well.
Not all values are available in log and linear, and in some cases you may even find that for a particular type, you can get them in any value you want, as long as it's 100k (for example).
Trimpots suffer a similar fate.
The only way to know what you can get from your local supplier is to check their catalogue.
In reality, everything is available, but you may have to go a very long way to get it.


Power and Voltage Ratings
For most audio applications, these are of little on no consequence.
In many other applications however, exceeding the specified ratings could lead to the destruction of the pot or yourself !
Neither can be considered a good thing.

Power - A pot with a power rating of (say) 0.5W will have a maximum voltage that can exist across the pot before the rating is exceeded.
All power ratings are with the entire resistance element in circuit, so maximum dissipation reduces as the resistance is reduced (assuming series or "two terminal" rheostat wiring).
Let's look at the 0.5W pot, and 10k is a good value to start with for explanation.

If the maximum dissipation is 0.5W and the resistance is 10k, then the maximum current that may flow through the entire resistance element is determined by ...

In fact, 7mA is the maximum current that can flow in any part of the resistance element, so if the 10k pot were set to a resistance of 1k, current is still 7mA, and maximum power is now only 50mW, and not the 500mW we had before.

Voltage - Two separate issues here.
One is directly related (in part, at least) to the power rating, and is important to ensure that the life of the pot is not reduced.
Knowing about the other might save your life.

Voltage across resistance element - The maximum voltage across the example pot from above is 7mA x 10k, = 70V.
This will rarely (if ever) be achieved in an audio system, but is easy with many other designs.
As the resistance increases, so does the voltage - a 0.5W 1M pot will pass only 700uA at maximum power rating, but the voltage needed to create this current is 700V.
Unless the pot is actually rated to withstand 700V across the resistance element (rather unlikely), it will fail - maybe not today, or tomorrow, but it will fail eventually.
Special pots are made (custom jobs, of course) for high voltages, and standard pots should never be used beyond their rating - assuming that you can find out what the rating is, of course.

Dielectric Voltage - The dielectric (insulation of pot "guts" to the body) rating is especially important if the pot is connected to mains operated, non-isolated equipment.
Wall mounted lamp dimmers and such are typical examples.
This is not commonly specified, but for safety, should be at least 2.5Kv
A common way to achieve this is to use a plastic shaft, with the body of the pot insulated from the chassis, and inaccessible by the user (even if the knob falls off or is removed!)
This point cannot be stressed highly enough.

Most standard pots will safely withstand (maybe) 100V or so between the resistance element and terminals, and the body and shaft.
Miniature types will usually be less than this.
Never, ever, use a standard pot with a metal shaft to control direct mains operated equipment.


Potentiometer Types
"But we already covered that, didn't we?"
Not really - I merely glossed over the basics.
Now, we shall look at a few examples of pots you may come across.
Firstly, there is the resistive material and some typical characteristics ...

Material Manufacturing Method Common uses Power (Typ)
Carbon Deposited as a carbon composition ink on an insulating (usually a phenolic resin) body Most common material, especially for cheap to average quality pots.
Has a reasonable life, and noise level is quite acceptable in most cases.
(DC should not be allowed to flow through any pot used for audio control)
0.1 to 0.5W
Cermet Ceramic/metal composite, using a metallic resistance element on a ceramic substrate High quality trimpots, and some conventional panel mount types (not very common).
Low noise, and high stability.
Relatively limited life (200 operations typical for trimpots)
0.25 to 2W
(or more)
Conductive Plastic Special impregnated plastic material with well controlled resistance characteristics High quality (audiophile and professional) pots, both rotary and linear (slide).
Excellent life, low noise and very good mechanical feel
0.25 to 0.5W
Wire wound Insulating former, with resistance wire wound around it, and bound with adhesive to prevent movement High power and almost indefinite life.
Resistance is "granular", with discrete small steps rather than a completely smooth transition from one resistance winding to the next.
Low noise, usually a rough mechanical feel.
5 to 50W
(or more)

additionally: