When they're all bent up, you can lift the shaft and phenolic wafer assembly out of the back housing like this.
All the fun stuff appears to be inaccessible under that black plastic cover. Actually, that cover performs a couple of very useful functions; it serves to hold the wiper in place, and also interacts with the stop on the case to set the mechanical rotation limits. If you remove the tab on the plastic disk and reassemble the pot, it will now rotate freely degrees. Looking closely, you can see that the shaft is held onto the plastic disk by being mechanically squashed into place.
That is quick, and makes for cheap pots, but it can't be removed and successfully reassembled. That is a peculiarity of this brand, as some pots can be disassembled and reassembled successfully. In the next picture, I've used a big pair of diagonal cutters to remove the plastic disk and the metal wipers. This exposes the bare conductor assembly. The whole works of the pot, excepting the rotating wiper are printed on the phenolic wafer.
The connections to the ends of the resistive material are a conductive metal material, as is the circle of conductor that connects to the center contact. We can now begin to see our control's kinship to Ohm's variable resistors. The resistive material is a circular band of carbon-containing gunk that is printed onto the phenolic wafer, overlapping the metalization to the two outside lugs at either end. The wiper is just a sliding bridge from the center metalization ring to the outer resistive ring.
The contact to the metalization is fairly reliable, but the contact to the resistive ring may be intermittent, so the connection to the outer ring is made by three contacts in parallel, as shown by the three tracks the contacts make in the top of the resistive material. Those gouges and scratches are a product of the delicate manner in which I removed the plastic wafer, so ignore those. With this view, a couple of things fairly leap out at us. This is a linear pot, which is exactly what we should have from a continuous, homogeneous strip of resistive material.
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That is to say, the wiper traverses an equal amount of resistance in ohms per degree turned. If we wanted to make a non-linear pot, where the amount of resistance per degree turned were not linear, we could mess with the resistive strip to make it nonlinear.
This is in fact how it's done. The strip can be made to get skinnier toward on end or the other; a narrower strip has more resistance per unit length and so the resistance per unit degree turned will change. This is not desirable from reliability reasons, as we'd still like to have three contacts to the resistive strip, and that isn't possible if the strip gets skinnier. We could also change the thickness or composition of the material from one end of the resistive strip to the other.
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It turns out that printing a varying thickness is very difficult to do cheaply. Also, varying the composition in any kind of smooth fashion is very hard, not a repeatable manufacturing step. What makers of pots actually do is to use straight-line approximations to a nonlinear resistive curve, and print in sections.
This allows the maker to come reasonably close to some taper curve. Other things are that we can see the way pots wear out. The wiper literally wears a path through the conductive strip. When all three contacts get all the way through, they don't make contact any more, and the pot quits functioning.
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During the normal life of the pot that is, the wear-out process the bits of resistive material gouged and work from the resistive strip stay around and can actually lift the wipers from the resistive strip. If there is a DC voltage across the pot when this happens, the wiper loses and then regains contact at a different DC level than it left, so it makes a scratch or click. Pots used to be made with taps part way around.
This was just a fourth terminal connected to a strip of metalization running under the middle of the resistive material part way round. The maker can print metalization "dead zones" in the middle of the rotation where nothing much happens resistively. It's very flexible. You could hypothetically scrape material off the resistor band and make a higher resistance, odd taper pot, but there are better ways to do that.
Old timers will recall a time when Radio Shack was really a place to get electronics parts. Radio and TV repair places had electronics parts, and distributors had a large variety of odd miscellany. You could usually find parts, even if they were not hot sellers, because the parts suppliers ordered a range of parts that they thought would appeal to their customers, and kept stock items because they were useful, even if they didn't sell for a long time.
Enter the MBA. If some item I'm selling is not returning as good a rate of profit as some other things, I can make more money be not selling the less profitable items and concentrating on only the things that are hot sellers with high rates of return. This does work. Businesses that adopted it were more profitable than their slower peers were, and in many cases the slower peers went out of business. These are the seeds of destruction for the electronics tinkerer, though. A lot of times tinkerers want devices that are not used in millions.
This means that increasingly, it's almost impossible to find odd values, odd tapers, or unusual terminal arrangements in pots. Distributors will not stock pots that aren't high sellers; this leads to some self-fulfilling prophecy. Eventually, the manufacturers note that odd value or taper pots are not selling - pesky distributors don't order them anymore - and they quit making the odd ones.
This leaves us where we are today. They'll happily make whatever you want, if you want enough of them.pierreducalvet.ca/196388.php
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The few of us that hand solder stuff together have to make do with whatever the distributors do order. And today, that is increasing only linear pot, maybe a few audio tapers. What is taper? It's just the ratio of the resistance already passed as the pot turns to the total resistance of the pot, described as a curve.
For instance: we want to make a variable power supply with an adjustment pot that smoothly varies the voltage from one to ten volts, so we want a control that lets us do that. We have no idea whether we'll want mostly low voltages or high voltages, so we want to adjust it equally well anywhere in the range. In this case, it's most natural for the control pot to have an equal change in resistance or voltage divided per unit of rotation - we want the control to feel linear.
This much of a turn is one volt, no matter whether it's near 0V or near 10V. Volume controls are different. The human ear does not respond linearly to loudness. It responds to the logarithm of loudness. That means that for a sound to seem twice as loud, it has to be almost ten times the actual change in air pressure. For us to have a control pot that seems to make a linear change in loudness per unit of rotation, the control must compensate for the human ear's oddity and supply ever-increasing amounts of signal per unit rotation.
This compensating resistance taper is accurately called a "left hand logarithmic taper" but for historical reasons has been called an audio or log pot. In these pots, the wiper traverses resistance very slowly at first, then faster as the rotation increases.
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The actual curve looks exponential if you plot resistance or voltage division ratios per unit of rotation. Likewise, if we used a linear pot for volume control, the volume would come up dramatically in the first half of pot rotation, and then do very little change in the last half. The dark horse taper is reverse audio, or more strictly "right hand logarithmic" taper. This taper traverses resistance very quickly at first, then more slowly as it is turned further.
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