E Gadgets 6: Numbering Lugs of Potentiometers in My Schematics

Last Updated on December 5, 2020

First Published on December 3, 2020

In my schematics, I use potentiomers. I think numbering lugs = legs of a potentiometer should be depending on an assembling basis. I watched several schematics online that the numbering is different from my thought. In guitar works, however, the 3rd pin on the flip side is for grounding. When you solder a potentiometer, you need to consider of the function, i.e., how the potentiometer works on turning clockwise or anti-clockwise regardless of numbering in schematics and boards.

* Many knobs to control the volume of electric guitars rotate between 7 o'clock and 5 o'clock. The knobs indicate 7 o'clock as the value of 0 in 10, 12 o'clock as 5, and 5 o'clock as 10, i.e., clockwise motion to increase the volume.

In fact, many datasheets of potentiomers number lugs with the reverse way from my thought. However, potentiometers are assigned in my schematics generically (without any specified product), and wiring lugs is needed to be cautious because the function is easily reversed on turning with incorrect wiring.

I apply some potentiometers as rheostats which are just variable resistors. For example:

Download PDF File: Schematic of Preamp

Download PDF File: Schematic of Preamp 2

Both RV2 of Preamp and RV3 of Preamp 2 have the same function as rheostats. However, wiring of the RV2 is different from the RV3. In the RV2, I connect between the first lug and the second lug. The second lug is connected to a movable brush in a potentiometer. Several datasheets recommend that you connect lugs like the RV2 of Preamp to hide chattering by the contact gap. Besides, datasheets recommend that you connect the second lug to the positive voltage on direct current; otherwise, the carbon resistor connected to the first and third lugs in a potentiometer makes anodic reaction on the positive voltage. In my schematics, basically, the first or the third lug is open because of the practice on assembling. I think when chattering occurs on an potentiometer, it's time to replace the potentiometer with new one. Besides, I don't care about the second lug to the positive voltage on direct current, because carbon doesn't react/ionize as well as gold. The small possibility of ionization means the small possibility of oxidation.

The maximum rated power of potentiometers seems to be examined the inner resistor from side to side. You may think that using movable brush in a rheostat makes the rated power reduce because the resistor isn't used from side to side. However, I think it doesn't reduce the rated power. In datasheets of big rheostats, I don't find that the rated power become variable on changing the resistance. Carbon is an enough thermal conductor to diffuse heat to the whole material.

I opened the case of Supertech Electronic's 1K ohms B curve potentiometer. The resistor (on the lower right corner in the lower photo) is shown on the outer circle. The resistor seems to be graphite, a form of carbon, and its base seems to be silver or aluminum. The board seems to be made of a phenolic impregnated paper. The inner circle seems to have graphite. I tested resistance on both circles, and found that the outer circle has 1K ohms from side to side, and the inner circle seems to be zero ohms. The difference of resistance seems to come from thickness of graphite and impurities. The movable brush (on the lower left corner in both photos) is a metal spring. I concluded that this product doesn't have polarity I mentioned above because both circles have graphite on these contact points. I also tested an electrical circuit with the outer circle by a 9V battery to check its heat conductance. In checking the circuit with the resistance between 7 o'clock and 9 o'clock for 2 minutes, assuming 200 ohms in 1K ohms, significant increment of heat doesn't occur. However, electric sparks occur in the checking. I think that electric sparks come from impurities in graphite. For more safety in using a carbon film potentiometer, placing a current limiter (resistor, etc.) is an option not to give high voltage to the resistance of a potentiometer.

In conclusion, applying potentiometers is practical and has to be tested actually even if your schematic seems to be safe. I recommend that you test your potentiometers in every case before using these in your DIY gadgets.

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