I am coaching 4th and 5th graders in an Elementary School Science Olympiad event about circuits. I am not an electrical engineer or hobbiest, but volunteered that I could probably handle this event if no one else stepped up to coach (I am a materials engineer).The kids are being tested on DC circuits - lamps, resistors, LEDs, diodes, capacitors, etc.

One of the other elements is a potentiometer. We have covered what it does, how it can be used as a variable resistor as well. One of the kids asked what the 4th terminal was for (I had wondered, too). I had actually opened one up to show the kids how it worked. We could see, and verified with our multimeter that it didn't seem to be connected to the resistor. I told the kids I would try to find out its function.

So - my question is: Why is this mysterious 4th terminal there?

Thanks in advance,

Nancy

 

If it is an audio POT, it can be a separate tap. Used for stereo volume control.

Do you have a picture or a brand name and model number?

 

Might that terminal have been a locating lug to go in a hole to orient the pot when mounting in a panel? A photo would be interesting.

 

True, it may be for stability when soldered to a pcb. That way it wont wiggle when its turned. Some power jacks use an additional pin for mounting rather than power.

 

Wow - such quick responses.

I have uploaded a couple of pictures - poor quality, sorry (only have my cell phone camera right now). My guess was that the 4th terminal had to do with mounting.

Thanks again.

 

Still a mystery. That lug is a solder terminal, not a locating pin. When you rotate the shaft all the way CCW, is there a click?

Can you get a name and part number off the case? All I can see is that it's an inexpensive carbon pot with a strange lug.

 

It's not a typical pot -- they have 3 terminals or 5 if it includes a switch. My guess is it was an OEM part for some assembly and the user wanted the extra solder terminal to help in the wiring of the assembly.

 

No clicking. When assembled nothing seems to touch the lug.

Definitely an inexpensive part - we get kits with a fair number of different components for $10.

Numbers on the case

100L-13-1204-00
R1379511 The last "1" is a different size

No name. It is a 100 Ohm pot.

 

Nothing to work with. It seems to be a cheapo 100 ohm pot with an unusual extra lug. As to the purpose, giving a couple of adults a small unsolved mystery seems to be about it.

 

Nothing to work with. It seems to be a cheapo 100 ohm pot with an unusual extra lug. As to the purpose, giving a couple of adults a small unsolved mystery seems to be about it.

Measure the resistance from the orphan terminal to each of the other three terninals. See if it's directly connected to any of them.


Eric

 

Thanks everyone who replied.

I don't feel quite so stupid now. I will show the kids that it is not electrically connected to the other terminals and tell them it is not typical for a pot to have this extra terminal.

 

Since the kits are $10.00 maybe surplus parts are from anything.
Someone had good Idea how use surplus to make a buck.

 

Someone had good Idea how use surplus to make a buck.

That is probably the best explanation. The pot got made to perform some function, but that terminal did not get connected. Instead of a trash heap, it got put in a parts grab bag.

 

I think it is another terminal where you can tap....That's what I think

 

One of the kids asked what the 4th terminal was for (I had wondered, too). I had actually opened one up to show the kids how it worked. We could see, and verified with our multimeter that it didn't seem to be connected to the resistor.

That may have been the intent, but it does not seem to have worked.

 

If the extra terminal happens to be connect electrically to the outer case of the potentiometer's metal housing, it could serve a couple of purposes. One would be a convenient solder point to the chassis (assuming that the chassis is metallic). Another purpose would be to provide a connection point to the case to allow shielding the pot against EMI susceptibility or emission. The latter purpose would be particularly useful if the pot is mounted in a plastic (non-conductive) chassis.

hgmjr

 

Perhaps they use the same PCB for both tapped audio pots (in which case it would connect to a tap on the resistive element) and for untapped pots (like you have). That way, the manufacturer only has to stock one type of PCB.
/mike