EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: engrguy42 on May 29, 2020, 05:33:33 pm
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So I wanted a quick adjustable voltage divider for a 10 volt source, so I reached into my boxes of components and grabbed one of those cheap potentiometers (see image). It was like 5k, and I assumed yeah, not a problem since that's only like 2mA thru the full resistor.
Well, not so much. Now the thing is all flaky and inconsistent and probably got zapped somehow. Okay, I should have used a decent wirewound in the first place, but I'm a wildman. A renegade. I throw caution to the wind.
Anyway, I'm curious if anyone knows if these things have any sort of rating that would clarify exactly what their capabilities are. I'm sure part of the problem is mechanical too. Depending on how hard you press when you turn affects the value or whether it's open circuit. BTW, I tried this on another one and got the same results.
Thx.
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That cheap part is wide open and has no protection from dust or mucking with the contact spring. You might have to clean the element with some solvent. Are you drawing current from the wiper?
I don't understand what problem you are having.
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That cheap part is wide open and has no protection from dust or mucking with the contact spring. You might have to clean the element with some solvent. Are you drawing current from the wiper?
I don't understand what problem you are having.
Bob, baby...I took the cover off so you could see what I'm talking about. :D It has a cover.
Anyway, I just hooked it up to a 10V source across the entire pot, then connected the tap to an IC circuit (no amps drawn) to apply variable volts. The output voltage from the pot was flaky and not smoothly variable.
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Problem is not with the wiper contact, but with your soldering. ;)
You got excessive flux under the overheated riveted lugs, creating intermittent contacts. This is a well known failure mode with this style of pots. No matter which brand.
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Problem is not with the wiper contact, but with your soldering. ;)
You got excessive flux under the overheated riveted lugs, creating intermittent contacts. This is a well known failure mode with this style of pots. No matter which brand.
Yeah, that makes sense. Us old guys with our crappy eyesight.
Thanks.
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Your problem is likely to be the contact between the center lug and the wiper, not the wiper and the resistor. Although I have no idea if what Yansi has said would apply--the soldering doesn't appear to have caused damage that I would worry about.
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That pot is indeed incredibly poor quality, but not necessarily from an electronic point of view.
From your photo, the shaft is misaligned wrt to the threaded bushing. This implies there is a lot of mechanical clearance/slop there, letting the shaft position itself radially at will when adjusting.
You'll never get a reliable output from such a thing.
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Some potentiometers are much better than others even before you get to the wirewound precision types.
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That pot is indeed incredibly poor quality, but not necessarily from an electronic point of view.
From your photo, the shaft is misaligned wrt to the threaded bushing. This implies there is a lot of mechanical clearance/slop there, letting the shaft position itself radially at will when adjusting.
You'll never get a reliable output from such a thing.
Yeah, but the shaft and wiper assy are separate from the threaded bushing. I think that since the contacts are kinda spring-loaded against the resistive "film" or whatever I'm not sure a misalign with the bushing really matters.
Anyway, we can hypotheticalize for the next month about what might be the problem, my question was around the capabilities of the resistive element. Does anyone have any insight into that?
Thanks.
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At 10V, the track is only dissipating 20mW!
Irrc. Most carbon pots are rated for around 250mW (track, not wiper).
I don't think you're in danger of having to go to wirewound, you renegade wildman. ;)
P.S. Just a thought - you did check that it was a linear pot and not an audio or log taper?
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Those carbon film pots are usually 1/8 watt. Please note, I said usually.
Also, the power rating is for the whole element. If you’re using half of it, its rating will only be half of the total.
Edit: Indeed, 1/4 watt.
1/8 watt is for the smaller trimpots.
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Okay, apologies. My bad. Cockpit error. Potentiometer works fine. Smooth as silk.
Nevermind. Move along. Nothing happening here. Sorry to waste your time.
BTW, I hate breadboards. |O
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Anyway, we can hypotheticalize for the next month about what might be the problem, my question was around the capabilities of the resistive element. Does anyone have any insight into that?
Thanks.
I would say RTFM, or in this case, RFTDS, but I can't think of what you would find on the data sheet that would answer the question. What sort of capabilities? Power?
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Okay, here's the deal...
Y'know how everyone's talking about AI and how awesome it is cuz you get those freakin' useless self driving cars and an Alexa that knows what channel to turn to? Y'know, totally useless crap.
Why don't they use it to do something, like, USEFUL?? Huh? Y'know, like let's just say someone is wiring up, say, a breadboard or something, and, y'know, his eyesight isn't what it used to be. And y'know how those holes are freakin' tiny and next to each other?
Well why not have some AI thingy that says "hold on you idiot, you put that wire in the wrong hole. It's supposed to be one hole over"
Now THAT is some useful Alexa stuff.