resistance in potentiometer

Discussion in 'General Electronics Chat' started by Lee Bui, Mar 10, 2005.

  1. Lee Bui

    Thread Starter New Member

    Mar 10, 2005
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    Can anyone help to explain if it is suffice to measure the linearity and output smoothness of a pentiometer without needing to measure the continuous of the resistance?

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Yes, if you check the linearity of the pot you will also be able to see any deviation from the continuous function. Any jump in resistance will point to it.
     
  3. Lee Bui

    Thread Starter New Member

    Mar 10, 2005
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    Are you sure? I thought the linear output only show how the electrical travel within a control limit. That is has nothing to do with the mechanical resistance contact between the wiper and the conductive surface. Thanks a lot
     
  4. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Listen to beenthere, he knows what he's talking about.

    Generally, potentiometeres have lead 2 as the wiper, and leads 1 and 3 are the total resistance of the pot. If you had a 5K, 1 turn pot, then you should see 5K between leads 1 & 3, no matter where the wiper is, assuming that you've isolated the pot. When you have the pot turned all the in either direction, place your ohm-meter leads between 1 & 2, and slowly turn the pot all the way to the opposite direction. If you started at 5K, you would see a slow, smoothe decline from 5K to 0 ohms. If you now change the ohm-meter leads to 2 & 3, and turn the pot in opposite direction you will see the same thing again.

    Any jump or other erratic readings in you ohm-meter indicates that the pot is bad. The pot has to be electrically AND mechanically sound to work properly.
     
  5. Lee Bui

    Thread Starter New Member

    Mar 10, 2005
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    Absolutely understanding the way you described. What I would like to know is, when people measure the voltage linearity and output voltage smoothness, is it really measure the resistance continuity at the same time? Another word if the output voltage smoothness is bad, does it mean the resistance continuity is also bad? or output smoothness has nothing to do with resistance continuity? Thanks alot
     
  6. Nettron

    Member

    Jan 22, 2005
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    To help elaberate on "smoothness" , keep in mind there are two types of pots; linear and logarithmic. With a linear taper pots each increment of the wiper, say 1/8 of a turn=1K, is pretty much the same resistance increase(or decrease). With a logarithmic taper pot each equal increment will increase ( or decrease) the resistance logarithmically ( commonly used in audio circuits ).
     
  7. Erin G.

    Senior Member

    Mar 3, 2005
    167
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    I'm not too familiar with logarithmic pots. In this discussion I was referring to taper pots.

    Lee Bui, if yours is a taper pot, when the pot is adjusted, if the output voltage changes are not smoothe, then chances are, your pot is bad. Though, there could be other factors in various circuits that may cause abrupt changes in your voltage readings when the pot is adjusted. (I've never seen that happen with a taper pot, but it is possible.) That's why it's important to isolate the suspect part and test it de-energized, when ever possible.

    Remember that with a taper pot, the voltage output is directly related to the resistance inserted in the circuit through the wiper in the pot. If the pot has a resistance that jumps around when the wiper is adjusted, your voltage output is going to jump around as well.
     
  8. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
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    A taper pot is a logrithmic pot. The term derives because pots were originally wirewound and the resistance wire was wound on a strip former and then bent into a circle. For a linear pot, a parallel former was used. For a log pot, the wire was wound on a tapered strip which gave larger (longer) turns at one end than the other.

    In the past there have been linear, semi-log and log types. Inverse semi-log and log types have also been available for special uses

    There used to be a cheap pot for domestic use that used a track of thin card sprayed with carbon. These were notorious because with continual use the sprayed carbon wore through. I think most pots now probably use a conductive plastic track so the chances of them going open cicuit or high resistance are slim.

    If a pot is used as a pot, ie. as a potential divider to get a variable voltage source then the condition of the wiper is not important. But if the pot is being used as a variable resistor, ie. current is taken through the wiper than any discontinuity between track and wiper will show up as noise.

    Various cleaners have been devised for dodgy wipers, like Servisol and Electrolube. But I have used WD40 in an emergency and found it satisfactory.
     
  9. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Pebe, I used to think I knew a thing or two about pots. The ones I've used most have been for positive positioning applications, where we used the resistance of the pot as an analog input. The variable resistance was coupled to an analog input card for processing, so no real voltage was applied across the pot. Only about the middle one-third of the pot was used, and that's where it always failed, because of the almost constant motion of the device. These are plastic molded enclosures, with no way to access them for maintenance and cleaning. We just replace it and throw the old one away.
     
  10. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
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    So you were using the device as a variable resistor. It sounds like the pickup wiper is failing. The company I worked for many years ago produced a very good pot (built to military specs) with a carbon brush in the wiper. They were very reliable and noise-free. One particular model was used in continuous rotation (no end stops) coupled to a radar aerial as a bearing indicator.

    As a matter of interest, what make of pots are you using?
     
  11. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hello everybody, :)

    what had been discussed was really an eye-opener for members whose knowledge abouts potentiometer commonly called pots.

    just to toss in somemore info on pots, there are pots with negative temp. coefficiency and positive temp. coeff. :)
     
  12. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Pebe, They are Warner Electric model MSC-605-1, 5K-ohm, 1 turn, and they work through a range of about 60 degrees.
     
  13. Lee Bui

    Thread Starter New Member

    Mar 10, 2005
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    Thanks Erin. I think some1 lied to us about how to test their products. But don't wanna go there

    I used an Ohmeter to test the pot (plastic). As i turn the shaft the resistance either increase or decrease accordingly. What i found is the resistance in some of the pot sometime skips the reading from 3.1K then 3.3k - missing the 3.2K. I believed this is an open circuit. However, they kept saying the test they do should cover the open circuit issue. So the part pass their test of output smoothness, then I should not have a open circuit pot right?
     
  14. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Lee Bui,

    I am assuming that you read 2.8, 2.9, 3.0, 3.1, 3.3, 3.4,...
    If you experience a "skip" from 3.1K to 3.3K with your ohm-meter, then when the circuit is energized, your voltage will "skip" as well when the pot is adjusted through that range. The pot may not be open in that range, but rather, it probably has a bad spot that keeps the wiper from making contact at that point. I've seen this many times in the Warner pots I described earlier in this thread. How much of an effect this has on your circuit operation will depend on how sensative the circuit is. Remember, if the pot was truly open in any place, then you would not be able to read the total ohms of the pot from leads 1 to 3.
     
  15. Lee Bui

    Thread Starter New Member

    Mar 10, 2005
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    Thank you!!!

    There are 3 terminal points (1,2, and 3) on the pot for us to solder the wires onto. Is it possible for us to burn the pot when performing the soldering? assuming the solder tip is approximately 500 degree C and the contact time of the hot tip and the terminal is 5 seconds. Thanks again.
     
  16. Erin G.

    Senior Member

    Mar 3, 2005
    167
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    It is possible, but unlikely, expecially if the pot was designed to be soldered. In the future, I would turn the pot all the way in one direction or the other prior to soldering. If the soldering does cause a bad spot, it would be at the farthest end of travel, rather than somewhere in the middle of the pot.
     
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