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 lotOriginally posted by beenthere@Mar 11 2005, 01:49 PM
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.
Listen to beenthere, he knows what he's talking about.Originally posted by Lee Bui@Mar 12 2005, 01:15 AM
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
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 alotOriginally posted by Erin G.@Mar 12 2005, 10:50 AM
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. The 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.
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.Originally posted by Erin G.@Mar 13 2005, 08:22 PM
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.
Thanks Erin. I think some1 lied to us about how to test their products. But don't wanna go thereOriginally posted by Erin G.@Mar 12 2005, 08:02 PM
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.
Lee Bui,Originally posted by Lee Bui@Mar 17 2005, 01:28 AM
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?
Thank you!!!Originally posted by Erin G.@Mar 17 2005, 06:18 PM
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.
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.Originally posted by Lee Bui@Mar 19 2005, 12:37 AM
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.
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