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Old 4th March 2010, 03:54 PM   #3841
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In order to keep things in order. I STILL don't have any effective suggestions as to ADDING to the VARIABLE pot to extend its range, either up for less loading, or INCREASING its resolution. We do have, on the way, a P3 10 turn 10K hybrid multistage pot that should arrive within the week. I suspect that its resolution will be too tight for and easy 130 -138 ohms adjustment, for example. I know that people have worked on this for years, but I never bothered, because I have almost always used a variable pot,exclusively. Any suggestions out there?
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Old 4th March 2010, 04:29 PM   #3842
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In order to keep things in order. I STILL don't have any effective suggestions as to ADDING to the VARIABLE pot to extend its range, either up for less loading, or INCREASING its resolution. We do have, on the way, a P3 10 turn 10K hybrid multistage pot that should arrive within the week. I suspect that its resolution will be too tight for and easy 130 -138 ohms adjustment, for example. I know that people have worked on this for years, but I never bothered, because I have almost always used a variable pot,exclusively. Any suggestions out there?
Well, some of us tried, some tried to get an idea of what range you needed, but nothing was forthcoming. Then you said that the TO5 G-o-G relay was non-negotiable. Next you say you want to explore alternative switching elements. Then Simon suggested a switching approach and you told him, go do your own project. First you tell us the 1k pot is not negotiable. Now all of a sudden it's a 10k pot. You have an interesting engineering approach, I grant you that

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Old 4th March 2010, 05:55 PM   #3843
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I NOW have a SAMPLE pot on the way, and it is 10K. I would prefer 1K and the final design will probably be 1K. I can TEST the 10K when it gets here to access the practical resolution. Then later I will order 1K. Therefore, I am asking about 1K solutions. Does this clarify the situation?
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Old 4th March 2010, 06:28 PM   #3844
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Ed Simon, would you please answer your own question?
John,

You guess was pretty good, but there are lots of others who might want to try.

As to a 10K sample pot, about all you can do is parallel a 1K resistor, Then switch in or out a 100 ohm unit. A 1K trim pot may have about 1 ohm of termination resistance so you might want to put a few ohms in that leg. It should help low value tracking.
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Old 4th March 2010, 06:42 PM   #3845
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It is just a test pot, we probably will actually purchase a 1K pot. I regret if I have made the problem that I was hoping to address a little scattered. Here it is again: At this time (subject to change by my employer as it has already changed over the weeks) it is now 10 ohms-1K variable, then switched up to 100K. I am still looking for optimum switching from 1K to 100K. Any suggestions?
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Old 5th March 2010, 09:15 AM   #3846
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It is just a test pot, we probably will actually purchase a 1K pot. I regret if I have made the problem that I was hoping to address a little scattered. Here it is again: At this time (subject to change by my employer as it has already changed over the weeks) it is now 10 ohms-1K variable, then switched up to 100K. I am still looking for optimum switching from 1K to 100K. Any suggestions?
I don't know why your employer takes design decisions - isn't that why he hires people like you?
Anyway, from an engineering standpoint a 10t 10k appears a better choice. At 1k per turn there's enough resolution in the lower 100's area, and you could still use an (anti) log fake resistor to improve resolution even more. 10k should allow you to get away with less switching and less relays or whatever you want to use for switching. You could have one range 10k-10 ohms (with fake antilog or log law) and then 100k-10k. Are you foreseeing to divide that 100k-10k in pieces with switches or somehow with the same pot?

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Last edited by jan.didden; 5th March 2010 at 09:18 AM.
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Old 5th March 2010, 11:43 AM   #3847
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How about something like this? Different combinations of relay switching allow smooth adjustment from 0 to 80k.

Perhaps a bit of overlap would be good e.g. 4 relays with fixed resistors of 10k, 18k, 33k and 56k.
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Old 5th March 2010, 02:47 PM   #3848
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Jan, I have previously stated that the 'sweet spot' has to be in the 130 ohm area. 10K is fairly far away, that is why I am NOW considering 1K or 2K as a FINAL adjustable pot.
I would like to point out that I have used just about every pot that is normally available, over the years, and I have samples in my lab. Including P&G rotary, linear, and military, Alps, both Black and Blue, TKD, Bourns, AB, etc.
I even have a fair number of wirewound 10K pots. I have tested them already. My best hope is hybrid and probably a lower value than 10K.
Spot on Godfrey! That is just the sort of input I was hoping for.
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Old 5th March 2010, 02:55 PM   #3849
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I don't know why your employer takes design decisions - isn't that why he hires people like you?
Anyway, from an engineering standpoint a 10t 10k appears a better choice. At 1k per turn there's enough resolution in the lower 100's area, and you could still use an (anti) log fake resistor to improve resolution even more. 10k should allow you to get away with less switching and less relays or whatever you want to use for switching. You could have one range 10k-10 ohms (with fake antilog or log law) and then 100k-10k. Are you foreseeing to divide that 100k-10k in pieces with switches or somehow with the same pot?

jd
Jan,

You forgot rule #2. "When the boss is wrong refer to rule #1!"

Godfrey,

Yes that is one of the two basic circuits, and you got right that the resistors need to be a bit low to allow for tolerances. In the case of a 1K pot with an end resistor the resistors should be 988 ohms, 1.91k, 3.75K, 7.39K, 14.3k, 28K and 54.9K. The other method is an R/2R ladder see

Resistor ladder - Wikipedia, the free encyclopedia

The actual issue is how to minimize the use of relays. Ideally only one relay should be closed. So the serial method with more relays may be a good choice, except that relays are expensive.
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Old 5th March 2010, 02:59 PM   #3850
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Cost is not so important, minimum number of series relays is important.
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