Pots in Series

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Phrazemaster

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Joined
Oct 2, 2006
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Location
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Hi everyone,

I've been puzzling over what seems like a simple thing: 2 potentiometers in series. The purpose is to have a "course adjustment" and a "fine adjustment," with the first pot 100K and the second pot 1K. I'm using the PRR FET Tester circuit, and I'm finding having just the one 100K pot very challenging in matching FETS because at certain voltages you only breathe on the 100K pot and it moves. I need the fine adjustment.

Of course I have googled this and I'm embarrassed how many hours, how much money on parts, and how many which-ways I've tried to wire this.

Attached is the circuit, and in the next post I'll put the variations I tried based on a google.

Thanks in advance; I just really want to crack this nut and get this thing done; it's been at least a week (no joke!).
 

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A couple variations (among many) I tried, with a google schematic posted to the right of each.

While they kinda sorta work, they don't work as expected at all. At certain positions of the pots it seems to work, but I find that I'll turn the 100K and all of a sudden the voltage in the circuit posted goes from 0 to 9 in a tiny fraction of a turn - weird. I must be invoking some weird pot law stuff inadvertently. This seems like it should be so simple.

How do I replace that 100K in PRR's diagram with 2 pots, one for the course adjustment and one for the fine adjustment, 100K and 1K?

Thanks!

Mike
 

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Put the 1K between the top of the 100K and +9v point.
Sorry for the 3rd grade drawing...:)

 

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Thanks to both of you - both great ideas!

BlueBird I will try your solution tomorrow and I’m going to look at the pot you suggested Ian!

Thanks gang!

Ps 3rd grade drawing is awesome!!! I appreciate you taking the time to do that BlueBird!
 
There are multiple ways to make a fine trim from two pots, but another is to place a small resistor in series with one wiper, and a large value resistor in series with the fine trim pot wiper. At the junction of the two resistor the voltage divider will reduce the impact of the second pot for fine trimming.

You could just about eliminate the small resistor from the first pot, as long as it is not too close to either extreme, but using two resistors will insure it is well behaved over entire range. 

JR
 
JohnRoberts said:
There are multiple ways to make a fine trim from two pots, but another is to place a small resistor in series with one wiper, and a large value resistor in series with the fine trim pot wiper. At the junction of the two resistor the voltage divider will reduce the impact of the second pot for fine trimming.

You could just about eliminate the small resistor from the first pot, as long as it is not too close to either extreme, but using two resistors will insure it is well behaved over entire range. 

JR
Thanks kindly JR.

Any chance you might be able to doodle a schematic with suggested values?

If not no worries; I’m grateful for your pointer.
 
Here's my best attempt at putting together JR and BlueBird's suggestions. Please be kind; Electronics is not my native language.

Any thoughts?

Thanks,

Mike
 

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Phrazemaster said:
Thanks kindly JR.

Any chance you might be able to doodle a schematic with suggested values?

If not no worries; I’m grateful for your pointer.
I used to do stuff like that for $100 an hour, now I would probably charge more, because I don't want to do it.

It is pretty simple concept, and you will learn more by trial and error and melting some solder. I already know how to do it, and pointed you in (one) right direction.

JR 
 
JohnRoberts said:
I used to do stuff like that for $100 an hour, now I would probably charge more, because I don't want to do it.

It is pretty simple concept, and you will learn more by trial and error and melting some solder. I already know how to do it, and pointed you in (one) right direction.

JR


I saved it in my newly created jewels folder..... After seeing it yesterday I thought, I need to start saving stuff I catch here that is unusually special to me...
 
JohnRoberts said:
I used to do stuff like that for $100 an hour, now I would probably charge more, because I don't want to do it.

It is pretty simple concept, and you will learn more by trial and error and melting some solder. I already know how to do it, and pointed you in (one) right direction.

JR
Fair enough. I have already burned many fingers and melted a lot of solder on this, and spent several hours experimenting and googling, but I will forge ahead.

And I totally get when you make a living doing something and someone else wants your IP for free, it's a little insulting. So no disrespect meant.

Thanks kindly. You add so much to this forum, and I'll take your direction and BlueBird's and Magneto's and see what I can come up with.

Thanks,

Mike
 
The deal is you want to keep the voltage divider in tact. So you don't want to mess with the set resistance of 100k between the FET and -9v. Some of your first attempts changed this value a lot. Putting the 1K pot where I did only changes the value a little bit. You could even try a 5K if the fine adjustment isn't enough with the 1K. The multi turn pot like Ian suggested would definitely be the easiest solution, just cost some money and time.
 
Excellent explanation Bluebird - thx!

I'll be going to my studio later today and messing around with all of this.

Many thanks to you, JR, Magneto for the support.

Mike
 
I can't fathom why you would need anything like 100k of range. A 47k pot with say 22k fixed resistance at either end is a good place to start, imo. You can reduce either of the fixed resistances if range is too limited, simply by adding another resistor in parallel.

 
MagnetoSound said:
I can't fathom why you would need anything like 100k of range. A 47k pot with say 22k fixed resistance at either end is a good place to start, imo. You can reduce either of the fixed resistances if range is too limited, simply by adding another resistor in parallel.
Excellent - thank-you.
 
What are you measuring? (I don't remember.) Vgs?

What typical values are you finding? Like 1V?

Then change the "9V" to a 1.5V battery.

In general, put a pot across whatever battery and set the wiper to just over the maximum voltage you expect to need. Then use the other pot from there to 0V to dial-down to the voltage needed by the FET under test.
 

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PRR said:
What are you measuring? (I don't remember.) Vgs?

What typical values are you finding? Like 1V?

Then change the "9V" to a 1.5V battery.

In general, put a pot across whatever battery and set the wiper to just over the maximum voltage you expect to need. Then use the other pot from there to 0V to dial-down to the voltage needed by the FET under test.
Hi PRR, thanks Kindly for the schematic and ideas.

This is for matching FETS for an 1176; you had posted a long post about it, URL here: https://groupdiy.com/index.php?topic=1766.msg86599#msg86599

So the idea was to find out how well the FETS matched using the -9V on the gate, and finding the 10% drop voltage, the 50% drop, and the 90% drop. The voltage source was in mV, and the gate voltages ranged from -9v to almost 0.

Thanks for the ideas and I'll try to incorporate them.

Many thanks!

Mike
 
Phrazemaster said:
Fair enough. I have already burned many fingers and melted a lot of solder on this, and spent several hours experimenting and googling, but I will forge ahead.

And I totally get when you make a living doing something and someone else wants your IP for free, it's a little insulting. So no disrespect meant.
I am not sure I want to open this can of worms... BUT it is not because I value my IP so highly, but I prefer teaching people how to solve their problems, more than just giving them the answers.

It is also a minor PIA for me to scratch out a schematic (involving different computers), so I generally don't
Thanks kindly. You add so much to this forum, and I'll take your direction and BlueBird's and Magneto's and see what I can come up with.

Thanks,

Mike
I worry you did not get the concept I offered. Making a coarse and fine adjustment with two pots means the fine adjustment needs to have its sensitivity reduced. An easy way to do that is with a resistive divider between the two wipers (lower impedance in series with coarse wiper, higher impedance in series with fine wiper) .

In fact there are probably a half dozen workable solutions for this problem. If you don't care how to do this, but just want a solution (sounds like management material) I'm sure one of the solutions offered by a more helpful forum member will work.

JR
 

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