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How to find the value of POTs to use?

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pavjayt

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Hello,

I am trying to use a 10K pot to adjust a dual voltage range.

Lets say I have a -1VDC at pin.1 and 1VDC at pin.3, in this case I can get any voltage between +/-VDC on Pin.2 - the drop over Pot resistance (correct?)

My primary input voltage is +/-5VDC, I would like to have the user able to adjust his range rather than a fixed +/-1VDC using fixed value resistors at pin1&3. This can be accomplished by adding two more pots on both pins 1&3. But I am not sure how to calculate the value of the pots I need to use.

Can someone suggest me how I can achieve this?

thanks.
 
Are you drawing any significant current off the wiper? What is the wiper connected to?
 
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Wiper is connected to the NI input on one of the channels in a LT1014 which is set as a voltage follower. Where its output is connected to a IN0 input on a AD8170. So, there is no significant current draw here. But I want the user to have fine voltage adjustment between what ever range he sets across the pot.
 
So lets see if I understand correctly:

You have +/- 5 volts available.
You would like to be able to "set" the voltage at the wiper to be anywhere between =/- 1 to +/- 5 volts?
 
Do you have a ganged double pot? (look at my previous post again)

If not, I can think of a solution that uses two pots, one to select the endpoint +- values symetrically, and the other to select the voltage that varies between the endpoints.
 
So lets see if I understand correctly:

You have +/- 5 volts available.
You would like to be able to "set" the voltage at the wiper to be anywhere between =/- 1 to +/- 5 volts?

Yes, but the range could be any where from +/-100mV to +/-2V.
 
I dont have that kind of POT, if needed will buy one, if it makes things easier.

At the end I want to have the user able to adjust the voltages at pin 1&3 within a range of +/-100mV to +/-2V from a +/-5VDC source and have any voltage within the set range at the wiper with finer adjustment (10 turn pot).

thanks,
 
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I dont have that kind of POT, if needed will buy one, if it makes things easier.

At the end I want to have the user able to adjust the voltages at pin 1&3 within a range of +/-100mV to +/-2V from a +/-5VDC source and have any voltage within the set range at the wiper with finer adjustment (10 turn pot).

thanks,

The x axis is the value of "pos". The green trace is for fs=0, red fs=0.25, ... , violet fs=1
 

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The x axis is the value of "pos". The green trace is for fs=0, red fs=0.25, ... , violet fs=1

Thanks Mike, I will put together a test circuit on a breadboard and see how it performs. Hopefully this will work out fine for me.
 
If the 100mV level setting isn't too critical then here's a simple alternative way, giving a range from +/- 0mV to +/-4.9V.
AdjustableRange.gif
 
Expanding on Alec's circuit:
 

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Thanks Alec and Mike. Both circuits work fine. Could anyone of you guys explain me how you came up with those values which is what I am wanting to know.

thanks
 
One more general question regarding POTs, is it a good idea to use only pins 1&2 and leave pin 3 unconnected if you are using a pot as a trimmer (variable resistor) or using pins 1 & (2-3) shorted?

thanks
 
Expanding on Alec's circuit:

How did you come up with 6.19K, 243, 10K pot & 50K pot values? That way I can change them so that I can set the upper limit (now its at 2V with your values) to lower than 1V, that way I can get more resolution on the output.

Can I use a 40K trimmers in place of 6.19K resistors to change the upper limit without changing any other values or is there any other easy way to implement that?

thanks.
 
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I put the circuit in LTSpice. The goal was to find resistances that hit the 0.1V and 2V limits as U4 is varied. I could play with R1 (and R4), R2, and the ratio of pot U4 to pot U1. I just used trial and error, and it converged to the final values quickly.

I could have written three equations in three unknowns, and solved the equations, but I would probably still be trying to find where I made a math mistake... :eek:

A few more iterations gives R1=R4=16.5K and R2=732Ω
 
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One more general question regarding POTs, is it a good idea to use only pins 1&2 and leave pin 3 unconnected if you are using a pot as a trimmer (variable resistor) or using pins 1 & (2-3) shorted?

thanks

I short the wiper to one end. If the pot gets dirty, and skips off the resistive element, then if the end is floating, then the pot fails to infinite resistance. My way, it fails to the end-to-end resistance of the pot, which might be infinitely better... ;)
 
I short the wiper to one end. If the pot gets dirty, and skips off the resistive element, then if the end is floating, then the pot fails to infinite resistance. My way, it fails to the end-to-end resistance of the pot, which might be infinitely better... ;)

Thanks Mike, I ended up with those values for 1V at 16.4K, but haven't changed R2 though. So, what would be the range of R2 if I use 50K POTs in place of 16.4K, that way I can have a POT in place of R2 too.

One more last question regarding this, is it OK if I use four 50K POTs in place of U1 if I want to use this set range for 4 different channels? Soo basically it owuld be four 50K pots in parallel to Pin1&3 of U4?
 
Thanks Mike, I ended up with those values for 1V at 16.4K, but haven't changed R2 though. So, what would be the range of R2 if I use 50K POTs in place of 16.4K, that way I can have a POT in place of R2 too.

One more last question regarding this, is it OK if I use four 50K POTs in place of U1 if I want to use this set range for 4 different channels? Soo basically it owuld be four 50K pots in parallel to Pin1&3 of U4?

Paralleling four 50K pots would result in an effective resistance for U1 of 12.5KΩ, so that would necessitate re-figuring everything. Just use four 200K pots in parallel; that makes 50KΩ as before...
 
Paralleling four 50K pots would result in an effective resistance for U1 of 12.5KΩ, so that would necessitate re-figuring everything. Just use four 200K pots in parallel; that makes 50KΩ as before...

Thanks again Mike. Thats a very good suggestion.
 
Paralleling four 50K pots would result in an effective resistance for U1 of 12.5KΩ, so that would necessitate re-figuring everything. Just use four 200K pots in parallel; that makes 50KΩ as before...

I removed the post since I found low cost 200K pots from other vendors other than Digikey.
 
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