Expanding a pot's working range

[Futterama]

Hi,

This is a bit hard to explain and the title may not be very accurate but here goes:

I have a normal pot that achieves full range by turning it about 270 degrees. The pot is used as a voltage divider with Vss and VDD at each outer pins and the divided voltage on the middle pin.

I need some circuit that can generate this full Vss to VDD range when I turn the pot only 90 degrees. But I really don't have any good clues of how to do this. I was thinking an op-amp but I haven't worked with those very much, so I don't know if it can be easily done using an op-amp or two and some resistors.

I am sure some of you guys can help me out.

Thanks.

Best regards,
Futterama

 

[Grossel]

Hi.

This can be achieved in one of three ways:

1. Opamp configured as noninverting amplifier. The opamp need to have rail to rail output. If not, it won't be able to provide all voltages from -Vcc to +Vcc
2. A simple gear attached to the pot's axe.
3. Get more voltage on the end pins on the potmeter that is lower than -Vcc and higher than +Vcc.

In any cases above, you need a scaling factor of 3/1.

Right now I cant say wich one is the best solution for you.

 

[Futterama]

Grossel, thanks for the answer.

The two last suggestions are not possible so I will try the non inverting amplifier. I have read a bit about it and still have no idea how it really works. But I will give it a try and play around with resistor values, maybe this will give me a better idea of how it works.

 

[Grossel]

Hi.

Good choice by the way. In order to make amplification 3 times, you need two resisors, one that have twice the resistance value of the other.

Then I assume you want to find it out by yourself? I could have provided a more straight forward answer, but need to know some more about your avaiable voltage and also what output voltage range you want to adjust within.

 

[Futterama]

Grossel, thanks, I will make some measurements and tests tonight (GMT+1) and post my results. Maybe I would need a bit more help, but lets see about that tonight

I guess a LM358 could be used? These are the only op-amps I know for sure I have.

 

[Grossel]

Be aware that the output on LM358 can't go all the way up to +Vcc. Following the Motorola datasheet (assume it applies to same chip but other brands too) if +Vcc is 5V, then 3.3V <= Vo <= 3.5V.

 

[Futterama]

Well, it seems like I don't have a suitable op-amp then. Any dual op-amp with rail-to-rail output swing and 5V single supply you can recommend I try?

 

[Grossel]

No, I haven't actually had use for rail to rail output opamps so far.

You can always do a web search and see if you find one that you find suitable.

 

[Futterama]

Hi Grossel,

I finally got my hands on some rail-to-rail opamps (LT1490A from Linear). I have connected one as non-inverting amplifier and using the middle pin of the pot as voltage input. I can't seem to get the desired results, so I need help

Supply voltage: 5V
Pot used as voltage divider, voltage range: 1.7V - 3.2V (this is what I need "converted" to 0-5V).

Used this schematic for non-inverting amplifier:

**broken link removed**

 

[ADWSystems]

I think I see your problem. Are you getting 5.1 to 9.6 volts? Of course not, the plus rail is only 5V. Silly me. Can you make the opamp amplify the 0-1 volt portion of the pot's range?

The issue is the 1.7 starting voltage. Converting 1.7-3.2 to 0-5 is going to be akin to converting celcius to farenheight. You'll need to subtract off your 1.7V starting point first which will make it 0-1.5 then convert to 0-4.5 (3x).

 

[Roff]

Here's how you do it.
The opamp needs to be rail-to-rail.
All three resistors need to be the same value, but you can use any value between 10k and 100k. Higher values will get you closer to the rails.
The pot can be any reasonable value.

EDIT: Vcc can be any value that is within the working range of the op amp.

 

[Futterama]

Thanks, I will try this tonight.

 

[Futterama]

Roff, I have tried you circuit with 10k and 100k resistors, and I get the same results. With a supply of 5V, when turning the pot, I get 2.5V to 5V.

So basically it doesn't work somehow :-(

 

[Futterama]

Roff, I have tried you circuit with 10k and 100k resistors, and I get the same results. With a supply of 5V, when turning the pot, I get 2.5V to 5V.

So basically it doesn't work somehow :-(

Oh, I read the circuit wrong, it works now!

THANKS!

 

[unclejed613]

on the inverting input, reference it to +2.5V with a voltage divider rather than ground, this will cure your problem of the input range shift. it's that simple (surely not as complicated as converting C to F)

 

[Roff]

on the inverting input, reference it to +2.5V with a voltage divider rather than ground, this will cure your problem of the input range shift. it's that simple (surely not as complicated as converting C to F)

I posted a schematic in post #11. Are you suggesting an improvement?

 

[Futterama]

I posted a schematic in post #11. Are you suggesting an improvement?

I think it is a response to post #10, and commenting on the non-inverting amplifier where I didn't have a voltage divider on the inverting input as I have on the circuit from you.

 

[Roff]

I think it is a response to post #10, and commenting on the non-inverting amplifier where I didn't have a voltage divider on the inverting input as I have on the circuit from you.

Yeah, it was the last post on the page. Unclejed probably didn't realize there was another page. I've done that myself a few times.