Scale output of a potentiometer. 1

[drummerboy827]

Hey everyone, I'm new here and am a Mechanical so I know very little about this subject and am hoping to find my answer here.

So I'm replacing an old control system with a new one, the only problem is the pot's seem to give different ranges. When 5vdc is applied the output on the old controller gives from 2vdc full rev, 2.5vdc neutral, 3vdc full ahead.

I tested the new controller and w/ 5vdc applied it gives 1 vdc full reverse, 2.5 vdc neutral, and 4 vdc full ahead. The problem is the controller is expecting of an input range from 2-3 w/ 2.5 being neutral and the new controler is giving off 1-4.

Is there any way to scale back the input while keeping 2.5vdc as the center voltage?

Thanks!

Chris

 

[VEBill]

There are several possible solutions. The obvious solution is to use op-amps to scale the signal while maintaining the same offset.

But an even simpler solution might be to wire in additional fixed-value resistors at each end of the pot so that the wiper explores a smaller section of the 0-5V range. Given the 3-to-1 gain issue, then perhaps these two additional resistors would be the same nominal resistance value as the pot itself.

This solution is based on my interpretation of your issue. I hope that I've interpreted it correctly.

 

[OperaHouse]

I assume you have replaced the controller and are trying to use the same pot. Keeping this simple, you could start with a 2.7K resistor in series with the wires at each end of the pot and see how close that gets you. That is just a guess not knowing the resistance of the pot. Better still would be use a 5K pot in place of the resistors. Through several iterations you could come up with the correct values.

 

[ScottyUK]

The controller spec probably states what the value of the control pot should be. From the sound of things the existing pot does not match the requirement. Check the manual.


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

I don't know exactly what you're working with but I'm guessing it's a controller with the pot as an input and the
voltages you posted are measured on the output?

If so, then I'd think that the controller likely has some way to scale the range internally.

If not and you are measuring the voltage right at the pot, then the new pot voltages should not matter. What happens in the controller and how it uses the new pot voltage is the important part.

 

[hacksaw]

there are commercial instrumentation modules that allow you to perform this conversion with various bells and whistles to filter the signal.

It is one thing if you are dealing with lab fit up or a home brew but in an industrial envirnment get something with a warranty and manf. support

 

[roydm]

From the little information you provided I assume it'a some sort of analog controller you are working with, and you are using the original potentiometer. Perhaps on the circuit board of the new controller it has a couple of trim pots that allow you to adjust the voltage going to the potentiometer.
Post us the Make/model etc. someone will recognise it.
Regards
Roy

 

[zappedagain]

It sounds like you just have too large of a potentiometer; maybe the old pot degraded to a lower value than it says on the side (not that I've heard of that if you still have linear response on the pot).

0 --1K----3K pot---1K-- 5V
| | |
1V 2.5V 4V


0 --1K----1.5K pot---1K-- 5V
| | |
2V 2.5V 3V

More likely your circuit connecting to the pot has lower resistance:

0 --2K----3.0K pot---2K-- 5V
| | |
2V 2.5V 3V

(grr, I wish this site used the same font to write the message as it does to display it - use Courier New font to get the images above to line up again)

Assuming the pot is connected via resistors to 0V and 5V, disconnect your pot and measure the resistance from one side to 0V, and then measure from the other lead to 5V. You should see the same resistance on both sides for balance around 2.5V. Your pot value wants to be 1.5x that resistance.

Note this is one possible case of OperaHouse's solution.

Z

 

[LionelHutz]

Zapped, was this what you wanted to show?

0 --1K----3K pot---1K-- 5V
        |    |  | 
       1V  2.5V 4V    


0 --1K----1.5K pot---1K-- 5V
        |    |     | 
       2V  2.5V    3V   

More likely your circuit connecting to the pot has lower resistance:  

0 --2K----3.0K pot---2K-- 5V
        |    |    | 
       2V  2.5V   3V

Put "code" "/code" tags around the text you want to have fixed formatting and it should display the same as how you typed it in. Use [] brackets around tags instead of "", I have no idea how to post the tag without it being processed unless I turn off TGML processing which doesn't show anything processed.

This
45678901234567890123456
^ ^
| |
Start End

Will look like this which is how I actually typed it;

45678901234567890123456
      ^        ^  
      |        |
    Start     End

 

[zappedagain]

Fancy! Thanks Lionel, that's a star for you!

Z

 

[automatic2]

I was going to say 'most'. but I'll venture and say all controllers have some scaling capacity. You'll need to walk through the documentation. Sounds like you might be able to 'scale' the new pot by altering it's driver's mechanical ratio.