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choosing potentiometer

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nicky.r

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If I have a 0-3.3v adc with 3.3v analog input pins and I want to use a potentiometer to give analog inputs in the range of 0-3.3v,what rating of potentiometer should I use.

Thank you
 

potentiomenter can be of any rating. there is not restriction...

if you select single turn pot the accuracy will be less. and for multiturn it will be more...''
you need to connect pot one terminal to 3.3V and other terminal to ground and the middle pin to output.

vary the pot, and measure the output it will measure 0V at one extreme and 3.3 V at other extreme....

if you use 10K pot 0ohms will be 0V and 10K will be 3.3V like this for any pot....
 
If the analog input is an Analog-Digital converter (ADC) or a microcontroller's analog input, check the specs for the chip. It may have a specification for internal resistance or leakage current. You will want a resistance low enough to compensate for the leakage but not so low as to strain your 3.3v supply.

For many modern day analog inputs, a 1k or 5k potentiometer is probably sufficient. 100k may be too big to give you stable results.

If your circuit is battery powered (or regulator current limited), the value of potentiometer may be a big concern. Use Ohm's Law (V = R * I) to determine the lowest resistance value you can use. Example: If your 3.3v regulator or battery can spare only 5 milliamps for this part of the circuit, that's:

R = V / I
R = 3.3 v / 0.005 A

R = 660 ohms

I'd use a 1k potentiometer in this case.
 
If you want to use a high value pot to minimise power consumption you can often satisfy the ADC maximum impedance requirements by putting a capacitor to ground from the pot wiper if the resistance is high,

Keith
 
keith1200rs said:
If you want to use a high value pot to minimise power consumption you can often satisfy the ADC maximum impedance requirements by putting a capacitor to ground from the pot wiper if the resistance is high,

Keith
Click to expand...

I don't understand how it will help to use a capacitor this way,
the ADC needs a low source impedance to charge fast the sample & hold capacitor and usually a value below 10k is needed or even 1k.
If we assume a pot with a value of 50k which is in the middle position then you will still have a resistance of 25k between the signal input and ADC and also the external capacitor you have suggested from the ADC input (wiper) to the gnd,
in addition this capacitor may add filter characteristics (depending on the cap value and frequency) which may not be desirable.

Alex
 
It works because the 'impedance' is low and the ADC internal capacitance cannot discharge the external capacitance very much depending on the values. Where it may not work is if you want extremely high sample rates but I am assuming that isn't the case if you are connecting a pot to the ADC - there is a limit to how fast you can physically move a pot. I have often used 100k pots for manual inputs to a microcontroller.

Keith
 
so this external capacitor is used as a "storage" of the voltage set by the pot and then it provides
that voltage as a low impedance source when the internal ADC sample & hold circuit is in the sampling phase.
In a way this bypasses the high input resistance because this capacitor can be charged while the ADC makes
the conversion and provides the result, yes it makes sense.

Thank you
Alex
 
Thank you very much for all your suggestions
 

Hi all

One clarification is required. Does PIC controller accepts above 10K for ADC. I think PIC does make any differnce above 10K. Please correct me if I am wrong and also weather it depends on different controllers like ATMEL, TI etc

Thankyou
 

Potentiometer may get saturated and show no variation after certain value.. So a 10k Is usually used.. you need to find out the saturation value and use a proper value pot.....
 

Thankyou Sir,

I asked this becuase most of us tell to use 10K as a thumb rule. I wanted to know weather really we should use 10k or we can use above that etc.

Thankyou once again.
 

prmurthy said:
Hi all

One clarification is required. Does PIC controller accepts above 10K for ADC. I think PIC does make any differnce above 10K. Please correct me if I am wrong and also weather it depends on different controllers like ATMEL, TI etc

Thankyou
Click to expand...
hi prmurthy....
I think there is no relationship between potmeter value vs Mc ADC.... (Particularly ADC potmeters). You can use any value for any microcontroller.
**broken link removed**
What potentiometer should I use with the Arduino Mega? - Yahoo! Answers
Potentiometer | electronics | electric | jtag | pcb design
go threw this link it may usefull to u...
 

There are circumstances where it won't work well with high values. The ADC in the PIC32 microcontroller needs <500 ohms if you want to sample at the highest rate.

Keith.
 

Senthilkumar_rjpm said:
hi prmurthy....
I think there is no relationship between potmeter value vs Mc ADC.... (Particularly ADC potmeters). You can use any value for any microcontroller.
Click to expand...

I have to dissagree on that, for example the AVR datasheet says in the ADC section

"The ADC is optimized for analog signals with an output impedance of approximately
10 kΩ or less. If such a source is used, the sampling time will be negligible. If a source
with higher impedance is used, the sampling time will depend on how long time the
source needs to charge the S/H capacitor, with can vary widely. The user is recom-
mended to only use low impedance sources with slowly varying signals, since this
minimizes the required charge transfer to the S/H capacitor."

When the capacitor is used as keith1200rs has proposed then the source impedance seen by the ADC input will be low and you can get away with higher pot values but input resistance for an ADC is something that matters and also depends on the ADC used.

ckshivaram said:
Potentiometer may get saturated and show no variation after certain value.. So a 10k Is usually used.. you need to find out the saturation value and use a proper value pot.....
Click to expand...

What is a saturated potentiometer, i have never heard that characterization for a potentiometer.

Alex
 

keith1200rs said:
If you want to use a high value pot to minimise power consumption you can often satisfy the ADC maximum impedance requirements by putting a capacitor to ground from the pot wiper if the resistance is high,

Keith
Click to expand...

Would this requirement be necessary for a 10k pot?.
Thank you Sir
 

It depends on the ADC specification (and the sample rate) as Alex says.

Keith.
 

It depends on the input characteristic of your specific ADC, what does your device datasheet say about the recommended impedance,
if your input is a constant DC voltage and you use the pot to change the value of the ADC input then the capacitor won't hurt.
It would only be a problem with fast changing input signals.

Alex
 

alexan_e said:
I have to dissagree on that, for example the AVR datasheet says in the ADC section

"The ADC is optimized for analog signals with an output impedance of approximately
10 kΩ or less. If such a source is used, the sampling time will be negligible. If a source
with higher impedance is used, the sampling time will depend on how long time the
source needs to charge the S/H capacitor, with can vary widely. The user is recom-
mended to only use low impedance sources with slowly varying signals, since this
minimizes the required charge transfer to the S/H capacitor."

Alex
Click to expand...
Thanks for your valuable comment mr.alex. Now only i hered this news about AVR thanks....
 

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