The acceptable range of potentiometer resistance depends on two factors:
1. Characteristics of the supply
2. Input resistance of the analog input (when configured as voltage input)
1. Every source od DC supply is limited by the current delivery. If the potentiometer is supplied from an internal 10.5 supply (provided by ATV) you have to check the max current load for this internal supply. In the case of ATV61, 71 and ATV process, the max delivery current is 10mA. For this reason, the potentiometer resistance cannot go below 1 kOhm , which is the value if we calculate 10V/10mA.
In case you have an external 10V supply, able to deliver for example 1A, then the minimum potentiometer resistance could be 10V/1A, which gives theoretically 10 Ohms, but practically such potentiometer would suffer from excessive heat losses (U^2/R=10V*10V/10 Ohm = 10W). Potentiometers are not designed to dissipate such losses. Consider max 0,5W on a real potentiometer or better to check max losses from the potentiometer datasheet. Do not use potentiometer with resistance lower than 250Ohms (U^2/R with 10V and 250 Ohms will be 0.4W)
2. Input resistance of analog input does not limit the potentiometer resistance selection, but bear in mind the linearity. If we designate the upper part of the potentiometer as R1 (resistance between potentiometer beginning and current position of the selector), the lower part of the potentiometer as R2 (resistance between the selector and the end) and ATV input resistance as Ri, then the circuit represents "R1 - R2||Ri" (where - is in series and || means parallel). For your information, ATV61-71 analog input resistance is 30 kohm. Now lets imagine a potentiometer with a resistance 60kOhms and its selector in the middle position. So R1=30 kOhm. R2=30kOhm, Ri=30kOhm. If we supply potentiometer with 10V, the voltage on R2 (the same on Ri) will be : UR2= 10V * (R2*Ri) / (R2+Ri) / (R1+ (R2*Ri) / (R2+Ri)) = 15kOhm / (30kOhm+15kOhm) = 3.33V. You see that the middle position does not lead to 5V but only 3.33V. To minimize the effect of non-linearity, it is recommended to have potentiometer resistance lower than analog input resistance divided by 5. Even better if the ratio: of potentiometer resistance / analog input resistance is below 1:10.
Practically for ATV61-71, the ATV process, the potentiometer resistance should be below 6kOhms. The recommendation is 2.5kOhm or 1kOhm potentiometer.
1. Characteristics of the supply
2. Input resistance of the analog input (when configured as voltage input)
1. Every source od DC supply is limited by the current delivery. If the potentiometer is supplied from an internal 10.5 supply (provided by ATV) you have to check the max current load for this internal supply. In the case of ATV61, 71 and ATV process, the max delivery current is 10mA. For this reason, the potentiometer resistance cannot go below 1 kOhm , which is the value if we calculate 10V/10mA.
In case you have an external 10V supply, able to deliver for example 1A, then the minimum potentiometer resistance could be 10V/1A, which gives theoretically 10 Ohms, but practically such potentiometer would suffer from excessive heat losses (U^2/R=10V*10V/10 Ohm = 10W). Potentiometers are not designed to dissipate such losses. Consider max 0,5W on a real potentiometer or better to check max losses from the potentiometer datasheet. Do not use potentiometer with resistance lower than 250Ohms (U^2/R with 10V and 250 Ohms will be 0.4W)
2. Input resistance of analog input does not limit the potentiometer resistance selection, but bear in mind the linearity. If we designate the upper part of the potentiometer as R1 (resistance between potentiometer beginning and current position of the selector), the lower part of the potentiometer as R2 (resistance between the selector and the end) and ATV input resistance as Ri, then the circuit represents "R1 - R2||Ri" (where - is in series and || means parallel). For your information, ATV61-71 analog input resistance is 30 kohm. Now lets imagine a potentiometer with a resistance 60kOhms and its selector in the middle position. So R1=30 kOhm. R2=30kOhm, Ri=30kOhm. If we supply potentiometer with 10V, the voltage on R2 (the same on Ri) will be : UR2= 10V * (R2*Ri) / (R2+Ri) / (R1+ (R2*Ri) / (R2+Ri)) = 15kOhm / (30kOhm+15kOhm) = 3.33V. You see that the middle position does not lead to 5V but only 3.33V. To minimize the effect of non-linearity, it is recommended to have potentiometer resistance lower than analog input resistance divided by 5. Even better if the ratio: of potentiometer resistance / analog input resistance is below 1:10.
Practically for ATV61-71, the ATV process, the potentiometer resistance should be below 6kOhms. The recommendation is 2.5kOhm or 1kOhm potentiometer.
Released for:Schneider Electric New Zealand
