Technical FAQs

# What is the potentiometer range (min/max resistance) compatible with the Altivars?

The acceptable range of potentiomer resistance depends on two factors:
1. Characteristics of the supply
2. Input resistnace of the analog input (when configured as voltage input)

1. Every source od DC suply is limited by the current delivery. If the potentiometer is supplied from internal 10.5 supply (provided by ATV) you have to check the max current load for this internal supply. In case of ATV61, 71 and ATV process, the max delivery current is 10mA. From this reason the potentiometer resistance cannot go below 1 kOhm , which is the value if we calculate 10V/10mA.
In case you have external 10V supply, able to deliver for example 1A, then minimum potentiomer resistance could be 10V/1A, which gives teoretically 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 a losses. Consider max 0,5W on real potentiometer or better to check max losses from 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 doesnot limit the potentiomer resistance selection, but bear in mind the linearity. If we designate upper part of potentiomer as R1 (resistance between potentiometer beginning and current position of the selector), the lower part of potentiometer as R2 (resistance between 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 potentiometer with 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 middle position does not lead to 5V but only 3.33V. To minimize the effect of non-linearity, it is recommended to have potentiomer resistance lower than analog input resistance divided by 5. Even better if ratio : potentiometer resistance / analog input resistance is below 1:10.
Practically for ATV61-71, ATV process, the potentiometer resitance should be below 6kOhms. Recommendation is 2.5kOhm or 1kOhm potentiometer.