I will make an interface for a 10k wind-vane recommended in this document. I will follow the recommendations but I don't understand the explanations of the document and I have at the end four short questions regarding the same interface which are confusing to me.
I want to use 12V supply for the opamp buffer and around 8.5V(through a linear regulator) for the vane. So I wanted to simulate all these in LTspice where I mimic the potentiometer of the vane by varying R4 and R3 in the following circuit:
Here the plot of vane and buffer outputs for 10k range:
Below the zoomed plot of vane and buffer outputs when pot is around 1 ohm::
Finally the zoomed plot of vane and buffer outputs when pot is around 10k ohm:
If my drawing to LTspice is correct(?):
I have the following questions:
1-) The document says:
Similarly, the 1 K ohm series resistor shown in the wiper lead prevents short circuits from causing over-current damage to the potentiometer. If the sensing circuit has an input impedance greater than 1 K, the additional resistor is not needed.
I don't understand what meant here. Totally lost. Can you give an example how can be a short circuit? The opamp has already huge input impedance why then they use 1k just after they claim the additional resistor is not needed if the sensing circuit as an input impedance greater than 1k?
2-) The document says:
The 270 k Ohm pull-down resistor is desirable to make sure that the input reads North when the wiper is in the deadband, and is open circuited.
I can understand that 270k will pull down the input signal to the opamp when the wiper is floating. But why 270k? Why not 1Meg for less error?
3-) The document says:
By adding 100 Ohm resistors between the excitation and the potentiometer, and between signal common and the potentiometer, the potentiometer range is reduced slightly to compensate for the sensor deadband.
I cannot picture what is meant here as well. Is it possible to explain this by a simple drawing? I don't get why potentiometer range is reduced and why the deadband is compensated.
4-) The document says:
For most data acquisition systems, an amplifier is desirable to buffer the sensor signal from the any loading in the data acquisition system.
First of all the document adds resistors to overcome deadband issues and shorts ect. and this introduces error. But the same document suggests buffering. Most of data acquisition systems have at least 100Meg input impedances which would not cause loading. What can be the benefit of buffering here?