here at the second page: http://www.skyeinstruments.info/index_htm_files/W200P%20Windvane.pdf

there is an external 470k resistor in parallel with the measuring device. it seems like the manufacturer wants users to connect this before measuring. but doesnt data acquisition boards and other measuring devices have already huge input impedance? what is this resistor for?

  • \$\begingroup\$ I was disappointed to see this is made by a company in Wales, despite the name "skyeinstruments". But at least with a max windspeed of 170mph it should survive the Isle of Skye, as long as it's salt water resistant... \$\endgroup\$ – Brian Drummond Oct 26 '14 at 23:36
  • \$\begingroup\$ does 470k resistor make sense? \$\endgroup\$ – user16307 Oct 26 '14 at 23:38
  • \$\begingroup\$ i think that pull up resistor is to make the readings zero/gnd instead of floating. have a look at here: ftp.campbellsci.com/pub/csl/outgoing/uk/manuals/w200p.pdf. but i still didnt get why particularly 470k why not 800k why not 200 ohm for instance. \$\endgroup\$ – user16307 Oct 27 '14 at 0:54

[This does not contradict what EMFields wrote above. This is just another point of view.]

The purpose of the pull-down resistor is to give the output a known state when the wiper is in the gap located at the North.

Have a look at the figures from p.2 in the datasheet.

connection diagram

enter image description here

  1. Assume that the wiper is at NW. The output would be around 90%.
  2. Now assume that the wiper is turning clockwise from NW. The output will be growing.
  3. Then the output will reach 100% when it reaches the point T1 on the circuit, which is at 358.25°.
  4. The output will stay at 100% as the wiper keeps going clockwise between 358.25° and 358.85° .
  5. At 358.85°, the wiper becomes an open circuit.

If there is no pull-down (or pull-up), the output would be floating. It would act like an antenna for interference an leakages. The reading is unstable. The measuring equipment may even see 2.5V because of leakages and think that the vane is pointing South while it's actually pointing North.

So, the pull-down gives the output a determined state when potentiometer is open circuit.

Why 470kΩ ?
I think, it's a ballpark number, and it's driven by a compromise.
On one hand, you'd like a strong pull-down to reduce the effects of interference. Strong pull-down means smaller resistance value.
On the other hand, the pull-down is in parallel with the lower leg of the potentiometer, and it's introducing non-linearity into the measurement. So, a weak pull-down is desired. Weak pull-down means larger resistance value.

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  • \$\begingroup\$ is there a way to calculate that non-linearity effect? \$\endgroup\$ – user16307 Oct 27 '14 at 1:51

I'd guess it's to keep the impedance across the HI-Z stuff reasonable when the pot wiper goes open-circuit around North.

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  • \$\begingroup\$ but what happens when wiper is at deadband around north? what is the problem that 470k can take care of when the wiper goes open? \$\endgroup\$ – user16307 Oct 26 '14 at 23:30
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    \$\begingroup\$ With no resistor across the input to the HI-Z stuff, you wind up with a couple of long, insulated conductors which could charge up to a voltage high enough to damage the equipment. The 470k is at least small enough to keep a static charge from building up, yet large enough not to affect the linearity of the pot too much. \$\endgroup\$ – EM Fields Oct 26 '14 at 23:45
  • \$\begingroup\$ do you mean the insulated long cables will form a capacitor when the wiper is at dead region(open circuit)? and this capacitor will discharge on the HI-Z stuff and it will burn it? and when there is 470k the current will flow through this parallel resistor instead of HI-Z stuff because it is smaller? \$\endgroup\$ – user16307 Oct 26 '14 at 23:59
  • \$\begingroup\$ i couldn't find any info about this phenomenon in literature \$\endgroup\$ – user16307 Oct 27 '14 at 0:00
  • \$\begingroup\$ i think that pull up resistor is to make the readings zero/gnd instead of floating. have a look at here: ftp.campbellsci.com/pub/csl/outgoing/uk/manuals/w200p.pdf but i still didnt get why particularly 470k why not 800k why not 200 ohm for instance. \$\endgroup\$ – user16307 Oct 27 '14 at 0:56

I'm currently working on this very device and the 470K resistor is not just plucked out of nowhere. If you connect the device up to a micro controller you will notice that the readings close to or in the north gap are slightly erratic, even with a pull down resistor. The smaller the resistor, the more benefit will be gained but the more non-linearity introduced. If you can work out the non linearity then you can use a lower value resistor and get better readings. Although the calculation is based on easy parallel resistance maths the formula for % non linearity is not completely obvious: enter image description here

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