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#40C Output Signal Loading

David McNeill

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I am designing a custom interface circuit to process the AC sine wave signal from the 40C anemometer. I measured the winding resistance of the anemometer to be around 600 ohms, so I am ensuring that the input impedance to my circuit is significantly higher than that so that it doesn't load down the anemometer and affect the wind speed.


Do you have any data relating to loading on the anemometer output and how it affects the calibrated wind speed? Also, I am investigating the use of the 40C in an energy harvesting application where I would, by design, have a low impedance load to extract the small amount power generated by the anemometer. It would help greatly to know how that affects the wind speed and whether I would need two sensors or would have to multiplex between the sensor as a measurement device and as a power generator. Thanks for any help,


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Interesting approach...

The  #40C will generate AC power by the nature of its magnet-coil design, albeit at very low power. For example, in the 5 - 7 m/s wind speed range it will output a 500mVAC signal with about 500uA of short-circuit current. At 250 micro-Watts, this isn't a lot of power, but I suppose it could be harvested for low power application.

 We don't have readily-available information on how the anemometer's reported wind speed is affected by electrical loading. In R-NRG's application of this anemometer, the output is driven into a high-impedence input, so the electrical loading is insignificant.  Our manufacturing process and use of the sensor only concerns the presence of AC voltage from the coil; we do not calibrate the voltage level of the output per given frequency, only the frequency output per given wind speed.

Loading the output of a #40C electrically would render the provided calibration parameters inaccurate, and it is safe to say that the loading effects characterized for a particular sensor (from testing) could not be applied to another #40C, given that the voltage amplitude character of individual sensors is not captured or calibrated.

Generally, I'd suggest using two anemometers; one that is loaded, and another unloaded. If you would prefer to use one anemometer then you would have to perform individual voltage/wind speed characteristing testing for each anemometer. This would probably include varying the wind speed and loading in structured testing.

I would guess that electrically loading the sensor's output could affect the  slope/offset calibration parameters by as much as 25%, but I have not done any testing on this.

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