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Topic: 240 Volts ac variable voltage measurement (Read 1 time) previous topic - next topic

pbendel

Hi, how would you build a circuit that measures the voltage of a variable speed fan where the voltage varies from 240 volts ac at 100% fan speed to about 75 volts ac at 40% fan speed?  I don't want a non-linear response from a transformer, accuracy is important to me.  But a one second lag in response time would be fine.  I am stocked with rectifiers, resistors, and capacitors, but not a working cicuit....

robtillaart


Warning 240 volts is dangerous - you probably knew that allready - both for humans and Arduino's. So use an optocoupler.

If you attach a small 240 V lamp parallel to the fan and an LDR to the analogRead() from the Arduino you have an optocoupler that follows the voltage.

To solve the non linear mapping of the light to voltage, you could use - http://arduino.cc/playground/Main/MultiMap -

Another way could be: (you should check the numbers first.
=>> AC to DC   (some diodes, and a capacitor or so) followed by a voltage divider 50:1 [1 M? - 20K? ]  and you will get ~0~5V

my 2 cents...
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

el_supremo

Quote
accuracy is important to me

How much accuracy and why is it important?

Pete

Constantin


Hi, how would you build a circuit that measures the voltage of a variable speed fan where the voltage varies from 240 volts ac at 100% fan speed to about 75 volts ac at 40% fan speed?  I don't want a non-linear response from a transformer, accuracy is important to me.  But a one second lag in response time would be fine.  I am stocked with rectifiers, resistors, and capacitors, but not a working cicuit....


Well, I have used a AC-AC transformer with 1% results on a bog-standard 328P Atmel running the Arduino IDE and using the openenergy monitor basic sketch + suggested hardware (a few resistors, a cap, and an external AREF source to maximize the current sensing happiness). A transformer seems like a wise investment considering the voltages involved and the isolation it brings to your circuit.

jackrae

What makes you think a transformer is a non-linear device.  The turns ratio of a transformer is what determines the ratio of input to output and this is physically fixed during manufacture.  The series resistance/inductive reactance of the secondary winding will introduce some voltage loss when you draw current but for sensor purposes you may assume the current draw is near enough zero so the voltage loss is also zero.  Using a precision rectifier circuit on the transformer output will eliminate any diode loss.


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