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Old Jan 08, 2012, 08:09 PM
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Voltage sensor not reading correctly on ER9x w/FrSky

I got a FrSky D8R-II with my er9x setup and I got the voltage sensor plugged into A2...

I go into the telemetry page and set A2 to 7.4v for a 2S lipo battery.

I go into the main screen and on the telemetry screen, it's showing me 2.78

Now I know my battery's charge is 3.75 and 3.76 per cell, totalling 7.57 or something... why is this screen showing me 2.78? I don't understand...

I gotta bring the voltage upto 19.8 on the A2 screen for it to give me 7.4 which isn't right obviously

I don't think I'm doing something right...

the voltage sensor hasn't been changed, it's on the 'ground' and 3S prong. apparently it's good for upto 5S (19.8V) like that.

it says to use the 2S one for upto 13.2 volts

I actually tried out a 3S battery on the sensor and it read off as well... (6.3)
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Old Jan 08, 2012, 08:23 PM
Balsa addiction since age 3
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Upstate NY
Joined May 2005
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Quote:
Originally Posted by razorseal View Post
I got a FrSky D8R-II with my er9x setup and I got the voltage sensor plugged into A2...

I go into the telemetry page and set A2 to 7.4v for a 2S lipo battery.

I go into the main screen and on the telemetry screen, it's showing me 2.78

Now I know my battery's charge is 3.75 and 3.76 per cell, totalling 7.57 or something... why is this screen showing me 2.78? I don't understand...

I gotta bring the voltage upto 19.8 on the A2 screen for it to give me 7.4 which isn't right obviously

I don't think I'm doing something right...

the voltage sensor hasn't been changed, it's on the 'ground' and 3S prong. apparently it's good for upto 5S (19.8V) like that.

it says to use the 2S one for upto 13.2 volts

I actually tried out a 3S battery on the sensor and it read off as well... (6.3)
19.8 or 19.9 is what mine is scaled at... with stock FrSky 3S sensor position.
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Old Jan 08, 2012, 08:27 PM
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Boca Raton FL
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scaled at? so that's normal? that's really weird!

so this is normal to scale up to 19.8 to get it to read what you want? if I start dropping the voltage on the battery, it will also go down by 1:1 ratio?

I'd hate to see it drop by let's say .2 volts for every .5v my battery loses ya know?
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Old Jan 08, 2012, 10:18 PM
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Originally Posted by razorseal View Post
scaled at? so that's normal? that's really weird!

so this is normal to scale up to 19.8 to get it to read what you want? if I start dropping the voltage on the battery, it will also go down by 1:1 ratio?

I'd hate to see it drop by let's say .2 volts for every .5v my battery loses ya know?
That unit works up to 5S setup and uses a 6:1 resistor divider setup...

Max voltage at the receiver is 3.3V which is why the voltage divider is needed and the analog input is resolved to 8 bits as I read their manual.

I believe you can resolve to approx .1 volt RESOLUTION ON YOUR TRANSMITTER display/readout with this setup, do you need better?

If you use the 6 Channel receiver with built in input on receiver voltage, you will need a different scale factor on that input.
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Last edited by ScottSails; Jan 09, 2012 at 09:15 AM. Reason: left off the RESOLUTION ON YOUR TRANSMITTER
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Old Jan 09, 2012, 06:50 AM
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Quote:
Originally Posted by ScottSails View Post
That unit works up to 5S setup and uses a 6:1 resistor divider setup...

Max voltage at the receiver is 3.3V which is why the voltage divider is needed and the analog input is resolved to 8 bits as I read their manual.

I believe you can resolve to approx .1 volt with this setup, do you need better?

If you use the 6 Channel receiver with built in input on receiver voltage, you will need a different scale factor on that input.
sorry, little confused =/
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Old Jan 09, 2012, 09:09 AM
Balsa addiction since age 3
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Upstate NY
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Quote:
Originally Posted by razorseal View Post
sorry, little confused =/
The receiver can only physically handle 3.3V max, it releases its "magic dust" if input voltages exceed this level - ie either clips the voltage or burns up. The setting you have used (default) on the voltage interface (which is really a resistor voltage divider) is a 6 to 1 ratio. So 19.8V on the red interface lead is reduced by a factor of 6 to 3.3V. 12.5V is reduced by a factor of 6 to 2.083V (examples).

In any case, this receiver 3.3V input (max) is converted to a range from 0 to 255 by the analog to digital converter (8 bit conversion) in the receiver. This 8bit (0-255value) is then transmitted by the receiver to the transmitter. The software in the transmitter needs to know the proper "scaling" to convert the digital number back to something that make sense to you and I instead of the 0 to 255 number. So in this case, with the 6:1 ratio, we tell the SW that 19.8 is the scaling factor for max voltage.

The resolution is roughly 19.8V/255 which is .077 volts (minimum change) so you will see ~.1 volt increments as the Lipo voltage drops down.

Make more sense now?

Scott

You can attach your DVM and calibrate your reported voltage even better since the SW lets you tweak the 19.8 value to 19.9 or whatever you like to allow your screen voltage to better match the DVM value since I am sure the resistors are not precision. You should only have to make small tweaks.
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Last edited by ScottSails; Jan 09, 2012 at 09:38 AM.
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Old Apr 14, 2012, 12:41 PM
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thnks Scott for this input, makes perfect sense now
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