Supply Voltage reported by XR8 Receiver

[mpjf01]

Out flying Sunday and a friend who was flying with a Taranis asked me to come over and read out his RSSI level to him as he had received a low RSSI warning. While doing so I noticed that his A1 voltage was reading high, 9+v from memory. After he landed I looked at his telemetry settings and the range was set to 13.2v. We had to adjust it to 7.2v to get the Taranis to display 5v, the BEC supply voltage.

I have looked now at my stock of 5 XR8 receivers and measured the relationship between the voltage being fed to the receiver and the voltage displayed by my Taranis. I have done the same measurements for 4 of my D series receivers that were easiest to access. I have also measured the relationship for one of my XR8s over a couple of hours. The figures are in the attached spreadsheet. Unless I have made some errors (I have the feeling that i have done something stupid) in my method (please tell me if so) it is pretty clear to me that the Rx voltage reported by Taranis in the case of the X series receivers is of little value and my alarms based on these are not reliable.

While this situation is true for my friend's 3 XR8 receivers as well, it is the case that they were acquired together in a single purchase and may represent an abnormal batch of product. Otherwise everyone would be noticing the same.

In looking at this I find that I am also curious why it is that the set alarm voltage changes with changes in the range. These changes are 0.3 to 0.5v. Limiting the range to a single decimal place also has the effect of preventing fine adjustment of the displayed voltage.

FRSKY RX volt measures.xls

(20.5 KiB) Downloaded 227 times

[Kilrah]

In looking at this I find that I am also curious why it is that the set alarm voltage changes with changes in the range.

The A1/A2 value received from the receiver is a number between 0 and 255.
Range defines what voltage 255 corresponds to.
The alarm is also stored as a number between 0 and 255, and is triggered at low level when A1 is smaller than the defined level. When you change the range you change the scaling everywhere. Precision is limited by the 256 steps.

[mpjf01]

Thanks for that. It makes sense when explained.

Any suggestions why all my XR8s are giving strange outputs?

[rotozuk]

Depending on what your voltage sensor is (you can make your own FBVS for example) you will want to adjust the offset on your telemetry screen (page 12 of 13 in the model setups) to reflect the correct voltage. This offset setting allows for voltage calibration. It is also a good idea to set your range and multiplier (FBVS) to a respectable level for better accuracy.

You will also want to double check the voltage on the Taranis battery itself, you can fine tune it's value also. (Voltage when off a full charge will be about 8.6 volts.)

Hope this helps.

-Wayne

[mpjf01]

Yes I am aware that I can adjust the offset, or the range, to reflect the correct voltage. The question is, what values do I use that will give me a reliable indicator of the voltage being supplied to the Rx?

In the case of the D series receivers, the range value of 13.2v gives a pretty accurate voltage measure back at the Taranis most of the time in my testing.

In the case of my 5 X series receivers the value that I need to use is not 13.2v, it varies from 13.35 (if it was possible to enter that) to 14v depending on the actual receiver being used, the voltage of the supply battery, the length of time the receiver has been switched on, some combination of these or some other factor(s) that I have not identified.

So the question is, why is the value of 13.2v that has worked so well for the D series no longer valid for me and why does the Taranis reported Rx voltage value change when the actual input voltage has not? How can I reliably set LV alarms when the information being used to initiate them appears to be wrong?

I wonder if the semi-secret firmware upgrades for XJT and X8R would fix it?