Thursday, November 28, 2019

Cheap and cheerful current sensor for Rx with an analog port

this might be useful for people wanting a very lightweight and compact readout of current on the receivers that have A1 and/or A2.

It's one of these things.

It replaces the XT60 connector on the speed controller so adds virtually no additional weight or bulk. Not bad for about US$8. The sense resistor appears to have an opamp to amplify the sense voltage. It puts out about 1.3V at 80 Amps which is well within. the 3.3V limit of the A1/A2 ports.

In answer to the questions:

1) The display resolution when displaying current values up to 80 Amps appears to be +/-0.2Amps. There is a bit of fluctuation at all values of about +/-0.4 Amps around the reading. I suspect there might be a bit of drift at constant high current caused by heating in the sense resistor.

2) The actual shunt resistor is 0.5mOhm so the dissipation at 80 Amps is 3.2 watts which is well within its 5W rating.

Attached is a calibration test.

Currents from 5 to 80 Amps at 5 Amp intervals were created using an adjustable 220Amp constant current load and a 12V 100Watt power supply.

The discharger was calibrated against a Medusa wattmeter at 10A using an accurate 10A FSD digital meter and then used the Medusa to calibrate the discharger up to 100A. The maximum observed error in the discharger was about 0.3% error. In practice the current values on the x axis are probably better than 0.5%.
It looks like this little sensor will give you a reasonable result for mAh consumed as well as the current.

I did a calculated consumption field in Companion but used A2 as the sensor source instead of the usual Curr.

I have attached a picture of the Taranis screen. At this particular instant, the load was a nominal 50 Amps but the Taranis and EmeterII were in close agreement at 48.3A and 2167mAh consumed.

I have also added a plot of the Taranis display vs. EmeterII measurement over the range 0-80Amps.

I suspect consumption is fairly sensitive to the Offset Value you chose when setting up the A2 telemetry field. With a bit of patience and mucking about with Ratio and Offset I think you would be able to get it even more accurate than this as the sensor seems quite linear. The secret I suspect is that once you have found a value of Scale that gives the correct slope you adjust Offset for a zero reading when there is no load. And of course you can set up a Logical Switch to have a Special Function play an alarm or track when the consumption exceeds a certain value.

On most of my models I have a voice reminder every 10% of pack capacity below 50% and then am brusquely reminded "It is time for you to land" when I have used 80% of the pack. 
XT60 sensor update 2

It looks like this little sensor is a bit more sophisticated than I gave it credit for. It doesn't just use a simple Op Amp to amplify the 0.5mOhm shunt resistor voltage.

The Package is hard to read but it is marked "B38" and it will be a Texas Instruments INA 138 current shunt monitor. I have attached the data sheet if anyone is interested.

The only question I have is whether the input impedance of the FrSky A2 analog pin is sufficiently high to avoid any need for buffering. I'm guessing by the linearity in the graphs that I plotted that it is.