Using a Pixhawk with 6S Batteries


I thought I should post this I am 99% sure this page is wrong and part could be dangerous. I am talking about this page

Using a Pixhawk with 6S Batteries … batteries/

The part that could be dangerous and I am sure is wrong is using a 90A Autopilot sensor to measure up to 150 amps. they say the resistor selection is set to measure 150A but as far as I can tell it is not. the resistor that is used on the 90A boards (actually all the autopilot boards) is LVK25R001FER, if you look at the datashet you will see the LVK25 series is rated at 2 watts. there are 2 of these in parallel on the 90a board giving you, two .001 ohm 2watt resistors in parallel = .0005 ohm 4 watt resistor. so to figure max amps for this circuit is
Imax = sqrt(Pmax/R)
so the resistor can take sqrt(4/0.0005) = 89.5 A

If you draw over that (89.5A) for long, well it is out of spec’s at 150 amps. could be dangerous.

the other thing that I think is wrong is the voltage divider shown on the same page shows a divider on the output of the current sense line, this one made up of R1 and R2

at first I thought this was a good idea, till I tried it. It did not work for me. I wanted to know why. If you look at the autopilot circuit at the bottom of this PFD … Current/DC Voltage and Current Sense PCB with Analog Output.pdf
you will see there is a 73.2k ohm resistor between current sense and ground, when you add this voltage divider you end up parelleling R2 with this 73.2k resistor. the 73.2k resistor is used to determine the gain for this circuit, I believe this divider lowers the gain very low, probably (if I am correct) from a gain of around 73 to a gain around 15 which will ot give the output expected.

actually the divider is not the best way to go to convert the autopilot 90a to 180a, just add 2 of the LVK25R001FER resistors, in parellel with the 2 that already on the board, one on top one on bottom. it is the oly difference between the 90a and 180a versions of the board (that will correct the scaling too so the divider is not needed.
I just thought someone should look at this page and correct it.



John, we can set you up to edit the wiki and make the changes. Just PM me if you are interested.