I recently upgraded to arducopter 3.0.x on my APM2.0. I have run through all the calibration, including radio, compass and accelerometer. I can arm my quad fine when connected via USB, disconnect USB and fly with no issues. However if I cold start and attempt to arm without the USB, the red LED continues to flash. The blue LED is solid, indicating GPS lock.
I have bypassed the pre-arm safety check in EEPROM, and I can arm without issues. I cannot use APM planner to look at the pre-arm safety check reason, as doing so prevents the problem manifesting.
To me, it smells like a voltage problem. The 5v from the USB is enough to get through the pre-arm checks. Unfortunately I don’t have a multimeter with me to check right now.
TCIII: Powered from ESCs on the output pins. I had J1 installed.
I’ve think I’ve identified the problem after a bit of poking around with a multimeter. A while ago I blew the schottky diode attached to J1, I replaced it with a “Schottky Diode 30mA 30V SSMini2-F5-B” from RS, and a bit of modwire (+1 to open source schematics).
Unfortunately for some reason this diode is showing a 1.5v forward voltage drop, instead of the 0.4ish it should have. This brought the input 5v rail down to 3.5v, causing it to fail the voltage part of the pre-arm check. Plugging in USB boosted the voltage up enough to pass the check. I’m surprised anything worked at that voltage.
I suspect I’m seeing the higher drop because the replacement diode is in a different package, it’s probably got a lower current rating, and it’s dropping more volts than it should. For now I have removed J1 and put a jumper lead between the input 5v rail at the output 5v rail. I’ll just have to remember to remove it when powering from ESC and USB.
If that Schottky is really only rated for 30ma, it is not the part to be replacing the original Schottky diode. The original diode is rated at around 500 ma and a forward drop of around .3 v.