This is a discussion re the bad Accel, Gyro and Baro values that we're seeing with ArduCopter-3.1. The increase in the SPI bus speed from 500khz to 8Mhz has exposed a hardware problem on some boards. That hardware problem is that the 3.3V regulator has been blown so all sensors are running at 5V instead of the intended 3.3V.
How have these regulators been burnt out?
- Attaching a radio receiver or MinimOSD to the APM while the APM is only powered through the USB (see video below)
- Some clone boards seem to come from the factory with blown regulators. 3DR boards might also come with blown regulators although they do a specific check of the regulator as part of the regular QA process.
- It is not (as far as we know) actually caused by the AC3.1 software itself, it just exposes the problem. You could prove this to yourself by checking the 3.3V regulator (see video above) before and after the upgrade.
How can we fix the regulator?
Option #1: If it's a new board (so that it's less likely you burned it out yourself) you could report the problem to the retailer that sold you the board and ask for an replacement. If it's 3DR it's called an "RMA".
Option #2: if you're handy with a soldering iron you can replace the regulator yourself. On the APM2.5.2 (and higher) boards it's not that difficult. On the APM2.5 it's far more difficult.
For APM2.5.2 : TPS79133DBVR
For APM 2.5: MIC5219-3.3YML TR
How can I stop it from happening again?
Do not connect any devices such as a radio receiver, MinimOSD, GPS, etc while the APM is powered especially while powered only through the USB cable.
Attaching a 100uF capacitor across any of the APM's radio input's 5V and GND pins will stop the regulator from being blown by plugging in a receiver. video here!
There are very few reports of regulators being blown twice and no reports of it ever failing in flight.
Below are some graphs of the types of values that we are seeing on these boards.
Replies
Thanks for doing some real testing. How fast is your scope?
A couple things that would be great to try if you could:
1) Try to connect the BEC such that it generates a long spark. Same on disconnect.
2) Are your power wires long or short? There's a connection between the wire length, inductance, and spiking on inrush current.
3) I believe having some capacitance at the load also causes more spiking?
You should try this again, but on the output of the BEC, put 6" of wiring, and maybe 10uF of capacitance. Then attach the probe to the capacitor.
I guess it's great if these BECs are well behaved, but it still leaves us scratching our heads.
Here's a discussion I started last year about the issue with servos.
http://www.diydrones.com/forum/topics/helicopter-avionics-power-supply
It's an even bigger problem on helis which tend to use big, fast, powerful servos, and they often all move at exactly the same time.
My board blown twice. Yes it is.
Basically, if you use the 3DR power brick, there is a Diode, a fuse, and a Zener to keep the voltage below 6v
Unfortunately this protection only protects from small spikes and assumes that any over current caused by clamping the Zener will be protected by the fuse and diode on the PM input.
If power is connected via the input rail, it is applied directly to the vcc rail, so if more than 6v is applied for too long, D4, the Zener, can be caused to heat up, and fail.
This removes all protection, and will allow the 3.3v reg to be exposed to any vcc spikes.
Now, the Servo rail is connected to the PM via the jumper, so a BEC connected here will be protected from over voltage, but unfortunately we have discovered another source of very nasty voltage spiking.
We have found voltage spikes coming from Digital servos (of various brands). In our testing spikes up to 11v were coming out of the servos. While the jumper is connected, these spikes were managing to get through to the 3.3v regulator, displayed as spikes well over 6.0v.
So I found that connecting a 1A 5.1v Zener across vcc, and powering the board with the 3DR power module protected from digital servos feeding back into the board. (I had the digital servo on servo plug 11 for this test, but keep in mind, you are never meant to power a servo from the vcc rail at that point)
If you use this Zener, ensure that your Bec has an average voltage below your Zener, or it will overheat)
This situation could also be replicated by powering a servo directly off your RX, while the RX is still connected to the APM.
So I hope this gives an idea as to how NOT to connect your APM.
It may be totally unrelated. But I recently lost all sensors (at least test ins and test compass results in the MPU6000 failed to initialise error) after updating my genuine APM 2.5.2 board. The board had flown several times with no major issues. It wasn't on a frame when it died. I've checked the 3v3 reg though and it's not blown. We've bought a few Pixhawks now, so hopefully they won't suddenly die on us.
The board is available for free to anyone that wants it (shipping at recipients cost from Australia). I would just like to know what the problem is and likely cause.
Well. Thanks very much to Philip for his expertise. This issue with my board was simply operator error/ignorance.
I'd parked a jumper between what I thought were GND pins, turned out I had connected Vcc to the SPI MISO pin (they're in the same line as the headers marked GND) effectively killing off all the sensors. It still works though so Philip is kindly sending the board back. Hopefully the last time I assume that parking a spare jumper on the board is a good idea :j
Two pins at edge of board to left of the USB header
The offer was accepted and is therefore no longer available. Thanks.
Hi. I already shipped to Philip Rowse.. Thanks for your interest though. Hopefully he might figure out what went wrong so we can avoid these seemingly common problems with quality of APM.
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