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.
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A normal RX has to take quiet some abuse if I see that :)
Update: We replaced the voltage regulator on the board, and the the message still appeared intermittently but it did not affect the simulation but eventually the issue came back and the autopilot would no longer fly the plane correctly in Xplane. I also compiled the 2.75 firmware with HIL and while we don't see the "bad gyro health" message it definitely is not functioning correctly. So we thought the new regulator was blown. However, to make this issue even more confusing, when running the regular firmware 2.78b(not HIL) the gyro appears to work just fine and respond to all movements and update APM accurately.
So is this an issue just in the HIL firmware?
I'm going to try and find small probes to test the regulator and also different firmware versions.
Background: We were running HIL firmware 2.78b on an official 3DR APM 2.6 when the "bad gyro" issue initially occurred, over the summer of 2013 we had tested the board many times with the HIL sim successfully.
Since BECs are always discussed I was wondering how different models would perform and just grabbed all different BECs that I had laying around here to do a quick test during power-up with no load. Assuming that a no-load condition would cause the greatest overshot.
I also hooked up a Fluke multimeter trying to catch the MAX voltage value.
As result I would say none of those three showed any extreme bad behavior and none overshot more than expected.
I also uploaded all pictures here: http://1drv.ms/MWz8in
First sample was a Castle Creation 10A BEC - slowist rising voltage with aprx. 80ms to come close to the 5V but also the most accurate with 5.074V max caught on my Fluke multimeter.
The CC BEC wins the most accurate 5V price!
Next was the famous HobbyKing LOW RF NOISE 5A BEC. It had the weirdest rising curve of all three starting first slow and getting steeper towards the end. With 20ms a quarter of the time needed compared to the CC BEC.
I did notice that the voltage jumper was set to 6V when taking it fresh out of the bag. So my first test actually ended up being close to 6V - thinking that BEC really overshoots that bad - but that was because the jumper was in the wrong position.
With 5.316V max the HK LOW RF NOISE BEC was also the most inaccurate BEC of the 3 units I tested.
Last one was a HK SBEC 26V 5A. This one was the fastest of all three. Almost instant 5V. It overshot a bit before settling at 5.127V max.
So I would say this one was middle class for accuracy but extreme fast - don't know if that is such a good thing for whatever you want to power with it if you are slamming it with sudden 5V. I assume that would also create an extra inrush current on your device while for example the CC BEC gives a very mild power on.
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.
I was reading through the recent posts and realized (at Robert L.'s posts) that there is a over-voltage protection if APM's PM input is used but if one uses an external BEC powering the board directly (from input side of the board for example) then there wouldn't be any over-voltage protection in case the BEC fails and lets the high voltage pass through...
So my question is that how does this over-voltage protection circuitry works? Say for example, you are using 3DR PM and something happened and PM started feeding over 6volts to the board. Would the over-voltage protection circuitry protect the board?
How? for how long?
Alex the over voltage protection is done with a 500mW zener diode that has a nominal voltage of 6V, which is D4 in the schematic. If you feed it over the output header 5V over JP1 or the power module connector JP_Vcc you will go over the polarity protection diode D1 and the fuse F2. A zener diode basically gets a low resistance once you go over the rated voltage and more or less makes a short circuit which in return is supposed to blow the fuse F2. Since you loose over the polarity diode D1 and the fuse a few millivolt I guess you would need to feed at least 6.3V in order to trigger it. The same thing would happen for over-voltage on the USB port just with a different fuse (F1).
The protection is destructive because it will blow the fuse and since the zener diode is rated at only 500mW probably die the heroic protection death if the fuse is not fast enough :)
If you feed the APM over the input rail you bypass the fuse and that for depending on the power of your 5V supply kill the zener diode in case of over voltage. All that will most likely happen with a blink of your eye. After the zener diode died and your supply voltage did not drop you might damage the rest of the board as well if he voltage was high enough.
Thanks for the explanation Detlef and Philip. How would all this apply if only USB is powering the board?
Is there any protection when the board is hooked up to computer via USB?
The same thing would happen for over-voltage on the USB port just with a different fuse (F1).
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