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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Agreed, that's why I chose 100uF, small enough not to cause too much grief on the Power module or USB, but big enough to safely help in this case.

looks good :)

I agree with you on that one!

I have not done any testing (I ran out of regs now) but on my workstation that I have usually used for configuring the APM and hot plugging stuff I have a external high power multi USB port and never had an issue. This week I use the first time my laptop with a non powered weak external multi USB port and have in no time blown almost 2 regulator.

Its a good idea to harden the APM against those things even if its for something you should not do since it seems as simple as a cap!

Detlef, Philip, Randy,

Is the 3300uf cap iskess linked above same or better solution than using 100uf ceramic cap?

Do you guys have an answer for this?

Never mind guys. My question is answered below. When I was posting my question, the below posts were on the next page and I didn't realize...

Going with the 100uf cap. for sure if that is what we need...



Paul, there's absolutely nothing wrong with what you've written here.  This is the essence of open source.  Getting different perspectives on these issues.  And we don't always have to agree, as long as we can disagree nicely.

BTW, your link is broken?


independent of APM or not if you you need such size capacitor to protect you against brownouts then I would rather opt for a bigger/better BEC in your RC system. If you believe that a big size capacitor helps to safe your model with issues in the wiring like a bad contact - then such things show up when it gets that bad that the capacitor will be not be big enough no more to buffer those connection failures. Because before you won't notice that you even have a wiring issue.

Besides that it is during power-on at first a full short circuit for your power system in my opinion size is not helping unless you have a real odd load with occasional high currents on your RC system that you want to smooth out. Whatever load spike you pull out of your buffer capacitor your BEC has to put back in it the next moment.

Rob -

My link was to a pretty detailed build post in the RCG APM 2.5 thread started by jabrams, where I ask for further help evaluating my LT1117-3.3 mod. He evidently felt my input wasn't of any value and decided to delete it with extreme prejudice, along with another post where I suggest that buffer caps across the power rails at any plug-in header is commonly accepted sound engineering practice. This even after admitting he doesn't fully understand the engineering practices I refer to. :rolleyes:

As you may imagine, I'm more than slightly annoyed. ;) However it is his thread and he's allowed to commit as much revisionist history on it as he sees fit. This is the weakness of the open forum environment; folks can start a thread ostensibly as a knowledge base, then delete relevant material on a whim. I'm much more annoyed at myself though; for not keeping a copy of my hard work elsewhere. Lessons learned.

In all honesty, I don't feel we are disagreeing that much in general; both our work generally leads to two important points, even though they come at it from different angles.


(B) Buffer caps at the power rails of any plug-in header is a good idea.

(3) Any questions, See (A) above.

Simply adding those buffer caps may be all that is needed; with them in place, the additional clamping zener may not even be necessary. But I feel the buffer caps need to be soldered in place to be properly effective; when you are dealing with ESR this low, plugged-in just doesn't cut it, especially over the long haul.

I understand needing to make a Plug & Play solution for those who don't want to solder to their expensive APM board; but we should offer a solder-in recommendation for those who are willing to do so for more reliable results.

The only point we really disagree on is the choice of regulators in the design; my perspective is that "If the old tried and true will work, why replace it with something much more expensive? Who cares if it is 10x as much as we need; it's 10x cheaper and known to be nearly grenade-proof." and that this issue with the TPS79147 supports that.

The devs felt that the TPS79147 was a better choice as it is intended for the expected operating loading, voltage range, and sensitive devices attached to it. As I said, I'm not privy to all the reasons for the choice; I gladly concede that. I just wanted to know that reasoning so I can evaluate whether the LT1117 is in fact an acceptable break-fix repair substitute given the unforeseen failures of the TPS79147 in the wild. I'm most concerned with issues of whether the power it provides is stable and clean enough for the sensors it is attached to; I've tested that as far as I know how to, and really need the help of those who have more experience with this product and have better test equipment than I have to answer that.

I promise; what I'm interested in here is NOT placing blame, as seems to be the nature of much of the discourse around this issue. I just want to come up with a simple, durable fix for those oodles of us stuck with the resultant broken boards.

We can't RetCon the design; these boards have been deployed. We CAN come up with a revision that makes it so we only have to fix it once; I'd like to parallel the existing development re: buffer cap and clamping diode with testing of the LT1117-3.3 as a substitute, but that requires handing it off to those who know the product better than I. In that, I can only hope I've offered sufficient evidence that it is worth their effort.

I'll recreate the lost work and post links here; the mod is REALLY easy to implement and easily reversible. This was why I felt it was worth the time if I can get the interest of someone able to do further testing; not a lot of time or effort is required to set up the test mule.


I read your post - I don't know whats not to like about that write-up you did other than that it was not jabram's idea... I guess you can't make everyone happy :)

I thought it would be a nice alternative fix for a broken board for someone who wants to go a more sturdy route!?

Unbelievable.  That's actually what I thought might have happened.  I won't say anymore to avoid igniting another war.  I feel bad about what happening at RCG.  But there's nothing I can do to stop it.  I tried, but had the exact same results as you.

As I mentioned, I've been soldering caps to the APM power rails for a long time.  Just seemed like a good thing to do.  As for changing the regulator, I don't know the answer.  My guess is that it might work just fine.

Nice work! 

The main article could do with a little more clarification of problem, cause, and solution though imho.

Okay... I've recreated the lost work and added it to my Blog at RCG, where I'm reasonably sure it will remain intact.

Any suggestions?

Any volunteers to carry on my research?


Rob - I agree... and in retrospect, if you look at it from a different perspective, my work actually tends to support his assertion that more is required to fix the board than just a buffer cap at the power rails. What the duck?!? As you say... "No mas..."




Okay... I tried to add a reply to my original post here: Comment # 1590992 to let folks know where to find the deleted material, but it wound up all the way at the end of this page, where clearly it doesn't do any good. I'm just going to formally ask this here then:

I've started testing on a substitute regulator here: LT1117CTS-3.3 Regulator Mod and I've taken testing as far as I can with my resources. I am now asking for electronics-savvy volunteers who are equipped to abuse-test this mod and report back if the regulator provides adequately clean, stable power for the various sensors that are commonly used with the APM 2.5.2 and clone boards.

I have several of the required regulators on hand; I'll gladly mail one to anybody interested in furthering this research.

Thank you for your time,




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