Frequently asked questions are about powering an APM2.5 or why the same does not work or why it has failed.
Here is a simplified schematic showing the major components of the 5V/Vcc circuit on the APM2.5
Target Audience: Readers who do not have Eagle or are confused about using it.
You Need: Basic electronic schematic reading ability
Note that I have separated the PWM Output section into the upper left corner with a heavy purple line that is bridged by the jumper JP1 on the APM2.5. This is to emphasize that diode D1 passes current through JP1 (and drops voltage). The remaining energy is then called 'Vcc'. Everything outside the purple corner is Vcc.
This is why the USB will power the APM2.5 when connected to a computer/USB hub. Note the fuse on the APM2.5 side of the USB connector AND another fuse on the output of D1. Another change from APM2.0 is the D4 6.0V Zener diode. If an over voltage is applied to the board, D4 will crowbar what it can to protect devices.
As with APM2.0, any 5V source connected to any of the indicated Vcc/5V points outside the purple corner (with GND of course) will power the APM2.5.
Also of note is the 'Mystery Port'. This port takes 5VDC before D1 and the fuse and is called 'JP_VCC'. Other clues to the mystery are the lines to 'ADC 12' and 'ADC 13'. A mystery indeed!
Any errors in this drawing from editing are mine.
I hope this can be used as reference for solving problems.
-=Doug
27-FEB-2014 Eagle files are now located HERE
Replies
Dear Doug,
before testing the above as you mentioned I would like to tell you that yesterday night I replaced my APM unit with another new APM 2.8. Now, when I connect the battery and everything connected to APM, in the same measurement points (between the top and bottom pins of an Output pair) I get around 4.9V and it fluctuates until 4.5V. So in the Mission Planner Auto Analalysis I get a Fail for Vcc because the Difference between the Max and Min Vcc voltage is bigger than 0.3V. So things are better than before but still not as it should.
By having replaced the APM and still having those Vcc voltage fluctuations is it maybe that something else is the problem ???
I will make my tests as you mentioned in your previous post.
Thanks and Regards,
Interesting result and probably indicative of a quality issue on the 'first' APM unit.
If you have read upstream of this thread, you would see that the D1, F2 path is often a failure point for various reasons. The good news is that you are using ESCs with opto isolated inputs (no BEC). There have been several incidents where ground paths/wiring have melted due to either accidental shorts or other excessive current draw.
Even though APM is open source, the suppliers of the product are diverse and do not all hold the same standards of quality control that 3DR has *developed*. That is not to say 3DR was 'born perfect'. It has taken them time to hone and move toward a high quality standard.
I also understand the aspect of cost regarding these projects. If a student (or fixed income adult) has limited funds, looking to save cost is important.
At this point, it would help to know your background/education/work skills. I wouldn't want to drag you through heavy engineering stuff if you don't have the interest.
-=Doug
The 3.3V circuit was not part of this original thread. Many builders/makers were having trouble understanding the connections and limitations of the 5V circuit - many, many of them. Therefore this thread was developed and I was lucky enough to have it live here for going on 3 years. That is an eternity as compared to many topics.
Xbee was one of the original wireless technologies used by APM. It was displaced by what is now called the 3DR Radio (a couple versions until current version, but essentially the same).
I am not certain that the TPS79133 regulator is the type on *your* APM. You will have to check.
This screencap is from the APM2.5.2 Eagle set I have. It is hilarious to me that the G-search for the part has images of APM as part of the results. It has only 100mA of current supply. Take care what you try to do with the 3.3V from the chip. Welding is not recommended. ;)
Better, IMHO, to add a second higher current regulator for 3.3V to source other devices. Simply use either the Vcc 5V at a convenient location to feed the new 3.3V supply.
-=Doug
When you say 'out put pin' are you referring to the regulator pin or some other pin on the APM? I suspect if you can remove/replace this device, you know the pin-out of the device is not the same as the drawing representation.
You are correct. If Vcc is ~5 VDC on the input, the output should be 3.3VDC. If the aircraft were flying properly, arming, etc. it should have been OK to do it again. Unless you omitted part of the story.
I am confused by your explanation regarding the 1.77 volts.
-=Doug
The obvious answer is that a short exists on the board or outside it, between the two points.
1) Test with no connections/wiring to the APM (regulator still removed)
2) Meter on ohms setting and check the resistance between the input and output pads.
3) Between the Vcc and the 3.3V lines, you should have many, many ohms - ideally infinity, open.
If you have a few hundred ohms or less, something else has failed or a trace has been bridged.
Is this a 3DR APM or another brand?
-=Doug
If the replacement regulator is not a low dropout type, it may only pass the Vcc input to the output.
Can you breadboard one of the replacements and demonstrate it working to regulate 3.3V with Vcc input?
Odd problem.
-=Doug
I had heard about another problem with a level shifter. It was many months ago and I cannot remember the symptoms.
Keep at the effort. Your work will help someone else I am sure.
Well done.
-=Doug
Shashikumar - wrong spot for this question but, my initial opinion is your ESCs are not calibrated properly with APM.
The calibration can be tricky at times depending on your ESC and TX.
This does, of course, assume your motors are spinning in the correct direction for their assignments and that your propellers are on the correct motor and mounted in the correct up/down orientation on the motors.
-=Doug
There is a subtle difference between '5V' and Vcc in APM2.6.
5V is defined as the connection at JP3 ---> JP1(closed)---D1---F2--- then it becomes Vcc afterward.
The PM supplies JP_Vcc----JP1(open)---D1---F2--- same as above.
If JP1 is closed (jumper in) and a PM is used, the risk is potential voltage/current mismatch from the ESC/BEC and PM. They share a common ground(earth) and this is usually the failure point with the wire overheating and the insulation melting.
Another method is to simply remove the BEC output wire from the ESC. This way no mismatch can occur.
The condition does get your attention quickly. ;) :0
-=Doug
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