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
You should see a slight difference between the output of the PM and the Vcc points on the APM. It should only be the drop across D1. F2 should not drop voltage -- in a perfect world.
It could be that your F2 and/or D1 is dropping voltage. Leave your meter at the (-) neg connection of the PM and measure base on this point of reference. Move the (+) pos lead to the input of D1 first. It should be the exact same voltage +/- .001V . If not, then there is some bad connection in either the neg PM lead or the pos PM lead going to the APM.
If good going into D1, then check the output of D1, it should be slightly less than the input. Then move to the output of F2. Again, only move the pos (red) lead, leaving the neg (black) lead at the same reference point.
I suspect a poor solder joint due to quality issue.
Do you have a good magnifier or microscope to inspect the components?
I have marked up the simplified schematic to indicate how the power module is supposed to relate to the APM. The area circled in green above is where I suspect you are losing voltage.
You can do this test with nothing plugged into the APM except the PM.
If there is a bad connection in either the JP_VCC path or the GND path (not likely) you will drop voltage across it, but I still think it is being lost in the D1 F2 area.. probably.
-=Doug
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
Just come back from a flight. Plugged in my apm 2.6 to download the logs,
but cannot get it to connect, checked the 3.3 regulator and getting 4.8 volts on the out put pin
so I removed it to replace with new one thought I would check the output on the 3.3 pad on the board.
Still get 4.8 volts? How can the be when no regulator is present.
Also noticed that if no power while multimeter probe is connected to output pad then plug usb in I get 1.77 volts.
replaced the regulator and still 4.8 volts.
Dave
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
Hi,
Yes I am referring to the regulator.
I get the 1.77 volts while the apm is first plugged in boot up.
"REGULATOR IS REMOVED" for this test.
If I probe the board the input VCC pad I get 4.86 Volts witch is right, but the 3.3V pad I get 4.68 volts witch should be 0.0 Volts without the regulator in place
So why am I getting 4.68 volts on this pad if no regulator installed?.
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
Its a china one been a really good control till now.
Right I have probed the pads and I am getting 1.5 Mega ohms,
Seems OK to me.
Checked all diodes and they are fine.
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
-
3
-
4
-
5
-
6
-
7
of 15 Next