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.



27-FEB-2014 Eagle files are now located HERE


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Thanks again for helping to clear up and simplify/de-mystify potential questions.


Thanks for posting this Doug!

I cannot connect any longer APM 2.5 to Mission Planner  Board is recognised baud rate set ok  but Times out 

No Heat  spmething    message

I notice that the diode just above the IC2 socket has a small pin hole    maybe blown  yet seems ok with multimeter 

any one with the full schematic   

I think the ESC calibration caused this  connect disconnect Lipo send spikes all over the board especially with jumper JP1 on   it would be better to power the APM with a separate supply  no ?   Am I wrong???   help 

ok do you think by powering the APM2.5 with the jumper JP1 on meaning that the power comes from the ESC could this be the cause of a failure of the board when a ESC synch is performed connect disconnect etc to the LIPO pack

Is it not then advisable to have a separate supply maybe 3.3 v to the board to prevent this type of failure . my board doesnt communicate "connect" to the mission planner I notice just above the silkscreen IC2 a pin hole in that diode marker ..I guess...3.3 ???

do you think further damaged could occur ???
Don't know what you mean by IC2 socket! Anyway if there is a pinhole in a diode then you can be pretty sure it's not functioning right!

If the diode in question is the supply one next to the 2 pin jumper then it's not a vital component, just pull the jumper and supply the board from the input side 5v0 -+0v5 (where are you intending to supply 3v3 to?)

Much safer (and easier) to calibrate the escs when they are disconnected from the APM altogether...

The full schematic is available on the 3DR Store page. NOTE: V 2.52

You will need Eagle to view the *.brd and *.sch files.

The feature schematic is taken from the full schematic with minor modifications to help the 'flow' of the circuitry.



He is referring to the I2C socket ('I' squared 'C'). See below's pic.


Doug, Thanks for clearing that up, obviously a language problem!

D4 is a 500mW 6v0 zenner across the board supply, presumably in the case of severe over-voltage it's supposed to blow the 500mA fuse, Not surprising the fuse won!

vitaly, Enough voltage present to blow the zenner, It could be that your board is fried!
That diode is not a vital component to the working of APM, more intended as an over voltage safety feature.

I would be very surprised if plugging in ESC's would have caused this! The APM is protected by not being connected to the raw voltage of the battery, The bec output should just rise to 5v0

Sorry was out...back now and still trying to get the board to work..

yes it is that diode  there is a marking on it E  cannot read the xxx as there is a pin hole 

but looks ok if test with my Fluke  as a diode should be  I have version 2.5.1 board  I have founf the schematic as well as v 2  I think you sent me the link.

I just cannot understand why?

could it be that I plugged the USB to my laptop at the same time as the ESC's and borad powered from the Lippo  (with jumper Jp1 ON ?????)   

I think the chip is ok  it looked like a burn but was some flux on it....

it is actually diode D4 


i could replace it  but would diode stop the board to communicate via USB ???

Peter has it dead on.
D4 is a 'last resort' protection of the APM Output lines in the event the BEC/Battery exceeds 6 VDC.

Vitaly, because it seems your D4 is damaged, even though a static Ohm test seems OK, your device may be doing the job at the wrong voltage. D4 is connected to the 'Vcc' line. 'Vcc' also supplies the 3.3V regulator for the logic families that use that power.

Things to do:
1) Remove the BEC/ESC input and check the power output that was connected to the APM. 5.1 VDC best!

2) With BEC disconnected, connect USB and measure the Vcc value. It may be 4.75 to 5.1 VDC; depends on the PC and connection/wire resistance.

3) If Vcc is OK, APM should respond OK. If Vcc is very low.. less than 4.1 VDC, D4 should be *removed*. If Vcc OK (4.75 to 5.1 VDC), D4 was problem. If Vcc not OK, something else is bad.


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