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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Dear Goud,Bill,

I have installed the BEC as we said according to the schematic of my previous post, and here are my findings..

When I connect APM to Mission Planner, my hwvoltage value is now almost steady playing between 4.90V and 4.94V. So everything good.. Before the BEC install the hwvoltage used to fluctuate between 4.45V and 4.81V..

However when I measure again with the voltmeter in the same points as I used to do before installing the BEC, you can see in the following pictures my new measured values...

SO it seems like the hwvoltage value has been fixed but the voltmeter values have been damaged...

Please give me your comments...

Thank you

The ground needs to be connected from the radio to the APM board as well. GND must be there for the signal lines as a reference.

I'm not sure why it's fluctuating but probably means 5V from another BEC is connected. Do you have ESCs with BECs connected. You may need to removed JP1

Dear Bill,

ok i can try to connect the ground from radio to APM as well... 

I dont have any other BEC connected.. I am using the DHI E310 propulsion kit that uses the DJI 420S ESC. As far as I know those ESC's do not contain any BEC's... 

My JP1 jumper is already removed...


The ground (neg) is the common reference point for *all* electrical signals or power sources.

Essentially the PM point you are referencing *should be* the same as directly on the APM. If it continues to provide a difference after all grounds are connected, suspect resistance in the wiring.

Resistance (unwanted) is usually a bad crimp or solder joint - a quality issue.

Resistance is *intentionally* added in electrical circuits with a component called a resistor.

Good Luck!


Dear DOug,Bill

I now connected the ground from radio telemetry to APM (see below image) as well but the situation remains.. My measured values are almost the same

Is this a dangerous situation or as long as my hwvoltage value is steady 4.94V i could fly as is ??? 

How should I identify the resistance on wires ??? 

Exactly what BEC (actually just a DC to DC converter) are you using? Does it have jumpers to set input/output voltages?

What is the battery voltage on the red and black BEC input wires? (connect your meter to the red and black input)

Remove the cable from the HK radio to APM and measure the voltage on the BEC output wires, red and black.

Would the hobby shop you purchased the items from be able to assist you if you took the aircraft there?


The only reason for the voltage to fluctuate is if there are two competing voltages connected together. Can you provide a more complete diagram and or pictures showing the wiring?

Dear Bill,Doug,

sorry for my late reply but I was away from home for business..

I attach a picture that shows a more complete connection diagram (sorry for the quality). I did try to fly the quadcopter as it is as the Vcc value now is in an acceptable way if you also agree..

I also attach my yesterday;s flight log file. I would appreciate if you could give me your feedback as you are more experienced.. What I understand is that my quad is in the limit of its payload ability. A few more grams and it will be overweighted for good.

Also, can i be confident with y mechanical parts (motors) that can fly for many hours or do I have to replace them every some hours ??? I have DJI E310 2312

Best Regards,


This look like it should be fine and if you not are experiencing any issue, I say fly it!

Log analysis is good if you have problems....IF the parameters show the issue.

Putting the radio on a separate BEC should allow you good performance.


Dear Doug,

thanks for your comments..


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