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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this link shows the main power bus and the reason for the 0.3V drop

The reason you are having an issue with the 500mW radio is it probably drawing two much power. The same happens with the RFD900 radio. You need to power it with a separate BEC from the APM, and not the power module.


if you connect a separate BEC, you will need to disconnect the 5V line on the molex connector to the board

Bingo Bill, I think you nailed it!

If I had paid attention to  the wattage rating on his first graphic, that should have triggered an alarm.

DN - Your first APM is probably fine. I apologize for missing the clue. At least now you have two units to work with and we can add another bit of knowledge to the community.

All my telem modules are 100mW units. With 5 times the power use, the issue becomes one of source (PM) current regulation. A similar situation can occur if users try to drive higher current servos in an ArduPlane setup.

Thanks again Bill! I think DN can stop the hunt.


Dear Bill and Doug,

thank you for your help.. I have two questions... 

1) given that my radio module has both molex and microusb ports(see image below),what if I power the 500mW radio through the usb port of the APM after I have disconnected the 5V line from the molex connector?? Is that OK?? I tested that and it shows now steady values like it shows with the 100mW radio(4.8V for the output pins and 4.5V for the input pins)... I'm  asking this because I find it more easy to just use a microusb cable instead of having to install a new BEC.

 2) in the arducopter wiki link that you gave me, I see in the table(see image below) that for the output pins the voltage should be 5V +-0.5  and for the input pins 5V +-0.25.

In my case it is the opposite. As i mentioned above my input pin voltage is 4.5V which falls inside the 5V +-0.5 margin  while my output voltage is 4.8V which falls inside the 5V +-0.25 margin. 

Is there a chance that there is a mistype of the margins in the table and they are written opposite??  If no, then my input voltage of 4.5V falls out of the 5V +-0.25 margin.... 

I hope my questions are consistent... 

Best Regards,

DN, In regards to your questions:

1) This should be the same as using the molex, just a different path. You are still connected to Vcc and will still draw more current with the 500mW unit vs the 100mW unit. From the APM side, Vcc is still connected to the USB port via fuse, F1. The intended use of the USB is connector is not to source current/power. The intended use is for the PC to supply power with no battery/PM connected to the APM. I cannot explain why your values are good after changing cables -- unless there is a significant difference in the USB connection of the 500mW radio.  In fact, if you use the USB as a power source, you are double fused from the PM to the device connected to the USB.


2) The section you show, I believe, refers to maximum input voltages *applied* to those pins, not sourced from the pins. Any device supplying power to those pins is restricted to the values listed.

This problem is simply one of power budgets.

The PM limits are discussed HERE. I made some quick grabs of features on that page and specs of your Telem and FrKsy Rec...

The two devices alone can load 530mA. The Rcv is probably powered off the PWM input rails, but if you power through the USB port, F1 is only a 500mA fuse. That is a risky situation to be in.

You use the USB to source power at your own risk, especially at near the rating of the 500mA fuse. I recommend a separate BEC for the 500mW Telem radio.


It's 1A fuse in later boards, though the 2.5.2 schematic its 500mA. I'm not sure whats in the HK boards. 
If you connect the USB power directly to the PM 5V out you bypass the fuse, and stop it voltage drop across it from being as large. That's the real issue with the 500mA part is that it drops quickly if your near the limit. 

(A silicon fuse is a diode that has really high resistance at cut--off current. the 500mA part has quite the incline, hence the voltage drop, the 1A part is much flatter, before the cut-off (knee))

Dear Doug and Bill,

so if i understand correct, if my fuse is 500mA the only solution is to use a BEC??? If it is a 1A fuse I could use the usb to power my 500mW radio module. Correct?? Is there a way to check what fuse I have ??

If I have to use a BEC is there somewhere a schematic or a video available that shows how to connect the BEC with the radio telemetry module?? I see that the BECs have a 3 pin servo connector.. How can I connect this with the 5 pin molex connector of my radio telemetru module ??? 

Thanks and Regards,

Bill, thanks for the update on the fuse change for the USB. Clearly the 1A device (silicon fuse as Bill pointed out) would be another variation of use for the USB connector. At this time, USB electrical limits are defined as:

As the current rating of the source goes up, the wire size must be appropriate to deliver the current.

DN, Bill mentions connecting directly to the PM (theoretical source current from the PM is 2.25A). This would require custom wiring from the PM to the USB connector or the nearest pcb pad/trace that can handle the current - this is the same as wire size mentioned previously. If the pcb trace is too small, it will overheat and possibly melt with excessive current.

We will have to dig around the boards for information on connecting your HK radio. I looked on the HK site and there was little that hasn't been taken from this site. The 'PM to USB direct path' might be the simplest solution.

THIS page from the Arduplane group shows an ESC powering the Output rail with an Xbee radio (older unit) and the +5 VDC to the Xbee noted as disconnected. Scroll down to the section called 'Example Wiring Diagram'.


Dear Doug,

I have created a schematic of how the radio telemetry, the BEC and APM should connect to each other... Is is correct ??? If yes, where should the negative line of the BEC's output be connected ??? 

Thanks and  Regards,

black wire from BEC goes to GND on the radio

Bill beat me to it.

The important part is not having the 5 VDC from the BEC going to the APM.

The 5 VDC from the HK radio could be cut or de-pinned from the APM connector.

I find lifting the latch on the connectors and pulling the pin easy and clean.


Dear DOug,Bill

in order to conclude, according to your notes the final schematic I should follow is this ??

Is this BEC's characteristics appropriate for my case ??? -->

Thanks a lot for you valuable help  

Yes, the power connections look correct. I have not confirmed the data connections, but if you are only modifying the power wires of the original cable, the data lines should be fine.

The BEC looks fine. You will have to confirm the input settings for your BEC. They may not be automatic and some BEC have outputs that are adjustable. Your choice looks fixed at 5V.

The 3 Amp rating (3000 mA) significantly exceeds the need of 500 mA of the radio.

Let us know how this works out. Good luck!


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