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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Hi every one. I am newbie to Arduino, APM and all this stuff. Are there any way to connect xbee directly to APM2.5.2?

Can i use APM GPS port for it? I checked the schematics and seems ok. What is the purpose of 3.3v regulator in the APM2.5? It seems i can get 3.3v supply to Xbee but not clear about the voltage levels of TX and RX. Do i need a level converter ? I need to test my Xbee S2. For the moment i don't have breakout board and cannot have in the near future. I do not want to connect any other modules only xbee s2 and APM2.5.2 for a small experiment.

This 3.3v is not for external devices... You will break the APM if you do that :)

 You will break the APM if you do that

Sure?. May i know the purpose of that 3.3v regulator. I read somewhere that xbees use very small current. Also found some schematics show xbees directly connected to Arduino.

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.


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.


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.



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?


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.



I think I have tracked it down to a level shifter.

Cut the track on the vcc side of the TXB0104 and and checked the pad of the 3.3 and the volts was 0 volts, so I soldered the new regulator in and connected usb got 3.3 volts happy face.

but wont boot up just get red led next to usb LED7 I think.

maybe due to the TXB0104 going bad.

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.



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