Replacing the power diode on APM 2.5

I recently had a crash that bent a motor off the arm of my quad and am now only able to arm my 3DR quad when it is connected to my computer via USB. After some troubleshooting following input from folks on this forum I think the problem is a burned out diode. I assume the diode is fried since the quad does arm when I connect the two sides of the diode with a wire. That said, I still have some questions.

First off, can someone confirm that this is the diode I need to purchase: MBR120VLSFT1G

Does anyone have pointers for replacing this. I've never soldered anything that small or components without pins but am willing to give it a try.

Lastly I was wondering if someone could provide insight into some odd voltage readings I got when testing the diode. I was following the instructions and diagram on this link:

I tested the voltages as per the instructions and this is what I got.
Diode test:
Gnd to point A = 3.9 volts
Gnd to point B = 7.5 volts
Fuse test:
Gnd to point C = 7.5 volts

How can I get a higher voltage across the fuse than the voltage I measured at point A at the diode?

Any advice is appreciated,


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  • Purpouse of this diode?

    soo i came across a couple of apm clones with bad diodes. very large voltage drop as soon as any load (receiver, gps, osd) was attached. resulting voltage around 4.5v. causing osd to shut down and or be very unstable. Possibly causing some flight instabilities also. Replacing the diode seems simple enough. Just some careful soldering.

    But what i dont understand, is the purpouse of this diode. Comparing to a typical clamping curcuit, where the zener diode would offer an alternative path for excess voltage to flow to ground, this configuration offers no exit for excess voltage as far as i can tell. In the case of the apm, the diode is in series with the load. Causing the voltage drop. How does this protect the circuit?

    I'm tempted to simply bypass the diode as i have no spares on hand. Bad idea?

    • Hi Aku,

      I know it has been a long time since your last message but i just found your post.

      I have the same issue, and the same question in mind : 'could i just bypass the damn faulty diode ?'

      Did you find an answer ?


  • Hello I ve got a question concerning the two SPI connectors U7 of the Atmega 2560 and U8 on the AT32U2. Are those connected with each other or where do they go?

    For a better understanding of the whole electronics I tried to draw the schematic. Is this more or less correct?


  • Hi,

    I did not see the schematic (but you most probably did!) - I would have no reservations using the diode you found. It is Schottky and it can take 1 amp of current, perfect.

    To unsolder it really requires the right tools... the problem is to get both ends heated at the same time, else it will not move a micrometer. Ironically you may have success with a too large soldering iron that can reach both ends at the same time. Apply some solder to transfer the heat to and through the diode, and add a little more again to mix some lead into the lead free solder it was made with. You might then be able to carefully shift the diode aside with the iron. NO prying and pushing allowed, the pads will come off.

    Then clean off all the remaining old solder with fine solder wick and apply a little fresh solder on one pad. Solder the new diode onto that and then solder the other end.



  • Take a look at THIS discussion... I posted a new graphic of the circuit that should help you.


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