5.23 Volt output signal pwm APM2

Hi There,

As I understand this so far my Rx gives a PWM signal thats 2.90V peak and varies it in width from 1.0 to 1.8 mS (without trims etc) and my APM2 does about the same at 5.23Volts.

None of my servos respond to this signal.


             Min              Neutral             Max                Peak V

APM      1.2ms             1.8ms            2.0ms               5.23

   RX      1.0ms             1.4ms            1.8ms               2.90

All the first 4 outputs vary in width properly but they are the wrong peak voltage!

Any ideas anybody?

Thanks John Fletcher

Tags: PWM, output, voltage

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  • Hi There,

    Well, the solution goes like this :-

    First, installed the power is normally applied to the 5V rail on the OUTPUT side of the APM. APM1 does not care which side it's powered from and works even if the receiver is powered and the servos still work properly and everything is OK. APM2 does not work like this, powering the INPUT side or the receiver causes the APM to function normally and 5V to appear on the output 5V bus, but there is no power only volts. As soon as it's loaded the voltage disappears.

    This is a difference in design between the 2 boards, it's taken quite a few hours to discover this!

    The signal levels really are the way they are described in the table, and there really is a voltage gain of around 2.3V.

    It does not matter what state the link at JP1 is in for this issue.

    I really hope this helps someone and that it's of interest, APM2 is quite different to APM1.

    Many Thanks

    John Fletcher.

  • John,

    I seriously doubt that is the issue. The servos should accept a normal 5volt PWM signal. Further, common sense says the outputs of the APM cannot be higher than the input voltage so what are you powering the APM with? Obviously it's higher than 5 volts. Most servos will take 6 volts, but it's not normally recommended, and meanwhile in a normal RC plane or heli, the receiver would also be at 6 volts putting out 6 volt PWM, so either your servos are damaged or there is something else going on. Again, the APM is going to put out what you put into it and it had better not be over 5 volts. Maybe you have a 6 volt BEC?

    By chance are you powering the APM via USB? IF so the outputs are NOT powered so while the signal is there, 5 volts to power the servo isn't. IF you measured the signal and it's there on the output pins, then the servo should work period.

    It's also not clear how you "measured" both pulse width and voltage of the outputs. Even based on your own measurements and assumptions, the signal scenario is that the servo expects a 3.3 volt signal and your APM is sending 5, so the problem is the low part of the 5 volt signal isn't read as a 0 or low thus the servo just sees the APM signal at high all the time. I seriously doubt this on so many levels. My big concern is that we KNOW there is a voltage drop through the mega 2560 outputs, so if the voltage is that high, your input into the APM is even higher.

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