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Need help powering up 2 circuits on same battery

Discussion in 'Power Electronics' started by sideburn, Jan 7, 2014.

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  1. sideburn

    sideburn

    75
    2
    Jun 14, 2013
    Hi all,

    I have a circuit that runs off a USB charger pack. One board is a Raspberry Pi computer. I am sending a PWM signal out the audio port and then connected to the low output headphone jack I have an LM386 amplifying the signal and then the output is fed to a servo.

    So my digital PWM signal is driving the servo by way of the audio out from the computer.
    Everything works great execpt I am unable to power the LM386 and servo off the same power supply it shorts out or something.. I think it is related to the input coming off the headphone jack. But I have my grounds wired correctlty.

    Is there a way to isolate this and power them both off the same pack?
     
  2. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    Hello
    What servo are you using? Even small RC servos can take several hundred mA of current to start up. Could it be the circuit board / USB power supply or regulator can't supply enough power and is shutting down. What supply voltage are you using for the servo. I don't know what the audio output circuit of the PI is but if it is a dedicated audio device then sending digital data through it might not work very well but I doubt this would cause it to shut down as the servo PWM input is reasonably high impedance.
    Thanks
    Adam
     
  3. sideburn

    sideburn

    75
    2
    Jun 14, 2013
    I can post more detail pics but here's a video of the circuit
    The pwm signals for the servo com out the pi's 3.5mm audio out

    The servo is powered by the 3.6v AA size battery.
    The other pack is a 50000mah 5v USB battery.
    If I plug into that there's a short and everything shuts down.
    So I think it's a grounding thing.



    I'll try again with the audio jack unplugged and confirm.
     
  4. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    Make sure you are not powering the servo from a lower voltage level than the processor is running and also make sure when you do this you are within the max voltage of the servo . This could cause current to go into the lower voltage battery from the PWM pin. I don't know the insides of the servo but if they have protection diodes at the front end then this will cause a problem but still not sure that this will cause what you are seeing. I would like to think the micro would have current limit on it's pins. You may damage the servo if your not careful. I can't make out much from the video but what's that connected to the other USB connector not the mini one where the power is.
    Thanks
    Adam
     
  5. sideburn

    sideburn

    75
    2
    Jun 14, 2013
    I am boosting the power for the display from 5v (which is what the cpu is running at) to aorund 6 or 7 volts. Im using that to also try to power the LM386 and servo..
    so that is what you see going to the other USB connector, It is a Buck boost converter.
     
  6. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    Ah so the PWM is 5 volts going into the servo powered from 3.6V. This might be an issue. Also looking at the data the data sheet for the LM386 it says its output is set at half supply at rest. So this would indicate it might be used for controlling the power to the servo not the PWM signal. Try and use another port pin for PWM and power the servo from 5V. Most servos will run at 6Volts so you should be fine.
    Just a thought

    Thanks
    Adam
     
  7. sideburn

    sideburn

    75
    2
    Jun 14, 2013
    Well not exactly. The PWM is coming from the low power AUDIO output of the 3.5mm headphone jack. The signals are embedded as the audio track of the video movie that is playing.

    I am then using the LM386 to amplify the very low audio coming out the Rasberry Pi.
    that amplified signal (i think we are still way below 3v) is then fed directly to the servo from the LM386 output.

    I have attached some pictures.

    The pic of the LM386 and Servo being powered by the separate 3.7v battery is the setup that is working.

    The pic of the 3.7v battery removed and power coming from the same USB battery pack is the setup that is NOT working.

    This is a picture of the setup working as you can see the audio out (containing the PWM waveform) from the Pi, is feeding into the input of the LM386. The LM386 amplified the PWM audio signal and drives the servo:

    2014-01-08 15.32.13.jpg

    2014-01-08 15.32.25.jpg

    In this picture, the 3.7v battery has been removed and power is connected to the LM386 and Servo from the same 5V battery pack (it has 2 USB ports and is a 50,000 mah battery) that is powering the Raspberry Pi and Display. The servo does not work in this scenario:

    2014-01-08 15.33.46.jpg
     
    Last edited: Jan 8, 2014
  8. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    I think I know what it might be, the USB power supply in the photo has two labels on them which look like the maximum current. The large servos peak current for a fraction of a second could be over this. The protection circuit within the power supply could be over sensitive to transients like this and is shutting down, then the servo switches off and the whole cycle starts again. If this is the case then a small value resistor of say 2R 0.5W-1W in series with the power to the servo, just cut the wire and solder between the two ends. If this doesn't work double the resistance and try again.
    Try this and let me know how you get.
    Thanks
    Adam
     
  9. sideburn

    sideburn

    75
    2
    Jun 14, 2013
    I will try. But the servo is plugged into the 2.1A output on that battery.
    I would be surprised if the servos is drawing more than that.
    What is a 2R value resistor?
     
  10. sideburn

    sideburn

    75
    2
    Jun 14, 2013
  11. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    You still might get greater than 800mA for a split second and this could cause the issue. It would also explain why it works with the single battery. A 2R resistor is a 2 Ohm. It might be decoupling problems, but without having the circuit here I can't tell. Link to a 1R resistor buy a few from Maplin or your local electronics store, start with 1 and then add more if this doesn't work. . What about the cables carrying the power, could they be an issue they look quite thin.
    Thanks
    Adam

    http://uk.rs-online.com/web/p/through-hole-fixed-resistors/7078546/
     
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