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Power bank with bluetooth speaker - power draw issues.

Discussion in 'Radio and Wireless' started by GamingAddict, Sep 5, 2017.

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  1. GamingAddict

    GamingAddict

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    Apr 26, 2017
    I yesterday bought a power bank for my bluetooth speaker that i made myself which can be ran off of any 5v source, and i had attached a USB connector at its end. I came back from the store, plugged it in, charged it completely and plugged the speaker in, and seems like the amplifier chip doesnt draw enough current to keep the power bank from giving up the output ( with phone it works like a charm ). So i had an idea to make a 1W ( or maybe a bigger one ) resistor with 1/4W resistors, but which value should i use, and what would be safe for the power bank to not go bad and the mini amplifier gets its desired current? The amplifier is a PAM8403 chip integraded on a PCB together with a bluetooth chip ( dont know which, cause the top of it is filed down ) and the speaker attached to it is an old woofer from Acme 2.1 speaker set.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,160
    2,548
    Nov 17, 2011
    You'll want a resistor that draws just enough current for the power bank not to turn off. Not more because the energy dissipated by the resistor is lost for the amplifier.
    1/4 W at 5 V requires a 100 Ω resistor (P = V²/R). Start with one 100 Ω resistor parallel to the amplifier's power connection.
    If that suffices, you're fine. You may then try 2*100 Ω in series or even more resistors to find the point where the power bank shuts off.
    If that doesn't suffice, use a 2nd 100 Ω resistor in parallel to the first one to double the power dissipation.

    The current to the amplifier obviously is of little importance as it seems to be so small it is insufficient to keep the power bank on.
     
  3. GamingAddict

    GamingAddict

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    Apr 26, 2017
    Well, a youtuber called Great Scott! made a bluetooth speaker with the same amplifier some time ago ( tho he made a charger and step-up board for the batteries that were 18650s with i dont remember what capacity ), he used 2 18650 cells in parallel config, and got out around 36 hours of continuous playback of music. I dont know how to calculate how much it draws, but in my opinion ( im actually guessing lol ), it draws a fairly low current from the batteries.
     
  4. GamingAddict

    GamingAddict

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    Apr 26, 2017
    Also, is there a way to make a 100ohm resister from another type connected in series ? Cause as i remember, i wasted all of my 100ohm ones last week
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    11,160
    2,548
    Nov 17, 2011
    Rtotal = R1+R2+R3... any combination of resistors in series that adds up to 100 Ω or a ballpark figure within that range.

    or

    1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ... any combination of resistors in parallel that adds up to 100 Ω or a ballpark figure within that range.
     
  6. kellys_eye

    kellys_eye

    4,275
    1,147
    Jun 25, 2010
    The device is a 3W amplifier with 90% plus efficiency so at maximum power it will draw around 600mA maximum (and probably only at peak signal levels).

    The amplifiers 'standby' current (known as the quiescent current) is some 10-15mA which should be more than enough to keep a power bank output working - this current level is around that drawn by a single LED which should keep the power bank output available.

    Do you have the actual specifications of the power bank? where it might actually indicate the MINIMUM current draw that is required? Slapping in resistors without proper consideration for the needed value will waste energy (flatten the battery quicker).
     
  7. GamingAddict

    GamingAddict

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    0
    Apr 26, 2017
    I have its manual, but it doesnt state any minimal draw thats required to keep it on. Also, i dont know if this is how you meant it, but it doesnt have a standby mode, you give it power, its on, you disconnect it from power, its off.
     
  8. GamingAddict

    GamingAddict

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    Apr 26, 2017
    I just came across this thread on a different web page - https://forum.pjrc.com/threads/28624-USB-Battery-Bank-Prevent-Shut-Down

    And the second comment states this -

    "Not All battery banks do that and they have different thresholds - I have some of each. Sounds like you need a dummy load to draw power to keep it alive. How long does it live before shutting down - like 10 seconds? You might get away with a dummy that pulses 70mA just short of that period for a second. You'd have to 'make' that - you could do it with a Teensy 3.1 or an LC in some fashion if that is what you were aiming for. The Teensy could stay alive and watch the clock and light some LEDS to get the current - or maybe warm some resistors?"

    I could maybe get some LEDs, drill holes in the back of the speaker thats a piece of acrylic and glue them in there, My speaker would work then, the back would look cool, and the power would not be wasted. Atleast kind of.
     
  9. Audioguru

    Audioguru

    3,116
    696
    Sep 24, 2016
    The woofer from a 2.1 speaker system probably sounds awful by itself which is why it uses a crossover network to prevent the highs from shrieking from the woofer and uses a tweeter for the high frequencies.

    The stereo PAM8403 IC produces 3 Whats per channel into 4 ohm speakers. Whats are with overdrive clipping so the distortion is awful at 10%. Without overdrive there is no clipping and the power is only 1.4 Watts into a normal 8 ohm home speaker which is very low power, a little more power than a cheap block radio.
     
  10. GamingAddict

    GamingAddict

    38
    0
    Apr 26, 2017
    Sorry for the late update, but i added 4 100 ohm resistors in parallel, and it seems to work fine. The resistors become room heaters, but i tested them, and they should withstand the time of me playing something on the speaker.
     
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