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Noob needs help building a circuit board

Discussion in 'General Electronics Discussion' started by Michael Dammer, Mar 9, 2017.

  1. Michael Dammer

    Michael Dammer

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    Mar 9, 2017
    Hey guys. So I'm trying to build a speaker for work. I have the basic stuff down for it, but I want to build a circuit board that will charge 18650s and allow me to use them as a power bank to charge my iPod and power my speakers (don't worry about the correct voltage going to the speakers, I gutted old computer speakers and they came with the PCB to run them). I would buy one online, but I am too inpatient to wait for china, and I want the satisfaction of building it myself. What I'm looking for from you guys is a shopping list and a diagram

    Thanks in advance, if this is in the wrong section please point me to the correct one
     
  2. Audioguru

    Audioguru

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    Sep 24, 2016
    The voltage is very important. If it is too low then the sound level from the speakers will be low or very distorted. If the voltage is too high then the pcb amplifier circuit will blow up.
     
  3. BobK

    BobK

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    I doubt you can build it as quickly or as cheaply as you can get it on Ebay or Amazon. There are plenty of vendors even on Ebay who ship in the U.S.

    In fact, you will not be able to buy the 18650s for less money than you can get a power bank. If they are less, they are likely counterfeits that do not have anywhere near the stated capacity.

    Bob
     
  4. Michael Dammer

    Michael Dammer

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    Mar 9, 2017
    Most powerbanks I've seen are always being complained about do to not having the mAh they claim. That aside, I'm not worried about dropping a few bucks on some decent LG HG2 18650s. My goal with this radio is for it to power my iPod (which has to be plugged in at all times or it shuts off) and my speakers for up to 10 hours at least for while I'm at work. So I'm thinking if I wire 6-8 HG2 (3000mAh a piece) parallel, that will give me 18000-24000 mAh. Which I would think should be more than enough. Is there a way or formula to figure out how many mAh I would need to power said device for said amount of time?
     
    Last edited by a moderator: Mar 9, 2017
  5. Michael Dammer

    Michael Dammer

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    Mar 9, 2017
    How do I figure out the voltage I need?
     
  6. davenn

    davenn Moderator

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    if you buy an amplifier module as it has been suggested to you .... it will state the voltage required
     
  7. Audioguru

    Audioguru

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    If the computer speakers have a Name Brand then info about them is everywhere. But if they are cheap ebay junk then good-luck.
    On the web you search for the owner's manual of the computer speakers and the manual will say the voltage of the power supply (or USB 5V power). The power supply for the speakers will have a label showing its voltage. The speakers will have a label showing which voltage to use and the polarity of the jack.

    To calculate the mAh of the battery you need to know the average current used by the amplifiers, how loud you play them and the duration.
     
  8. Michael Dammer

    Michael Dammer

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    Mar 9, 2017
    I'll be using usb 5v, the speakers say 8ohm. 50w max. I found a calculator online, but it said to run this set up (just the speakers mind you) I would need 100,000man. That's seems high
     
  9. Audioguru

    Audioguru

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    Sep 24, 2016
    Many speakers are rated at 50 Watts. You need to know what is the real maximum power rating.

    If an amplifier is powered by 5V USB and is perfect and produces an output with no voltage loss then its voltage swing is 5V peak to peak. But the amplifier is not perfect and might produce only 3V peak-to peak. Then the voltage is 1.07V RMS which is 0.14W into 8 ohms. Almost half of that power is heat so the audio output is only 0.05W per stereo channel. Some amplifiers are bridged and produce 3.5 times more power which is 0.175W per stereo channel.
    Most USB outputs can provide a maximum current of 0.1A then the 5V power is 0.5W. An amplifier uses almost half of that power getting warm so the audio output power is about 0.15 W per stereo channel.
    Some modern USB outputs can provide 5 times more current. Then the amplifier output is 0.75W per stereo channel.
     
    Last edited by a moderator: Mar 11, 2017
    davenn likes this.
  10. davenn

    davenn Moderator

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    not going to happen from USB power
    read very carefully AG's post
     
  11. Michael Dammer

    Michael Dammer

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    Mar 9, 2017
    So I hooked the power wires to a male usb and then connected it to a wall outlet (which since both the wall outlet female usb and a power bank usb should both put out 5v and similar amps, if not more from my powerbank, there shouldn't be a difference) and it fired right up, played as loud if not louder than I want it to, no issues. Am I missing something here? (not being a smartass fyi)
     
  12. davenn

    davenn Moderator

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    the term "loud" means nothing in this case as you have no idea of the actual power being generated
    that is ... not a scientific result ;)

    if it works ... and you are happy ... good :)

    a powerbank or charger USB will output more than a PC USB
    they are capable of up to a couple of amps for charging

    a PC USB as AG said is nominally 100mA and can be upped to 500mA under special conditions ( device related)
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    There can be several reasons for this. Two of them (which don't rely on the mAh rating being wrong are):
    1. The mAh rating is that of the battery, not of the output.
    2. Misunderstanding of what mAh means
     
  14. BobK

    BobK

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    What AK said is true of analog amps, but class D amps can do much better. They can swing to nearly ±5V with little power loss to heat. That is about 1.5W into an 8Ω speaker.

    Bob
     
  15. Audioguru

    Audioguru

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    Sep 24, 2016
    The Moderator edited my post. I always say that ads rate speakers and amplifiers with phoney Whats instead of real Watts. I said, "Many speakers are rated at 50 Whats".

    I bought a little name brand amplified speaker system that was on sale and when I saw the ad say 150W then I knew it was 150 Whats not 150 Watts. Inside the small power transformer was labelled 9V/1.1A (9.9W) and I knew that almost half of that was heat so the audio power was only 6 real watts. But it sounds great.

    The difference in loudness of double or half the power is small because our hearing's sensitivity is logarithmic.
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I understand what you mean, but you need to be explicit about the meaning of your bespoke terminology or it will just cause more confusion.

    I went back and read both versions of your post, and the unedited one only makes sense to me if I have information that was not available at the time.

    I should point out that while I didn't edit your post, I don't consider the edit unreasonable in that light.
     
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