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5v from 12v is this viable?

Discussion in 'General Electronics Discussion' started by Truckdrivingfool, Dec 6, 2016.

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  1. Truckdrivingfool

    Truckdrivingfool

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    Sep 30, 2012
    I have some cheap usb bluetooth receiver dongles, I want to splice one into an LM386 amp to make a BT speaker to plug into a 12v automotive port.

    It works in ltspice but being a noob I want a 2nd option if this is a viable/good way to do it. Is there a simpler way to do it? Do I need the cap on the 5v supply side? Finally whatever 6.2v zener should work but I may need to adjust the resistor to achieve the correct voltage - correct?

    [​IMG]
     
  2. Staffan Cronstrom

    Staffan Cronstrom

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    Dec 6, 2016
    Simply use a linear voltage regulator. At least in among others TO92 package there are variants with fixed 5V output. Variants in SOT23 may need one or two external resistors to set the output voltage.
    /SC
     
    davenn likes this.
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    I would be very surprised !!

    well maybe just 12V is the max for a LM386N-1 the LM386N-4 can handle 18V max



    what 5V side ? the LM386 is seeing 12V !!
    All that stuff to the left of the 12V (V2) PSU isn't doing anything
     
  4. Audioguru

    Audioguru

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    Sep 24, 2016
    Your poor little LM386 low power amplifier IC will smoke and burn if you try to have it drive a 4 ohm speaker loudly from a 13.8V supply in a car. It will overheat with only a 10V supply and a 4 ohm speaker but it will produce mostly heat but only a little output power.
    The LM386 is designed to drive an 8 ohm speaker with no more than a 12V supply and will have an output power of 0.5 Watts like a cheap little clock radio.
     
  5. davenn

    davenn Moderator

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    true, but in all fairness he though he was supplying it with 5V from his regulator cct. Except, as I said earlier, it's in a position where it isn't doing anything useful ;)
     
  6. Audioguru

    Audioguru

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    Sep 24, 2016
    The stuff on the left side is supposed to be a 12V to 5V regulator to power the Bluetooth dongle but the 100k resistor barely powers the zener diode with no current for the transistor. Try 680 ohms.

    The datasheet of the LM386 shows it getting very hot driving a 16 ohm and 32 ohm speaker with a 16V supply.
     
  7. Truckdrivingfool

    Truckdrivingfool

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    Sep 30, 2012
    First off when I said noob I thought the clueless part was included thanks for not holding back in your replies.


    Thank you for that nudge in the right direction, how about an L78L05 for the 5v side?

    Yes the stuff to the left is for the dongle the 85K was the best I knew how to represent the load. Is there a better way to do that?

    Also to clear up a bit more I drew that up and remembered my speaker ohms wrong I will be using 8 ohm speakers. I will probably use the Little Gem Mkii for the amp itself as I've made many of those for guitar practice amps and had good results playing MP3 through them.

    Perhaps a better representation with your suggestion added.

    [​IMG]

    Still wondering if I was to use the above zener/resistor/trans for regulation would it need the C7 cap? (in the first schematic)
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    C7, yes.

    Now you have a bridge connection of LM386's so they're going to dissipate more power.

    In addition they are not protected from noise on your car 12V system and probably won't last too long. They may also pick up noise from the engine.

    edit: also, not having a DC path to the inputs used for the signal of those LM386's is going to cause problems. Perhaps a 100K resistor to ground would be a good idea?
     
  9. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Yeah, 5v via the emitter straight to a 12v supply!

    Ebay for a cheap 12v audio amplifier
     
  10. Audioguru

    Audioguru

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    Sep 24, 2016
    Now both of the LM386 amplifiers will overheat and melt. 10 or 15 years ago there were 76 bridged car radio amplifier ICs available but today there are only a few left. They have a metal tab for a heatsink to be bolted on and deliver 14 clean Watts to a 4 ohm speaker.

    You 12V to 5V transistor has its collector and emitter connected backwards.

    Why did you add the Jfet to the inputs? Each input on an LM386 already has a 50k resistor to ground and your bluetooth dongle should be able to drive the resulting 25k ohms.
     
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    and as I commented to the OP earlier, 12V is the max for a LM386 (unless the LM386N-4 is used) so at 13.8 - 15V from the car battery, they are going to fail pretty quickly
     
  12. Audioguru

    Audioguru

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    Sep 24, 2016
    When two amplifiers are bridged and feed an 8 ohm speaker then their output current and heating are doubled as if they are separate and each has a 4 ohm load.

    The little LM386 is designed to drive an 8 ohm speaker to 0.5W output. A car radio bridged amplifier produces 14W into 4 ohms, a huge difference.
     
  13. Staffan Cronstrom

    Staffan Cronstrom

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    Dec 6, 2016
    Take a look at the LM675 Power Operational Amplifier. Use the 5V to the Bluetooth circuitry for the "ground potential" for the circuitry around the LM675. Certainly good to tap via an RC lag.
    Remeber the input and output capacitors.
     
  14. AnalogKid

    AnalogKid

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    Jun 10, 2015
    85K is an equivalent USB load of 60 uA. That is so low that you don't really need a pass transistor.
    Decrease R4 to 1K.
    Decrease D1 to 5.1 V.
    Delete Q1. Take the 5 V output from the D1 anode.
    Keep C7.
    Add 0.1uF in parallel with C7.

    ak
     
  15. Truckdrivingfool

    Truckdrivingfool

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    Sep 30, 2012
    Thanks for the replies folks I think I'm learning something here, I'll start out this round by saying the components I'm using (correctly or not) is because it's what's in the parts drawer. (for the most part)

    I'll go back to Staffan's suggestion and ask again an L78L05 will work for the 5v side and how about the 9v version to keep the magic smoke in the LM386s and hopefully alleviate 12v source noise?

    That's in reference to the 1st schematic and is the job of the 10k R2 in the 2nd correct?

    Schematical typo thanks for pointing that out, I could also Ebay a cheap BT speaker and be done with it but the wife says I need a hobby.

    I'm working from pictures on the internet here, as I said that's a guitar amp circuit from said internet and have used successfully for both guitars and playing mp3s with an adapter and and cord. Being as the best I've found a guitar pickup puts out up to .5 volt and when measured my phone's output was about the same I figured it was necessary. If not then that can be less parts out of my stash and into the final build.

    As for the speakers I appreciate your words on matching them and depending on what I have space for I will either use two 8ohm in series or find a 16ohm.

    That's way over my head for now, perhaps once I get making fire with sticks I'll move on and come back to LM675, I would be grateful if you could look at the above posted ICs that I asked about in reference to your first suggestion, as I think I see why it was the first and probably best reply to my original question.

    What does this do? Would it not just add to the capacitance of C7? From the rest of your answer can I assume that Q1 is just there to provide current capacity(probably not the right way to state it) correct?
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    There is more magic here than you understand at present.

    To put it simply, low value capacitors have a better high frequency response and thus paralleling a smaller capacitor will help remove high frequency noise.
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    No, it would be on the other side of the capacitor from R2. But as Audioguru has already pointed out, the LM386 has this resistor internally, so it's not needed (I missed it when I read the datasheet).
     
  18. davenn

    davenn Moderator

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    Sep 5, 2009
    if your 5V side is only drawing around 100 - 200 mA a 78L05 will be ideal ( TO92 package)
    if you are needing/expecting to draw around 500mA to 1A then use a standard 7805 in the TO220 package

    a much better solution for the LM386 supply :)


    Dave
     
    Last edited: Dec 9, 2016
  19. Audioguru

    Audioguru

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    Sep 24, 2016
    The Li'l Gem amplifier with two bridged LM386 amplifier ICs driving an 8 ohm speaker and a 9V supply draws about 250mA. The maximum from a 78L09 is only 100mA so you must use a larger 7809 with a little heatsink bolted to it. Its datasheet shows that it needs an input capacitor and an output capacitor.

    The LM386 already has input resistors to ground so a 100k resistor is not needed. The 10k resistor is the important source resistor for the Jfet.

    I am glad that you understand that the transistor was backwards.

    The high input impedance of the Jfet will allow the electric guitar to resonate and boost at about 5kHz and make the "twang' sound. Why will you play an electric guitar in your car??

    Two 8 ohm speakers in series or a 16 ohm speaker will probably not be loud enough. Use one 8 ohm speaker, the Li'l Gem bridged LM386's amplifier and a 7809 9V regulator.

    Q1 and the zener diode are a 5V regulator. You never told us the current needed for the Bluetooth dongle so Q1 might burn up if the current is higher than about 50mA.
     
  20. Truckdrivingfool

    Truckdrivingfool

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    0
    Sep 30, 2012
    I know this is getting all over the place from the original topic so here's the important stuff:

    Main lesson learned depending on needed current I can get to the correct voltage from just a resistor and diode to an IC. So some ICs are on the way for the project and parts drawer.

    Filter capacitors - need a better understanding

    I'll report back with a final product

    The rambling:

    In the Li'l Gem circuit R2 the source resistor is what sets the gain of the Jfet correct? and R1 acts as a filter?

    Since I'm running it from a system that gets charged this intended version the efficiency doesn't matter to me as long as it doesn't burn up. Future versions however will likely be run from six AA batteries(9v) and it will be an issue so using an IC in that case should gain some efficiency correct?

    I've done versions with one and two 8Ω speakers and get good volume. Your comment about the speaker matching hit home as my ear tends to prefer the versions I've made with two in series.

    The videos are more about the instruments but these are both made with amps with two 8Ω speakers

    The amp portion starts about 2:05 followed by a sound demo

    Clip of a different amp using the same circuitry and speaker type
     
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