how to change 3V circuit to work with 3.7VDC battery?

Discussion in 'The Projects Forum' started by joefly, Apr 14, 2010.

  1. joefly

    Thread Starter New Member

    Apr 14, 2010
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    Hi All, I am trying to build a hearing aid for my father. I have gone through many trials of different boards, finally found one that seems to work well, but the problems is that it was designed to work on 3VDC and I want to use 3.7VDC LI-ion rechargeable batteries also to integrate it with other circuits that I am combining it with. I got the board built and with 3.7VDC it works fine but the chip gets very hot quickly, if I use 3V then it seems to be fine. so the question is, is there a simple way to modify this board as TDA2822M is designed to work with higher voltage or another simple method of dropping the source voltage from 3.7 to 3V?

    The circuit that I am referring to is http://www.crukx.com/education/bac/ee/projects/files/Hearing Aid.pdf

    BTW, I am a manufacturing engineer and not EE. So go easy on the technicals.

    Thanks in advance.
     
  2. Lesaid

    New Member

    Apr 16, 2008
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    Would adding a series silicon diode with around 0.6-0.7 voltage drop perhaps do the trick?
     
  3. joefly

    Thread Starter New Member

    Apr 14, 2010
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    Thanks Lesaid,

    I did a google search and ran across such use of diodes, But do you have a specific part number and how to use it. And when you say series, what do you mean?

    Thanks
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    The TDA2822M is designed to work perfectly with a supply from 1.8V to 15V.
    Its idle current with a 3V supply is typically 5.2mA and with a 4V supply its is typically 5.7mA so it dissipates only 4V x 5.7mA= 0.023W which is nearly no heat.

    Since yours gets hot then it is probably oscillating at a high frequency maybe beause it is built on a breadboard. Use a compact pcb as shown in its datasheet.
     
  5. Bychon

    Member

    Mar 12, 2010
    469
    41
    The diode would be part number from 1n4001 to 1n4007, but Audioguru is right, too. It won't hurt to try a diode because you can also remove it.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Those are rectifier diodes. Something like a 1N914 or 1N4148 signal diode will handle the low current very well, plus they are smaller.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    There is a DC path through the headphones between the amps. This is not good.

    See the datasheet. There should be 100uF caps between the outputs and the headphone.
     
  8. Bychon

    Member

    Mar 12, 2010
    469
    41
    Will a signal diode survive the start surge when the power switch connects 3 volts to 100uf?
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    The output coupling capacitors are needed when the amplifier is connected as a stereo amplifier. But here it is wired as a mono amplifier and is bridged so that each output has the same DC voltage and an output coupling capacitor is not needed.
    But (horrors) the earphones are probably connected out-of-phase!
     
  10. joefly

    Thread Starter New Member

    Apr 14, 2010
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    0
    Thanks all for all the help. It is a big relieve to finally get some helps as I been hacking around for a couple months on my own without much sleep. All the comments are helpful.

    Audioguru, Yes, it is getting hot on the breadboard, but the same thing on a PCB, here is a list of things I have altered which maybe is causing the trouble.

    1) I am using it to drive stereo headphones (would the headphone impedence or such cause heat also?
    2) instead of using VR1, I am using a 10k Resistor.


    UPDATE: I tried the diodes suggested above, 1n4007 did the job. Thanks for the help.


    Again, I am not sensing much heat when using 3v but hot when using 3.7V

    Thanks.

    Will try it today with the two different diodes
     
    Last edited: Apr 15, 2010
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    If the 4.7 ohm resistors are actually 47,000 ohms (4.7k) then the TDA2822M will probably oscillate at a high frequency and get hot.
    The same if C6 is missing.
     
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