Pretty Basic - dropping 3V to about 1.3v -1.6v

Discussion in 'General Electronics Chat' started by iONic, Nov 12, 2015.

  1. iONic

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    I have a 3V wall adapter (350mA) that I want to drop the voltage on to equal somewhere in the range of a single cell 1.5v battery. Thus I can accept anywhere from 1.25v to 1.60v. I suspect a diode or two could do this, but was wondering if there was any other "easy" option. The device I am powering is a portable(not any more) .mp3 player that operated on a single cell bat. and consumes aprox. 125mA max with high volume and back-light on. This device will be playing for 8-10hrs/day every night, so batteries, even rechargeable ones are really not an option.

    Your thoughts please!
    Thanks
     
    Last edited: Nov 12, 2015
  2. MikeML

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    LM317 with the ADJ terminal grounded or jacked up by 0.25V? Need to check the dropout voltage, though...
     
  3. spinnaker

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    IMHO diodes will be your easiest option.
     
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  4. iONic

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    I have tried three diodes. Vdrops of .65v, .65v, .22v = 1.52V leaving 1.48V available. Measured the open circuit voltage at about 2.0v -2.2v and does not power the device.

    The LM317 is adjustable and its Vdo is 1.2 V(min)
    Vin = 3.0V, R1 = 240, R2 = 22 for Vo = 1.365V
    [​IMG]

    Ya think Caps are necessary .1uF input, 1uF output???
     
    Last edited: Nov 12, 2015
  5. marcf

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    0.65, 0.65, and 0.22v?

    I think that you should investigate why the 3rd diode is only has 0.22V across it.

    What is the output of the 3V 'wall wart' under load, with say a 12ohm resistor across it?

    Do you measure any AC voltage across the load?

    If it is a good 3vdc 'wall wart', all three diodes should drop 0.6V under load.
     
  6. Bordodynov

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    upload_2015-11-13_14-29-18.png

    Stab1_5V.png
     
    Last edited: Nov 13, 2015
  7. MikeML

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    @Bordodynov 's sim is showing that the LM317 is dropping out because the 3V input is just not quite high enough... I doubt that his Spice model is good enough to model this accurately.

    This is from the TI data sheet:

    do.gif
    At room temp and at ~150mA, for proper regulation, the LM317 needs a min of ~1.8V differential across it.

    Any chance that the 3V wall-wart is actually higher? Is it regulated or just a full-wave rectified transformer with some arbitrary value of filter capacitor. If so, try putting more uF of filtering on it upstream of the LM317, and it might behave...
     
  8. GopherT

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    If you can accept down to 1.25 V, why not use a bare wire for R2? An LM317 is simply a 1.25V fixed regulator.

    And... If your load draws more than 10mA at 1.25 V, then you can eliminate R1 as well.
     
  9. iONic

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    It's a pretty cheep wall-wart, designed to power a "Game Boy"
    Open Circuit voltage is 3.07V
    Voltage across a 10ohm, 0.5W resistor read about 2.6V
    I measured the voltage drop on the diodes using the "diode" setting on my multimeter
    2 diodes @ .65v (45765) were the same, the third diode was a different part# (IN5819) which read .22v
    Don't think there is much AC, .65mV on the AC setting on the multimeter

    Yes, 1.25V will probably work fine. A 5ohm or even a shunt might work, given the poor input voltage.
    I'm gonna scan my Vregs, I should have one with a lower VDO.

    Update:
    Just found a couple of new options.
    1) MCP1700-1802E
    LDO 150mV-350mV
    Io 250mA
    Vo 1.2V - 5.0V
    Vi 2.3V - 6V

    2) An old 5V 1A cell phone charger supply if needed
     
    Last edited: Nov 13, 2015
  10. ScottWang

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    Aug 23, 2012
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    I was modified from my another circuit, but this one haven't try yet, if you want then you can try it, and the output is adjustable.
    1300-1600mVPowerSource-01_ScottWang.gif
     
  11. iONic

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    Thanks Scott, but since much of my electronics parts are boxed up for a move I am opting for a simpler solution, not that your solution isn't simple, but that it takes finding all the parts. Since I found the MPC1700 I can get pretty much the exact voltage with 2 parts. And I even had it operating prior to my soldering......which I am in the middle of right now. So I think I'm at the finish line, I believe!!
     
  12. ScottWang

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    MPC1700 is fine for the voltage you want, but its rating current only 250mA and it is better using less 150mA or even less, I will using it less 100 mA, how much current do you need?

    If the current is not enough then you can adding a npn bjt to provide the bigger current.
     
  13. iONic

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    My understanding is that the current is provided as needed, up to 250mA, and the voltage is fixed at 1.8V. By the way, I added a diode to the output of the Vreg to bring the voltage to 1.5xV. I am noticing, however, that there is a low hum when the supply is plugged in and passes through the speakers. This is not the case with the 1.5V battery.
     
  14. Bordodynov

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    May 20, 2015
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    See
    1_5V__.png

    I want to give a warning. Stabilizers "Low Drop" likely to generate under some load capacitance values!
    When a voltage of 1.5 volts maximum output power greater than at 1.3 volts.
     
    Last edited: Nov 16, 2015
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  15. ScottWang

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    Could you upload the asc files?
    Thank you.
     
  16. Bordodynov

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    Circuit for LTspice:
     
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  17. iONic

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    Wanted to understand where the "low Hum" was coming from as a result of the adapter and Vreg. I used a 3.7V Li Ion battery in place of the 3V wall adapter and the hum was not present. The adapter is Full-wave rectified and appears to have smoothing caps as well though. I then swapped the 3V adapter for a 6V one temporarily and did not get a hum there either. So the cheep little radio Shack GameBoy adapter might light a series of parallel LED's but not much more. But the MPC1700 Vreg Max Vin is 6V so I will opt for an old cellphone adapter/charger at 5V to stay on the safe side.

    The MPC1700(T-92) is a nice little regulator with fixed outputs of1.2V, 1.8V, 2.5V, 3.0V, 3.3V, 5.0V with a Typical Dropout Voltage @ Max. lout (mV) of 300mV and Max Iout of 250mA.
     
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