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Have 5v regulator need 9 volts

Discussion in 'Electronic Basics' started by amdx, Jan 24, 2013.

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  1. amdx

    amdx Guest

    Hi all,
    Tomorrow I'm working on a power supply to replace a 9v battery.
    All I have is a *5 volt regulator. I think can use a voltage divider on
    the output, then tie the normally grounded leg to the center of my
    divider to bring it up to 4 volts. So I have 4 + 5 = 9 volts.
    If I used 5.1K and 4.3k in series to ground. that would give me 9.1
    volts. However there is some current from the reference pin.
    So, I don't know how to calculate the proper ratio and how much
    current do I need to flow through my divider.
    Should I put a capacitor across the lower leg of my divider?
    How big?
    Thanks, Mikek


    *My local Radio Shack only carries 5V and 12 V.

    PS. In case this isn't feasible,
    Here's the scene, I have three devices, an infrared transmitter (9v)
    an infrared receiver (4.5v) and a walkie talkie (4.5v). I have tested
    both of the 4.5v units on 5 volts and I'm comfortable they will be OK.
    I'll be running this all from a 9v wall wart, we'll say it puts out
    14v at no load. The big load is the walkie talkie during transmit
    ~350ma. At idle it is less then 10 ma for all devices combined.
     
  2. Phil Allison

    Phil Allison Guest

    "amdx"

    ** All you need is a 3.9V zener in series with the ground lead.



    .... Phil
     
  3. Jasen Betts

    Jasen Betts Guest

    http://pdf1.alldatasheet.com/datasheet-pdf/view/89091/NSC/LM140.html

    There it is right on the front page.

    adding a capacitor between pin2 and ground will slow the voltage rise
    at power-up but and may improve regulation under variable load
    conditions,
     
  4. amdx

    amdx Guest

    Yes, but I don't have one locally and I want to get done today.

    Thanks, Mikek
     
  5. amdx

    amdx Guest

    John, I had thought about that but I had a concern that 10 volts
    would be to high for my 9v device. Guess I could stick a diode in series
    with the output giving me 9.4 Volts, pretty close to a fully charged 9
    volt battery.
    I think that might be an easy solution, need to reread thread, someone
    mentioned quiescent current is a problem.

    Thanks, Mikek
     
  6. amdx

    amdx Guest

    For now, I'll try the stacking with a series diode on the output to
    drop .6 volts.
    The wall wart is not a problem, I have a box full of various voltages
    and currents.
    The wallwart is where my problem started. I used a 6v 700 ma, with a
    no load output of 9.6 volts. All worked good until the walkie talkie
    went into transmit. Then the wallwart voltage dropped to about 7 volts.
    It still worked but I could hear hum on the receive walkie talkie.
    If I went to a higher voltage wallwart then the voltage was to high for
    the 9 v device. So, the need for a second regulator.
    Thanks, Mikek

    with s
     
  7. amdx

    amdx Guest

    All that looks great but the objective of this exercise is to make it
    work with the parts I have on hand today. The parts I have are two-5v
    regulators. I have many wallwarts, so I have one that won't fall to low.
    Hmm... I just recalled, there is an audio repair shop in town and the
    guy sells parts. it's worth waiting till he opens and checking if he has
    a 9v regulator in stock.
    Thanks, Mikek
    Thanks, Mikek
     
  8. I think that's pretty standard... did you look at any 5 volt regualtor
    spec sheets? I bet they have a circuit.
    I've got a LM79L05 spec sheet tacked to my wall. (I can never
    remember pinoputs.)
    It's got what you want on the back.
    R1 from out to 'adjust', R2 from 'adjust' to ground. (Adjust is the
    old ground)

    Vout = -5V -(5V/R1 +IsubQ)*R2
    5V/R1 > 3*IsubQ
    0.1uF cap across R2.

    All you need is the quesient current for your regualtor.

    George H.
     
  9. amdx

    amdx Guest

    Probably meant troughs rather than peaks.
    Ok, project complete, boxed and working. Picked up a 9v regulator at
    the audio repair shop. $3.02 :-(
    I used a 1000uf cap to filter the wallwart and .1uf at the output of
    each regulator. There are input filter caps in the devices.
    At idle, with every device powered the input is 13 volts with 100mv
    of ripple. When the walkie talkie transmits the input voltage trough is
    11.1 volts and 800mv of ripple. This is fine, there is no sign of any
    ripple or droop in the output of either 9v or 5v regulator.
    I used a 9v 1 amp wallwart.

    Thanks for everyone's input.
    Mikek
     
  10. amdx

    amdx Guest

  11. Well the pinout on my wall is for the 79XX so that's what I quoted.
    (I can remember the 78xx pinout. :^) Others have pin ups on their
    wall, I've got pin outs, wire gauges, screw sizes and Drill tables
    (And some drawings done by my kids when they were younger.)

    George H.
     
  12. amdx

    amdx Guest

    Hmm... I guess you just get hum if the troughs go below 10.7v. :)
    Mikek
     
  13. Phil Allison

    Phil Allison Guest

    "amdx"

    ** Your stupid problem - fuckhead.
     
  14. amdx

    amdx Guest

    Is that you Phil? It's seems a little weak.
    Imposter!

    Mikek
     
  15. Guest

    No, Phyllis is normally weak. ...or were you commenting on the
    "little"?
     
  16. amdx

    amdx Guest

    When testing, my line voltage was 123.3vac. When I got to my boat and
    checked the line voltage it was 112.9vac with my heater running.
    I decided to use a 12v wallwart instead of the 9v.
    The 9v Wallwart only had ~ 0.4 of headroom at full load.
     
  17. What's a walkie talkie? That sounds like some sort of gay sex device!
     
  18. amdx

    amdx Guest

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