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Convert 12-14 vdc to 9 vdc for a device only needing 0.3ma...

Discussion in 'Electronic Design' started by Default User, Dec 28, 2008.

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  1. linnix

    linnix Guest

    Even my wife knows about the car battery, but nothing else. My
    corolla has analog meter, but difficult to tell the exact reading.
    Her Luxus does not even have analog meter. After her last dead
    battery, she is well informed about monitoring the voltage. My meter
    plugs into the cigarette socket. She will let me know if the reading
    is below 11V. Just to be sure, I would probably wire a three color
    led based on the battery voltage. She wouldn't ignore it if the color
    turns yellow or red.
  2. Jasen Betts

    Jasen Betts Guest

    Yuasa makes car batteries with a charge indicator.
    One of the caps has a built in hydrometer and some optics.
    that change colour when discharged and (a different colour)
    for low electrolyte.
  3. Rich Grise

    Rich Grise Guest

    Good Grief! It was a strip of liquid crystal, which are famous for drawing
    well under one mA, more like microamps.

    Hope This Helps!
  4. Rich Grise

    Rich Grise Guest

    So, in other words, Lexi are crap? (assuming by "Luxus", you mean "Lexus")

  5. JosephKK

    JosephKK Guest

    A tad touchy? NOT creating warm fuzzy feelings about your real
    competence by my reading.

    Michael has given more to the world than you ass-ume. I have yet to
    see similar contributions from you.
    Not that i have much to brag about, but i have paid my freight.
  6. JosephKK

    JosephKK Guest

    I will personally notice Michael's demise, but i won't even deign to
    acknowledge yours, though it has already happened.
  7. Jamie

    Jamie Guest

    Excuse me?

    I guess you must one of the sheep being herded.

    I feel sorry for your incompetence to see between the cracks.

    I don't try to lead any one, especially ways that a few do in this
    arena, but if that is what you wish for, so be it.

    I stand on my original content of this post! Just because the present
    day design of those cheap meters no longer allow the measurement of its
    own supply rail as many do not. I have 3 in my possession that do and
    can confirm they are not the same board design as was posted from a
    poster here, which I can't remember at the moment who it was.

    It's all about how the meter is design, using a virtual ground from a
    signal source.

    Have you ever heard of differential and single ended systems?

    One requires a VG which makes it inadequate to measure it's own
    rails and the other, does not.

    Next time you're looking around for panel LCD's, pay close
    attention to the spec's that allow you a floating common and non
    floating. That
    mays all the difference..

    Now let me get back to something worth doing and it's not talking to
    sheep like you.


    I don't step aside for those that think the world owns them
  8. linnix

    linnix Guest

    You can externally generate the negative supply or reference voltage.
    Neither one is cost effective for cheap meters. By the way, my local
    Harbor Freight is out of "cheap meters". The new version with back-
    light is $9.99, so $8 for the LED back-light.
    LCD panels don't care about absolute voltage, as long as value between
    segments and commons are fixed. The chip can do both ways. It's just
    a matter of cost in the meter design.
  9. ehsjr

    ehsjr Guest

    That seems an untenable position, so you may have more
    in mind that what was posted.

    Your original post was: "yes, use a couple of diodes
    in series." in response to the question: "Is there an easy
    (read cheap) way to drop the 12-14 volts down to 9 volts
    without increasing the power consumption very much?"

    1) It has been shown that there are meters that won't work
    properly when measuring their own source. Your post makes
    no allowance for that.
    2) A couple of diodes will not drop 12-14 volts down
    to 9 volts.

    You might have something different in mind, but people
    are reacting to what you posted, not what you have in
    mind (assuming it is different). I'm interested in
    what your specific thinking is, assuming your statement
    about a couple of diodes did not truly reflect that.

    I'm in the (slow) process of adding metering to a physically
    (relatively) large cabinet that houses a 12-15V switcher. I
    have 3 different DC-DC converters breadboarded. The cmos 555
    version I mentioned in this thread works best, so far. In any
    event, adding two of those Harbor Freight meters is the cheapest
    way to go, so that makes it attractive. Now, if you have
    strong reason to know that they will work properly and not
    be damaged by a source that is 42% higher than nominal, I may
    be able to omit a zener from the DC-DC design. The switcher is
    already complete, it's just the meter supply that prompts
    the question.

  10. Jasen Betts

    Jasen Betts Guest

    it was a tapered thick film resistor acting as a heater behind a thermal colour-change
  11. Jamie

    Jamie Guest

    ehsjr wrote:

    yes, that was in response to dropping the voltage supply
    from 12 down to ~ 9 volts cheaply..
    Not all can measure their own rail, that's true.
    Those using virtual grounds will most likely not be able
    to do this.
    Each diode will give you a loss of ~ 0.6 volts.
    2 diodes will give you around 1.2 volt drop.
    so maybe more is needed, But the idea was put in place
    so a simply solution was at hand.
    Diodes are and have been a simply solution for years to drop
    the voltage level a bit.
    I recently saw a photo of the inside of a freight meter, it does not
    look like the one's I have. Those M-830B's have been around for years
    and i'm sure the internal board design has been changed several times.
    The 3 I have, can! and I repeat can measure it's own source. But mine
    are over 5 years old now. That does not mean the new ones are designed
    the same way.

    Why can't you just get one of those cheap LCD panel meters?"
  12. linnix

    linnix Guest

    That's why they are cheaper. They removed the external voltage
    My plan is to install rechargeable batteries on them, and charge
    through the 10A plug (cutting the internal wire, of course). I can
    still use them as regular meters, without turning them off or
    replacing batteries. They were steals at $1.99, but not anymore.
    They were all stolen from my local Harbor Freight.

    I installed the meter on the ignition wire tap in my wife's car.
    Knowing her, it will always be on or off. I prefer it always on with
    key in ignition.
  13. JosephKK

    JosephKK Guest

    You seem to need an excuse, but i will not grant you one today.
    The cracks in your garbage claims i do see through, but they are
    several feet wide.
    You lead? Now that is a joke.
    I do not recall any poster saying that not being able to measure its
    own source was a good design, not even yourself. I recall many
    reports of meters that could not do so.
    Maybe before you were born, must have been 50 years ago.
    I claim that "I have earned all that i have, some of it many times
  14. ehsjr

    ehsjr Guest

    Ok, thanks. I was hoping you might have done enough testing
    to be sure that running one of those meters at ~12.8V would
    not damage it and that it would work properly at that voltage.

    I can do that, and have in the past for smaller things. But you
    can't beat a $2.99 (sale) price tag for the Harbor Freight meter,
    and I have plenty of space in the cabinet to mount the entire meters
    in the front panel, if I want. And the test leads will be used
    to connect the supply to whatever it is powering.

  15. Jamie

    Jamie Guest

    I was just at the local junk store that carries like 75% of goods like
    harbor freight does and they have a whole shelve of them.

    The model number isn't exactly like my 5 year old ones but close.
    I think theirs is a DT or BT-830B, mine is M-830B.
    I couldn't tell you what the board design of that model is.
    I didn't feel it was necessary to buy yet another junk meter
    to have lying around."
  16. Greegor

    Greegor Guest

    Somebody said forward bias on 1N4001 diode would be .6V
    but I was taught that silicon diodes forward bias at .5.

    I wouldn't quibble about the difference but when using
    a whole string of those for a voltage drop wouldn't
    that become a big deal?

    Is forward bias on silicon diodes actually .6 v or .5 v as I was
  17. Eeyore

    Eeyore Guest

    Depends on the current.

  18. Jon Kirwan

    Jon Kirwan Guest

    Aside from structure and materials, the voltage drop depends also on
    the current. For base-emitter junctions on common silicon BJTs my
    experience says the drop varies at about 60mV per 10-fold change in
    current -- consistent with this model:

    Vd(I) = n k T / q * ( ln(I) - ln(Is) )

    Since Is is a constant, ln(Is) is also constantand drops out of the
    derivative. So you get:

    d Vd(I)
    ------- = n k T / q
    d ln(I)

    With q the electron charge, k Boltzmann's constant, T is temperature
    (Kelvin), and n is a fudge factor (okay, 'emission coefficient') that
    is mostly 1 for BJTs. kT/q at ambient temperature works out close to
    60mV (just under.)

    For diodes, my experience seems to be that 'n' is no longer close to 1
    and is not infrequently significantly closer to 2 than to 1. So I'd
    expect more like 100mV change in diode voltage for a 10-fold current
    change. For the common 1N4148 diode, I mentally estimate 0.5V at
    100uA and 0.6V at 1mA and work up or down from those.

    For LEDs, yet another story despite them being diodes, too.

    So the way I'd take all this is that it depends on the current under
    consideration and the part being used. You might see someone say 0.5V
    (and less) when they are talking about microamp ranges of currents and
    say 0.6V for low milliamps and still more if the currents are higher
    still. So... it depends on context.

  19. Eeyore

    Eeyore Guest

    And don't forget the tempco of ~ -2mV / C

  20. Eeyore

    Eeyore Guest

    I tend to expect Vbe of BJTs to be in the 0.55V area with collector currents of a
    milliamp or so.

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