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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. Default User

    Default User Guest


    I've got a battery box with a 115AH deep cycle battery for power in a
    tornado shelter.

    I had a thought that it would be nice to mount one of the cheap harbor
    freight $2 multimeters to it to monitor the voltage. They usually run on a
    9 volt batter and consume a nice and small 0.3ma. What I don't know is if
    12-14 volts would fry it or if it would do fine on that. Is there an easy
    (read cheap) way to drop the 12-14 volts down to 9 volts without increasing
    the power consumption very much?


  2. ehsjr

    ehsjr Guest

    Yes, but. (Actually, 2 "buts")
    The first "but": Those meters might not be able to monitor the
    same source that provides power to the meter, so you might need
    a dc-dc converter with an isolated 9V output.

    The second "but": You don't need to worry about minimizing
    power consumption. At .3mA those meters use very little power, so
    even using a 7809 voltage regulator won't be a significant drain
    on a 115 AH battery. If you don't have (and don't want to buy)
    a 7809, you can use 5 1N400x diodes in series to drop the voltage.
    That will work, if the meter is capable of measuring its own source.
    You can test that with a 9V battery installed, measuring the 9V

    Finally, why do you care? Just mount the meter and use it
    when you want to. It will last for a loooong time in tornado
    shelter use - probably close to shelf life. Buy a new 9V
    battery once a year and put it in the shelter if you want a
    little insurance. The shelf life is easily far in excess of
    1 year (Duracell says 7 years) and a 9V battery should be able
    to provide .3 mA for hundreds of hours. Duracell shows
    250 hours at 2 mA draw. Typical capacity is 500 mAh per
    At .3mA you'll use way less than that in a year in the
    shelter, unless you forget and leave the meter on.

  3. IanM

    IanM Guest

    Well for lead acid battery monitoring applications, you never need to
    measure below 10V (battery DEAD or nearly so) and you need to be able to
    read up to just over 16V for equalising. The nearest available analog
    meter scale would be 0-20 probably graduated in 2V intervals, each
    sub-divided in five 0.4V increments. This is *USELESS* for monitoring a
    lead acid battery in normal use. Other likely scales are worse. If one
    adds a circuit to turn it into a suppressed zero voltmeter and one makes
    a custom scale 10-16 graduated in volts each subdivided into 0.1V
    intervals, it will be as usable as a cheap digital meter.

    The simplest method of meeting the OP's requirements would be to take
    the cheap digital meter, keep its existing battery, jumper out the
    range switch and add a 'push to test' SPDT switch that both connects the
    meter's own battery and the 12V battery to be tested. Simple, cheap,
    fairly accurate and easy to use/read. What more could you ask?
  4. JeffM

    JeffM Guest

    The OP reminds me of the
    *To a guy with only a hammer, everything looks like a nail* thing.
    A text search there for uA or 1mA will give likely candidates.
    An 8.2V zener and a calibration resistor in series
    will give a useable range of ~8V - 15V.
    (The meter movement's scale will have to be re-labeled).
  5. Ben Bradley

    Ben Bradley Guest

    Others have adequately answered "How do I run a 9V multimeter off a
    12V 115AH deep cycle battery and measure the voltage of said*

    But it appears what you really want to ask is "How do I measure the
    charge left in a discharging 12V 115AH deep cycle battery?" This
    requires monitoring the current in addition to the voltage, and
    integrating (perhaps even weighting a higher current draw as more than
    a simple linear increase, as a higher current results in a lower AH
    rating) with respect to time. I'd think there would be such devices on
    the market, even calculting the time left on the charge at the current
    rate of discharge, but I haven't seen such. Perhaps researching it
    would be fruitful.

    Or you could program a microcontroller to do this very thing. Most
    manufacturers have "starter kits" for $100 or less. It's easy enough
    to get current draw down to 0.3 mA with most microcontrollers
    available today (especially doing this, the processor would run to
    measure voltage and current, do some calculations, maybe update a
    display, running for a cuple of mS then dropping back to wait or stop
    mode for 50mS, reducing the average current draw proportionately. Or
    just run it full out and pull a couple mA from the huge battery -
    that's still probably a lot less than the equivalent self-discharge
    current on the thing.
    * Yeah, I've been reading texts of patents.
  6. Jamie

    Jamie Guest

    yes, use a couple of diodes in series."
  7. Jamie

    Jamie Guest

    Screw you.
    I just happen to have 2 of those cheap meters the poster is
    referring to, that was given to me in a grab bag. I gutted one of them
    to use
    as a display in a junk project, the unit can read it's own source just
    fine if that is what every one is so worried about.

    If you want to talk about some one giving bad advice, take a look
    in the mirror assole.

    Unless they have changed the design of those 3..5 dollar DMM's
    at the freight, it'll work just fine..

    So go screw your self Terrell until you have something
    worth while to say, with out vile and excrement rolling out your pie hole.

    I don't give a shit how disabled you are and how that makes you feel
    that it give's you the ok to be a royal asshole with just about every
    one you intersect with.

    There are a lot of disabled people out there on their last leg, you
    aren't anything special.

    fringing ignorant old basted, thinks the world owes him something."
  8. linnix

    linnix Guest

    The proper question is whether an isolated power source is necessary.

    Couple of resistors will do.

    Thank you for your opinion.
    No need to get emotional.
    Mine can't. I guess some works and some don't.

    I just hatched a DC-DC converter and couple of 100 ohms resistors to
    power it. It works between 11V to 17V.
  9. linnix

    linnix Guest

    Sorry, hacked.
    Pay no attention to the date on the pictures. They were taken today.
  10. Default User

    Default User Guest

    Hi Everyone,

    Thanks for the ideas on the voltage regulator or diodes. I will check to
    see if it can read its own source or not.

    I wasn't looking for any sort of battery analysis, just the voltage and the
    reason I don't want to leave it on 9V is that I'd like to just leave the
    meter on all the time which would be no big deal to a 115AH battery.


  11. Jamie

    Jamie Guest

    Do you now have comprehension problems? Was my post that unclear to you?

    You must be among those ignorant saps that represents the greater
    whole of the moron society this country is quickly getting polluted

    Go dig a hole while you still can. Cause when its time for you to
    go, I doubt many will notice."
  12. linnix

    linnix Guest

    Yes, my fuke meter is SW7106A (QFP 40), but the Harbor Freight chip is
    42 pins. The chip itself can do single ended input, but not wired for
    so in the meter. Adding isolated DC/DC converter works, but the
    converter needs 20mA to run. I will try to rewire the board without
    the converter. For now, I can test drive it (in my car) with the
    converter. See:
  13. ehsjr

    ehsjr Guest

    Do you have a schematic of your converter? Wondering why
    it takes 20 mA to run. Jan has a circuit (haven't tried
    it, I have my own) that may take less - mine provides
    two isolated 9V outputs at up to 10 mA each, so it
    draws a lot. There was also one I stumbled on where the
    guy uses a cmos 555 that provides 1 mA so it may be low draw.
    I'll post a link to it if I find it again.

    And - I tried one of my Harbor Freight cheap meters and
    it won't measure its own source, either. If you mod
    yours successfully, please post.

  14. ehsjr

    ehsjr Guest

    Here's the link I mentioned in the other reply: supply.htm

  15. Jamie

    Jamie Guest

    Sorry, the design must of changed. I got mine about 5 years ago.
    M-830B. I still have 3 more of them lying around here. I got my hands
    on a box of them at a grab and give away as part of an event at a meeting.

    I looked at your picture, mine does not look like yours.
    Mine do not have continuity beepers and the fuse is just above the
    battery and the board goes all the way to the top of the meter covering
    the LCD..

    But it looks the same on every where else how ever except for the
    Oh well."
  16. Mark Zenier

    Mark Zenier Guest

    If it's a 7106 or 7126 type chip, I think that you can use a 3 resistor
    divider to measure it's own voltage. But you need to go in and isolate
    the COMMON virtual ground terminal from the IN-LO terminal.

    Theoretically (I never got around to trying this), if your circuit can
    isolate the IN-LO input (pin 30 on the dip version) from COMMON (pin
    32), and you toss the existing voltage dividers and run a resistor
    chain from the full V+, one between IN-HI and IN-LOW, and one from
    IN-LO to V-/ground, both IN-HI and IN-LO stay in the common mode range.
    (Doing this to Chip-on-board construction like the Harbor Freight meters
    may be a challenge).

    The ratio of R(IN-HI,IN-LO) to the sum of the other two resistors is
    either 1:99 or 1:9 depending on if the circuit is set up with a 100
    millivolt or 1 volt reference.

    At that low a current, I'd use a zener/resistor for the supply.

    meter battery V+
    6?-20 V |
    | 3.9 kohm 8 V zener
    | 7 Mohm 1 Mohm 2 Mohm
    | |
    | |
    Chip IN-HI Chip IN-LO

    Beware, if you try to measure one of these multimeter's own battery,
    hooking the negative probe to the + battery terminal might burn out the
    chip (depending on how much resistance is between the negative/ground
    probe and the chip's COMMON input). The COMMON input is a shunt regulator
    that sets a virtual ground to 2.8 volts (nominal) below the V+.

    Mark Zenier
    Googleproofaddress(account:mzenier provider:eskimo domain:com)
  17. Tom Bruhns

    Tom Bruhns Guest

    A simple way to do this that takes less power and is very easy to hook
    up is to just put an analog moving-coil meter across the battery. You
    don't need a whole lot of accuracy, since the battery voltage will be
    a function of temperature as well as state of charge anyway. You can
    easily expand the scale of a meter by putting a zener diode in
    series: get a reasonably tight tolerance 10 volt zener, a 100uA
    meter, and 50,000 ohms of net resistance (including the meter itself),
    and you have a meter that reads from 10V to 15V, and you should be
    able to visually resolve to less than 50 millivolts. You can use the
    meter's mechanical zero to calibrate one voltage accurately, and if
    your resistance is adjustable, you can calibrate the slope too.
    Typical current would be 40 to 60 microamps. Doing the same with a
    0-500uA or 0-50uA meter would make it even easier to read without any
    re-writing of the scale. If you really think you need finer
    resolution, you could give it a 2V range from 12V to 14V, or
    whatever. I'm aware that analog meters aren't as easy to find as they
    used to be, but they're still available.

  18. linnix

    linnix Guest

    That's what I had in mind, but I haven't got too far yet.
    I use 1 DC/DC converter and 2 resistors. Should add a zener across
    the converter input as well.
    The problem is the 20mA current to run the converter, for on-line
    I have been using it in my car. It reads 14V with engine on and 12V
    (plus low battery indicator) off.
    My car battery dies after a few days off anyway.
    I also have an AVR driving a LCD and reading the battery. It draws
    3mA and requires no isolation.
    But the $2 AVR is much more expense than the 20 cents 7106.
  19. linnix

    linnix Guest

    If I have my way, it will be in every car battery and dash board.
    Also, there will be audio input jack and usb power outlet in every
    junk box (that come with all car) they called the radio.
    Auto makers are decades behind the curve.
  20. Rich Grise

    Rich Grise Guest ;-)

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