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Voltage dropper

Discussion in 'Electronic Basics' started by [email protected], Mar 14, 2007.

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  1. Guest

    Hi Folks
    I'm trying to run a tiny 6 volt radio for as long as possible off a 12
    volt car battery. I came across a 7806 but was disappointed to find
    that it is nominally only 50% efficient. The heat dissipation (voltage
    drop times load current) being much the same as the load (out-voltage
    times load current)

    I read some primer on dc-dc converters that claimed that synchronous
    rectification could get to 95% efficiency. Is there some way to
    achieve a reasonable efficiency here? I even thought about centre
    tapping the 12 volt battery and using the two 6V halves in parallel.
    I suspect I might need a diode in this circuit and that would not bode
    well for the hopefully intermittent charging from a small solar panel
    in Winter. I will also be running a 12V, 1 Watt LED from this battery
    at night when the Sun definitely don't shine.

    I'm worried that if there is a weeklong rain period with no charging,
    I may flatten the battery too far.

    Perhaps two or three car batteries in parallel might be a better bet,
    but the ones I have are all different brands, sizes and ages, and
    would definitely self-destruct by trying to charge the weaker ones
    without diodes, but how do you rig up diodes to allow charging and
    draining?

    Thanks for any ideas or suggestions, jack
     
  2. Two choices. Find a 12 volt radio or use a switching supply to step the
    voltage down. The latter may generate noise if not well designed.
     
  3. default

    default Guest

    Some "bucking" switching supply for the high efficiency voltage drop.
    National Semiconductor has a line that is easy to implement and work
    well called "simple switchers." They are on line.

    You need a pair of diodes for each battery you want to isolate.

    From the charger plus lead wire connect the anode of the diode (arrow
    on a schematic). Connect the cathode or banded end of the diode to
    the battery. Do this for each battery the cathodes will go to a
    battery plus terminal and all the anodes will connect together and to
    the charger.

    The same idea works to isolate them for discharge. One diode anode
    connects from the Plus lead on each battery and goes off to the load
    and all the cathodes or banded ends go together to the common load.

    All batteries must have the same nominal voltage - don't put a 6 V in
    a bank of 12 V ones.

    The batteries should all have pretty near the same voltage during
    charge and discharge. The downside is that one shorted cell in one
    battery will prevent the entire bank from charging, by dragging down
    the charge voltage for the rest. A shorted cell won't matter to the
    load side. The low/shorted battery just won't support the load
    current and won't contribute any power.

    Each diode has to be rated for the maximum current it will carry and
    the maximum current could be the entire output of the solar panel or
    charging source or the entire current of the load - since there's no
    telling how hard each battery is going to be working in your
    application or the battery condition.

    Fuses may be a good idea, particularly on the load side - solar panels
    don't usually output much current so the charge diodes shouldn't be at
    risk.- but you still need diodes - they also prevent the panel from
    draining the battery when it is dark.
     
  4. Jamie

    Jamie Guest

    you can use one of those DC-DC converters how ever, i think you might
    find it more interesting if you were to make one of your own.
    using a Cmos version of the 555 timer which i think is the 7555 ?
    ,
    these use power, and then you can use this as a PWM (pulse width
    modulator) at high freq driving transistor into saturation mode into
    an small inductive coil that has a cap on the end to filter all of this.
    Then build your self a little regular to alter the charging cycle
    of the 555 timer..
    I know this all sounds like a lot how ever, you can actually get good
    efficiency doing this.
    I've do this before when looking around for a high efficient regulator
    just for the same reason you are..
     
  5. Guest

    Thanks so much for your help. I've found these and they look great.
    I'm wondering if there is a commercial product that is "plug-n-play"
    as I'm a total electronics novice. The cigarette lighter various
    voltage adaptors are likely to be inefficient as energy is not short
    on a car.
    This was understood, but I've got several car batteries that differ
    significantly in their charged voltage. There is one little battery
    that is a real performer. It is quite old, and I've abused it by
    would-you-believe reversing a car up a hill with the starter motor
    only. Because of it's chemistry, I asume, it consistently charges to a
    higher voltage than all the others including a brand new one.
    I'm leaning towards a large single 12V UPS battery.
    All these diodes are going to sap some energy I assume.

    My limiting factor is my solar panel which is rated at 20Watts
    (nominal). I have a proper controller for this which includes a diode
    for preventing reverse current flow in the dark. It also switches on a
    load when the panel charge voltage drops below 3.5V and switches off
    in the morning when the PV voltage reaches 5V.

    I want the radio on 24/7, so the smallest drain possible will be
    needed if the battery is not to go flat if the Sun don't shine for
    several days.

    Three days of 1W drain (83mA) will be 0.083 x 72 is 6Ah which at 20%
    discharge would require a 30Ah battery, and that is just for the
    radio. I may have a couple of W of lighting that will be switched on
    during darkness. This may require perhaps a 60Ah battery.

    Perhaps if I just buy the biggest wet cell marine battery that I can
    find, and hopefully only discharge it less than 20%? Just thinking
    aloud here. Hoping for a few comments or advice from you experts.
    Thanks again for your help, jack
     
  6. Guest

    Thanks, Homey
    Good advice.
    Seems like the simplest way would be that 12V radio.

    Anyone got any suggestions?

    What would be the likely power requirements for the smallest of
    non-hi-fi radios? Any ideas? jack
     
  7. A typical small radio delivers about 50 mW of power to the speaker. Double
    that as a guide to power consumed.
     
  8. ehsjr

    ehsjr Guest

    A radio will draw more current the louder you play it,
    so looking for the lowest drain radio is a non-starter.
    It needs to be lowest drain at the volume you have in
    mind.

    A battery lightly loaded will have more total capacity
    than a battery heavily loaded. Your 30 Ah battery into
    an 83 mA load will last longer than your figures. That
    30 Ah rating is figured at a higher load.

    A 12 volt radio will produce as much volume as a 6
    volt radio at ~1/2 the current, all other conditions
    being equal.

    Last, but not least, you should have an automatic
    shut off circuit to prevent your battery from being
    over discharged.

    Ed
     
  9. default

    default Guest

    Sealed Lead Acid batteries are hard to beat for long service in low
    current and float charge applications.
    Small portable radios use much less than a watt of power. If you're
    not too picky on fidelity - you can conserve more energy by using less
    bass. It takes more energy to play music with a lot of bass compared
    to voice .

    9 volt transistor radio batteries were designed to power radios in the
    100 milliwatt range with an average power of less than 100.

    Some of the newer "class D" audio systems will be even more energy
    efficient - they work similar to a switching power supply but the
    output voltage is modulated with audio. Most of the radio usage to
    date has been for cell phones and a few high end stereo systems.
    I'd use a sealed lead acid or even a pair of them - they are hard to
    beat. Marine battery or telephone system battery (wet cell) are
    great for keeping the cost low, but may need more maintenance.

    There's a lot of resources on batteries and off-grid living on the
    'net already. http://www.otherpower.com/ has some good information on
    generation, conservation and storage. Scroll to the bottom of the
    page for the categories.

    During hurricanes I operate an aquarium pump, TV and compact
    fluorescent from a pair of SLA batteries that I've had since 98 and
    they were sold surplus because they were manufactured in 1990 and
    hadn't been sold - intended to be used with trolling motors in boats.
    Once a year I recharge the pair, and again as a hurricane approaches.
    The TV runs off 12 volts the pump and light from a small inverter I
    cobbled together from radio shack parts.
     
  10. Bill Bowden

    Bill Bowden Guest

    You could try an old 12 volt car radio. I have a 1980s Ford AM radio
    that only draws 80mA with the dial light disconnected. It's very
    sensitive with good fidelity from a 4 inch speaker, and the push
    buttons are handy for changing stations in the dark. But you will need
    an external antenna and ground.

    Don't use any switching power converters if you want to listen to AM
    radio. They generate too much noise on weak stations.

    -Bill
     
  11. jasen

    jasen Guest

    car dadios aren't real efficient,

    A 9v battery radio would be easier to find (used) an probably more
    efficient even if powered from a 7809,

    my 3V pocket radio and run it from a buck converter could be another option.

    Bye.
    Jasen
     
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