24 VDC -> 12 VDC

Discussion in 'Beginner Electronics' started by Bluesman, Jul 9, 2003.

  1. Bluesman

    Bluesman Guest

    Hi,

    I'm looking for a circuit diagram of a voltage reducer, that is a DC/DC
    converter from 24 to 12 volts DC, current approx. 10 A, max. 15 A.

    I'd appreciate a cheap and simple, easy build solution and especially a
    PCB layout (got one day time to build this thing...).

    BR,

    Tom
     
    Bluesman, Jul 9, 2003
    #1
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  2. Bluesman wrote:
    >
    > Hi,
    >
    > I'm looking for a circuit diagram of a voltage reducer, that is a DC/DC
    > converter from 24 to 12 volts DC, current approx. 10 A, max. 15 A.
    >
    > I'd appreciate a cheap and simple, easy build solution and especially a
    > PCB layout (got one day time to build this thing...).
    >
    > BR,
    >
    > Tom


    How sloppy can the regulation (load and line) be? I have a small and
    cheap approach that has very poor regulation that is based on a self
    oscillating transformer. (Essentially a DC transformer with
    transistor and diode drops thrown in.) It produces an output that is
    proportional to input with some droop caused by losses, with no
    attempt to correct these flaws. might this be good enough?

    --
    John Popelish
     
    John Popelish, Jul 9, 2003
    #2
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  3. Bluesman

    Mark Jones Guest

    Bluesman wrote:
    > Hi,
    >
    > I'm looking for a circuit diagram of a voltage reducer, that
    > is a DC/DC converter from 24 to 12 volts DC, current approx.
    > 10 A, max. 15 A.
    >
    > I'd appreciate a cheap and simple, easy build solution and
    > especially a PCB layout (got one day time to build this
    > thing...).
    >
    > BR,
    >
    > Tom


    I've seen buck-and-boost regulators that will drop voltage in half with
    very little loss, but none at the currents you desire (although, it would be
    interesting to see this performed.)

    I did find an IC that will do adjustable output voltage and "up to 10A
    (50W)" per the title but this is entirely based on the P-channel output
    MOSFET used. (They can be paired for additional current capability.) Only
    problem may be the chip's tiny physical size. Check out this Maxim 1745
    datasheet for the general idea:
    <http://pdfserv.maxim-ic.com/arpdf/MAX1744-MAX1745.pdf>

    Regards,
    Mark
     
    Mark Jones, Jul 9, 2003
    #3
  4. In article <PyUOa.31$>,
    mentioned...
    > Hi,
    >
    > I'm looking for a circuit diagram of a voltage reducer, that is a DC/DC
    > converter from 24 to 12 volts DC, current approx. 10 A, max. 15 A.
    >
    > I'd appreciate a cheap and simple, easy build solution and especially a
    > PCB layout (got one day time to build this thing...).


    Here is a URL for a cheap and simple circuit that's fairly efficient,
    but it's only for an amp or so. Maybe you could do a bit of beefing
    up of the devices. Look at a04. near the bottom for a 12V version.

    http://www.romanblack.com/smps.htm

    > BR,


    > Tom



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    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
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    Watson A.Name - 'Watt Sun', Jul 10, 2003
    #4
  5. Bluesman

    Bluesman Guest

    John Popelish wrote:
    > How sloppy can the regulation (load and line) be? I have a small and
    > cheap approach that has very poor regulation that is based on a self
    > oscillating transformer. (Essentially a DC transformer with
    > transistor and diode drops thrown in.) It produces an output that is
    > proportional to input with some droop caused by losses, with no
    > attempt to correct these flaws. might this be good enough?
    >


    I need the voltage reducing 24 VDC -> 12 VDC for an inverter 12 VDC ->
    230 VAC. This might sound strange... Well, the fact is that I have this
    inverter which works fine in 12 V cars. I've got a lot of battery
    chargers with different output voltages (laptop computer, digital
    camera, mobile phone, walkman etc.) and this inverter has been very
    useful because I can use the same chargers at home and on the road. Soon
    I'll be traveling in a bus with 24 V electric system, so this fine thing
    would be useless if I can't get the 12 V voltage...

    Tom
     
    Bluesman, Jul 10, 2003
    #5
  6. Bluesman

    Guest

    Bluesman wrote:
    >
    > Frank Bemelman wrote:
    > >
    > > You are better off with a new 24VDC-230VAC converter. Sell the old
    > > one on Ebay ;)
    > >

    >
    > Ouch... Well, it's only one trip, then I would have to ask for a 12 to
    > 24 volts converter ;>
    >
    > If we still talk about a DC 24-12-converter with 10 A capacity, is it
    > possible to connect in parallel e.g. two linear regulators LM1084? It is
    > capable of handling 5 A current even at 12 V. Heat transfer is a
    > problem, but the current capacity is great.


    You can use a linear regulator chip like the LM1084, or one
    with lower current capability if you wish, with pass transistors
    to increase total current capacity. You heat sink the regulator
    and the pass transistors on a big sink, and add a muffin fan
    to ensure air flow if necessary.

    >
    > http://www.national.com/pf/LM/LM1084.html
    >
    > Actually I don't understand nor accept why it would be so difficult to
    > cut 50 % of the voltage (24 to 12) with a reasonable 10 A current.
    > Somehow. There must be simple solutions, but I don't know/remember so
    > much any more about electronics that I'd be able to tell how.


    A linear regulator is about as simple as it gets - but the cost
    is heat and power loss. You could also use an even simpler
    solution - a big resistor - at the same costs in terms of
    power lost/heat produced. That's the simplest - but there is
    a "show stopper" with that solution: it works only with a steady
    load.

    >
    > In a car, the starting motor might drain hundreds of amperes, so what's
    > the trouble with a much smaller current ;)


    The starting motor has BIG FAT wires in a relatively large
    (with respect to a linear circuit) area, so it can dissipate
    heat more readily and handle higher power. In addition, a
    starter motor is used for a short time, while a regulator has
    to be designed to run essentially forever.

    >
    > Tom
     
    , Jul 10, 2003
    #6
  7. > I need the voltage reducing 24 VDC -> 12 VDC for an inverter 12 VDC ->
    > 230 VAC. This might sound strange... Well, the fact is that I have this
    > inverter which works fine in 12 V cars. I've got a lot of battery
    > chargers with different output voltages (laptop computer, digital
    > camera, mobile phone, walkman etc.) and this inverter has been very
    > useful because I can use the same chargers at home and on the road. Soon
    > I'll be travelling in a bus with 24 V electric system, so this fine thing
    > would be useless if I can't get the 12 V voltage...
    >
    > Tom


    Just to throw in my two cents worth:

    Most inverters are designed to work off a car battery. They often have a
    very high inrush current and usually an undervoltage lock-out. If you run
    them from a current limited source you might find they lock out before
    establishing themselves. Typical running current is 1A per 10W output but
    at start up you might add 20A to 30 amps to this. Much better to float
    charge a small12V battery from the 24V to provide an unlimited source.

    Not wishing to advertise, but if you go to my company website,
    www.switchedmode.com and take 'technical' then 'battery issues' from the
    main menu you might see useful information.

    Regards,

    Pauline Aston.
     
    Pauline Aston, Jul 11, 2003
    #7
  8. Bluesman

    Bluesman Guest

    wrote:
    >>If we still talk about a DC 24-12-converter with 10 A capacity, is it
    >>possible to connect in parallel e.g. two linear regulators LM1084? It is
    >>capable of handling 5 A current even at 12 V. Heat transfer is a
    >>problem, but the current capacity is great.

    >
    >
    > You can use a linear regulator chip like the LM1084, or one
    > with lower current capability if you wish, with pass transistors
    > to increase total current capacity. You heat sink the regulator
    > and the pass transistors on a big sink, and add a muffin fan
    > to ensure air flow if necessary.


    Thank you, this is what I was looking for! Simple and robust but not
    necessarily so sophisticated nor efficient...

    Where could I find an actual circuit diagram for e.g. 10 - 15 A current,
    voltage reduction from 24 VDC to 12 VDC? It could be with any cheap
    regulator and other components. PCB layout pictures would be nice. I
    think that I have at home 7812s and maybe 3055's plus passive components.

    Tom
     
    Bluesman, Jul 14, 2003
    #8
  9. Bluesman

    ---MIKE--- Guest

    Your 24 volt system probably consists of two 12 volt batteries in
    series. Why can't you connect the inverter to only one of the 12 volt
    batteries?


    -MIKE
     
    ---MIKE---, Jul 17, 2003
    #9
  10. Bluesman

    Bluesman Guest

    ---MIKE--- wrote:
    > Your 24 volt system probably consists of two 12 volt batteries in
    > series. Why can't you connect the inverter to only one of the 12 volt
    > batteries?
    >
    >
    > -MIKE
    >


    When I asked this in a Finnish newsgroup sfnet.harrastus.elektroniikka,
    I got a convincing explanation why this is not recommended in the long
    run. If you understand Finnish, here's the authentic quote :)

    "Siksi että akkujen 'välistä' kuormitettaessa latausvirta ei enää mene
    juurikaan siihen ensimmäiseen akkuun (12V) ja sen napajännite laskee
    ..... ->11...12 V. Järjestelmä on kuitenkin 28 V laturilla varustettu...
    28V-11V = 17 V ensimmäisen akun napojen välillä (15 V kaasuuntumisjännite).
    Lisäksi koko kuormitusvirta menee tuo toisen akun lävitse 10...20 A ja
    sen napajännite on 17 V..."

    If not, then I try to translate: the whole 24 V system is equipped with
    a 28 V charger, and if you load partially only one 12 V battery with
    e.g. 15 A current, the charging current barely goes to the first battery
    the terminal voltage of which decreases to 11...12 V. Therefore the
    terminal voltage of the other battery rises to 28 V - 11 V = 17 V and
    almost the whole charging current goes through it. In the quote there is
    a term "gas voltage" which I actually don't know what it means, but it
    is inclined to be too high (max. 15 V).

    I know that the "intermediate 12 V output" is a commonly used technique,
    but according to electrotechnical laws, sooner AND later it causes
    problems :)

    BR,

    Tom
     
    Bluesman, Jul 17, 2003
    #10
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