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Help needed with 24vdc to 12vdc

Discussion in 'Electronic Basics' started by cabsandy, Jul 21, 2007.

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  1. cabsandy

    cabsandy Guest

    Hi all,
    Sorry if this seems a bit noddy but a few simple questions..

    I have a unit that expects 12vdc at about 500mA.I want to place it in
    a remote location but that location only has a 24vdc supply
    available.

    Can I simply connect it with no consequences?-does it matter if the
    input is regulated or unregulated? If not, I understand that I may
    need a DC to DC converter.The problem is , space and price are at a
    premium, and I want to keep this as simple as possible .I guess the
    converter is the "proper" option so I apoligise if this question
    offends anyone or causes them to shake their head ;-)

    Any advise gratefully received

    Sandy
     
  2. kell

    kell Guest

    If you have to ask, then it's a bad idea to hit a 12 volt device with
    24 volts.
    You're going to have to tell us what the device is. No definitive
    answer is possible with the information you've supplied.
    Kludge:
    If the device doesn't require tight regulation, you may be able to use
    a string of about 17 1N00X diodes, each dropping about 0.7 volt, to
    reduce the voltage from 24 to 12. Very inefficient, to be sure; the
    diodes will use up just as much power as your load. But it's quick
    and dirty, easy and cheap, if you have the power to waste.
    Best to use a converter, of course...
     
  3. You may be able to, if the unit specifies that it can tolerate 24V and
    regulate it as it needs internally... also assuming the 24V supply can
    provide the necessary current. But probably not.
    Does your unit require a regulated 12V supply? This is more a
    question for you to answer than anyone else. How could we know if
    your unit requires a regulated or unregulated supply??
    If it doesn't require a regulated supply??? Perhaps you are using
    terms you don't entirely understand?
    This is .basics.

    I think you need to provide more information. Your unit requires
    12VDC. And you have the figure of "500 mA," as well. Without you
    saying otherwise, normal assumptions would be that the 12VDC must be
    regulated, so that it doesn't vary by more than a few tenths of a volt
    over various loads up to something a bit over 500mA, but allowed to be
    much less as well. In other words, it holds the voltage regardless of
    the current required, up to some comfortable margin above the 500mA
    requirement. Also, I think usual assumptions might suggest that the
    power supply can take some reasonable time to come up to 12V DC on the
    regulated line -- perhaps on the order of 50 milliseconds or more.
    This may or may not be allowed by your unit.

    You don't say what your 24VDC supply is like. Is it also regulated,
    already? (If it varies a lot, by some volts let's say, this suggests
    something additional to watch, for the 12V DC regulator.) What
    current handling can the 24V supply support?

    What about heat? Can you tolerate losing 12V*500mA or 6 watts of
    excess heating in your "space premium" situation?

    How much space is allowed here? How much cost?

    Jon
     
  4. The advantages of a converter are probably isolated and
    regulated 12 volts at high efficiency (little heat and
    wasted 24 volt supply power.

    Wasting excess 12 volts while supplying 500 mA means that a
    linear regulator will have to dump 12*.5=6 watts of waste
    heat. A cheap integrated regulator (like the LM7812) and a
    few capacitors could do this job for a few dollars, if you
    have a heat sink surface available, or can add a heat sink
    that keeps the regulator below about boiling water
    temperature at the highest ambient temperature while being
    heated with 6 watts.
    Here is the data sheet for this regulator, that shows the
    recommended additional capacitors and possibly diodes for
    most applications.

    http://www.fairchildsemi.com/ds/LM/LM7812.pdf

    Here is the data sheet for an isolated 24 V in 12 V out DC
    to DC converter able to supply up to 1 amp, at 87%
    efficiency (12 watts out with about 1.5 watts waste heat),
    CC10-2412SF-E (about $23 from Digikey) on page 10 of:
    http://www.tdk.co.jp/tefe02/ea335_cc_e.pdf
     
  5. Note that the junction-to-case thermal resistance is 4C/W for either
    the TO220 or TO3 packages (the other arrangements are hopeless, being
    at least 32C/W.) 150C is the maximum junction temp and assuming some
    reasonable maximum for ambient (45C?), we are talking about at most
    100C rise over ambient. Less to be safe. Let's call it 80C rise.
    This figures to 80C/6W, or about 13C/W total. 4C/W is already used in
    the J-to-C, so the heat sink needs to achieve about 9C/W or less.
    These are in the 1.5" x 2" x .5" size range, I think.

    Jon
     
  6. For example the 7-345-1PP-BA has a rating of 9degrees C per
    watt:
    http://rocky.digikey.com/WebLib/IERC/Web Data/7-344,7-345.pdf

    It will fit in a 2"X.75"X.56" rectangular prism. Of course,
    it needs a lot more space around it and oriented so that the
    fins form little chimneys, to convect away the 6 watts.
     
  7. Jamie

    Jamie Guest

    get your self a LM7812 or any manufacture of a 7812 3 terminal 1 amp
    voltage regulator. Radio Shaft also sells them.
    mount the tab on a heat sink.. connect the ground/common (middle leg)
    to your - 24 volt supply. the #1 leg is the + input from your 24 volt
    DC supply and the #3 leg is the regulated 12 volt output ..
     
  8. ehsjr

    ehsjr Guest

    Simplest/best/cheapest/smallest *might* be a
    resistor in series with your 12 volt unit.
    It depends on how much current your unspecified
    "unit" draws and whether it draws that current
    all the time.

    Ed
     
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