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Voltage rating of LM317

Discussion in 'Electronic Design' started by [email protected], Nov 8, 2006.

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  1. Guest

    The datasheet of the LM317 adjustable regulator says that the
    max voltage rating of 40V is for the difference between input
    and output, instead of between + and - supply as for 78xx
    regulators. That's clear enough.

    But it's usually desireable to filter the output of any
    regulator with a capacitor, and the output terminal will be
    momentarily shorted to ground when it's first switched on.
    This will cause the input-output voltage rating to be
    exceeded in cases where the absolute input voltage is higher
    than 40V. The datasheet doesn't say anything about this.

    If this were a matter of dissipation, it would be of no
    consequence, but it concerns voltage breakdown. Can anyone
    clarify this point ? Can the LM317 be safely used with an
    input of, say 45V and a filtered output of 12V ?
     
  2. Mike Monett

    Mike Monett Guest

  3. Guest

    That's obvious. But that's a sustained fault condition.
    I'm talking about a momentary initial spike, under which
    many devices can withstand levels that exceed their
    continuous max ratings.
     
  4. Dave Moore

    Dave Moore Guest

    : The datasheet of the LM317 adjustable regulator says that the
    : max voltage rating of 40V is for the difference between input
    : and output, instead of between + and - supply as for 78xx
    : regulators. That's clear enough.
    :
    : But it's usually desireable to filter the output of any
    : regulator with a capacitor, and the output terminal will be
    : momentarily shorted to ground when it's first switched on.

    When what's switched on?
    If you mean the power source that feeds it then just
    make sure that the source has a longer time constant.

    : This will cause the input-output voltage rating to be
    : exceeded in cases where the absolute input voltage is higher
    : than 40V. The datasheet doesn't say anything about this.
    :
    : If this were a matter of dissipation, it would be of no
    : consequence, but it concerns voltage breakdown. Can anyone
    : clarify this point ? Can the LM317 be safely used with an
    : input of, say 45V and a filtered output of 12V ?
    :
     
  5. How fast does the input rise?

    If it's a power-line transformer and rectifier setup, the input is
    going to take at least a quarter cycle to rise. That's your salvation,
    as you only need to filter the output side for high frequencies, much
    higher than 240hz. The regulator's frequency response will take care
    of the lower frequencies.


    If it's a switching-mode source, you do have a problem.

    In either case I wouldnt push the regulator's limits. Capacitors.
    especially tantalum and deposited-film ones, have been known to
    flash-over and short for brief moments. You probably want your design
    to survive these conditions.
     
  6. Tim Shoppa

    Tim Shoppa Guest

    Where I have, I've always had my butt saved by the HV versions of the
    regulator :).
    At some point you have to draw a line in the sand and say you aren't
    going to protect component X if component Y if component Z fails.
    Failed PS actives take out capacitors and transformers, failed PS
    passives take out PS actives. Sometimes the circuit under supply will
    let its smoke out too, as long as the whole gadget doesn't catch on
    fire I don't see what the big deal is :).

    In time-proven commercial designs the manufacturer has good stats about
    warranty costs for the various failure modes and can make a sound
    economic decision about what to protect and what not to, with the goal
    of making a whole working device that is reliable and cost-effective in
    the real world. Those working outside the commercial world often have
    some truly ridiculous design constraints on their power supplies, many
    of them completely unrealistic.

    The example you point out (switching supply with fast uncontrolled rise
    time driving a regulator with too much capacitance on its output) is
    one of those unrealistic examples. In the real world nobody would hang
    big capacitors that are likely to fail (probably because of their low
    ESR's) on the output of a regulator, and they would avoid putting a
    linear regulator on a switcher output with a HV output drop. But it
    happens way too often. Somebody somewhere should've pushed back and
    said how foolish it is, and it's their fault for not doing so!

    Classic example of a design decision that is remarkably similar to the
    OP's question: transformerless 5-tube tabletop or clock radio. Turning
    it on makes a high-current surge through the cold filaments that
    wiggles the filaments. Every so often a filament will touch a
    cathode. If it tocuhes a cathode at the high end of the filament
    series-string, like the rectifier or output tube, the radio's PS's
    filter condenser is asked to filter 117VAC, and blows up, probably
    taking the failed tube, and maybe some outher tubes with it. This
    failure scenario is classic and common, but not common enough to have
    stopped billions of 5-tube transformerless radios being made over half
    the 20th century (most of which would still work today!)

    Tim.
     
  7. You're exceeding the maximum rated conditions, beyond which
    the manufacturer of the LM317 doesn't take any liability,
    Do you ? (Be brave).

    Rene
     
  8. There is also the reverse scenario, power off, where the output electrolytic
    is still charged while the input drops faster.
    IIRC long time ago I have seen an application note that took care of that
    with a reverse diode over the LM317.
    In the forward (first) case, you would need a say 30V high power zener
    (it would work as normal diode the other way) across the device.
    The zener would have to be able to handle the peak charge current.
    Maybe a transistor / zener combination.
     
  9. Eeyore

    Eeyore Guest

    I've used one to regulate many a 48V low current supply.

    A parallel zener of say 33 or 36 V solves the overvoltage issue.

    Graham
     
  10. Eeyore

    Eeyore Guest

    Don't use those capacitors !

    Graham
     
  11. Eeyore

    Eeyore Guest

    You'd be nuts not to fit a reverse diode across th regulator where significant
    capacitance exists on the output.

    Graham
     
  12. Tantalum is OK, has reasonably good RF decoupling, just do not reverse mount
    these ;-)
    I always use tantalum in 78xx and LM317 circuits.
    Some have lasted > 25 years, none have failed.
    Alu electrolitics will somtimes dry out over time.
     
  13. Mike Monett

    Mike Monett Guest

    See SOA

    Regards,

    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    http://silversol.freewebpage.org/index.htm
    SPICE Analysis of Crystal Oscillators:
    http://silversol.freewebpage.org/spice/xtal/clapp.htm
    Noise-Rejecting Wideband Sampler:
    http://www3.sympatico.ca/add.automation/sampler/intro.htm
     
  14. On Wed, 08 Nov 2006 13:49:47 GMT,
    This is a good idea for any linear regulator. I once blew out about 20 78L15's
    by acciedntally shorting out the (otherwise short-circuit-protected) +18V rail
    of a $50000 piece of lab equipment. Most of these were on a single letter-sized
    6HU board densely populated with tons of discretes. The hardest thing was
    finding every single last one of them damn' things.

    At first, of course, my heart sank when right after the short the room filled
    with burnt-epoxy stench and everybody looked at me...

    robert
     
  15. Mark

    Mark Guest

    I think there is an LM317HV version (high voltage)

    Mark
     
  16. Eeyore

    Eeyore Guest

  17. neon

    neon

    1,325
    0
    Oct 21, 2006
    Specifications are written to give you the designer some thing not to exceed as a safety guideline. Not all devices are created equal some devices exceed the max voltage if tested manufacture only test what they know will pass them through quality control without rejections. You may find one device surpassing the max spec but would i test for it and use after absolutily not. And yes if the output cap is huge put a diode across the device in-out to protect it that way. it is all there you must read and understand the specs.And finaly 40 volts surely exist when power is turn on and the output is zero.And there is no HV lm317 I wish it was.
     
    Last edited: Nov 8, 2006
  18. neon

    neon

    1,325
    0
    Oct 21, 2006
    with a cap on the input output a spike how from where and the fault for devices invariably is heat not current. GERMANIUM DIODE CAN PASS AMPS IF PROPERLY COOLED.
     
  19. Guest

    It's gratifying to see so many replies to my question
    while I was out for the rest of the day. Too many to
    reply to one by one, I'm afraid.

    Okay. Accepted that it's not adviseable to use the LM317
    with an input that may exceed 40V, no matter what the
    input-output difference is. But for the sake of argument,
    what about NatSem's statement that the device can be used
    to regulate hundreds of volts as long as the input-output
    differential does not exceed 40V ? This would then assume
    that the load has 0 capacitance. Can such a load even
    exist in practice, without even a pF or so of stray
    capacitance ? If not, then that brings us back to the
    matter of time-limited surge and rise time.

    The regulator is to be used with a transformer-rectifier
    input of 27 Vrms at normal line voltage, with load current
    ranging from a very few mA to about 0.6A. The transformer
    also has another load ranging from 0-3A. That's an input
    of nearly 37V to the regulator at light load. That leaves
    too little margin for mains fluctuations that may rise to
    more than +10%.

    Circumstances permit the use of only the most widely
    available parts. I hadn't thought of the LM317HV. It's a
    good choice. I don't have it in stock and will have to
    send for it from another city - which is not as simple a
    matter in certain parts of the world as it is in some
    others. Otherwise I'll have to use another transformer for
    the 12V supply, or take a different approach to the rest
    of the design.
     
  20. I love tantalums too. I've fixed at least three dozen Tektronix scopes
    where one to five of those orange gumdrop tantalums shorted out. Also
    fixed a $12K Fluke calibrator that had three shorted tantalums. All
    that equipment bought for pennies on the dollar, as they were
    completely dead.

    Try taking a gumdrop tantalum and turn up the voltage on it above its
    rating. A lot of them will avalanche into a dead short at about 125%
    of rating. Not encouraging.

    Yes they will. At anytime from a year to 75 years depending on how hot
    they got.
     
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