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Overvoltage protection

Discussion in 'Electronic Design' started by Louis Bouchard, Feb 17, 2007.

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  1. Greetings, my assembly contains the following HF amplifier (50 ohms in/
    out), whose max input power is 3W (about 12 volts on the input),
    http://catalog.rell.com/rellecom/Images/Objects/11300/11269.PDF
    but the HF300-0130 does not contain any overvoltage protection, so any
    overshoot could potentially kill it.
    Does anyone have recommendations on what I could use to provide some
    kind of
    overvoltage protection or voltage clipping to ensure the input never
    goes above 12v (3 watts)?
    This has to be 50 ohms impedance matched, fail-free, low noise and
    without introducing
    any transients (all risetimes should be less than 1usec).
     
  2. Guest

    The datasheet says "Max=3Watts RF input" (not voltage). Just use a
    variable attenuator that can handle the max power. M/A COM makes some.
     
  3. Rich Grise

    Rich Grise Guest


    Put a transzorb across the supply terminals, and a fuse in line from
    the power supply to it (and the +12 power to the board.)

    Googls "transzorb" - I'm not with the company, just a satisfied
    customer.

    Good Luck!
    Rich
     
  4. Guest

    ummm. the supply voltage is 50volts not 12 volts. AFAIK no one makes a
    50 ohm matched transzorb that works at HF , Please provide google
    link... ;^D
     
  5. Rich Grise

    Rich Grise Guest

    Max Foo, you need to learn to read. The supply voltage is 12V, the
    impedance of the signal is 50 ohms.

    Good Luck!
    Rich
     
  6. Guest

    READ the datasheet! the link the OP provided. supply voltage =52Volts,
    Current= 13Amps
    It's a 300Watt HF amplifier. where are you seeing this 12Volts on the
    spec sheet.
    The OP is confusing voltage protection with max input power...
    http://catalog.rell.com/rellecom/Images/Objects/11300/11269.PDF
     

  7. Okay, the confusion may be the 'overvoltage' terminology. The dc
    supply is
    52vdc 13 amps, and this is always constant (no need for overvoltage
    protection
    on the dc supply end), as the power supply is a very good one.
    The protection that is needed is on the 50 ohms RF input.
    The amp can be damaged if this exceeds 3W input. So 3 watts input
    into a 50 ohms port corresponds to 12 volts (Pmax=Vmax^2/R50ohms).
    That's where the 12 volts comes from. This amp is being fed by a 5W
    preamp, so the goal is to limit the output of the 5W amp, or,
    equivalently,
    to limit the input of the 300W amp. The attenuator solution proposed
    earlier will work, but I'm not sure it's the most efficient in terms
    of noise-
    adding passives (e.g. resistors) usually adds more noise.
    I was thinking about clipping the input to 12 volts, but I'm not sure
    this
    would be any better.
     
  8. Clipping will to the job (O.V. protection) but when the input is driven
    above the clipping limit, you will get noise (harmonics). You might have
    to add some filters downstream of the protection stage to keep the
    generated harmonics out of the amplifier.
     

  9. Harmonics are o.k., as long as they don't exceed 12 V. Would you
    recommend
    a particular produce as voltage limiter device, which can work in a 50
    ohm
    transmission line? (I have only used voltage limiters for dc
    operation)
     
  10. Guest

    Agilent makes power limiters but they are not cheap ($600-$1000).
    http://www.testequity.com/products/1378/
     
  11. Ban

    Ban Guest

    Well, you seem to be unexperienced with RF. A +/-12V limit will *not`* be
    2.88W into 50R(exept for a square wave). And the 52V/13A supply is the abs.
    max. rating, better to stay with the 48V given in the datasheet.
    To protect your amp you can reduce the supply voltage of the preamp, so it
    delivers the required power and drives the power stage to the specified
    level at onset of clipping at the operating frequency. If you cannot access
    the preamp, a variable 5-10W pad is the best solution. The noise floor will
    *not* be higher, since it will come from the preamp and will be attenuated
    as well.
    Much more important to match the load, whatever it is.
     
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