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Hard disk protection diode?

Discussion in 'Electronic Repair' started by pimpom, Jan 25, 2009.

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  1. pimpom

    pimpom Guest

    Does anyone have specific information about the protection device
    at the power supply inputs of hard disks?

    I assume that it's a zener diode or something similar in action
    such as an IC that acts like a precision zener diode. It's not
    difficult to see how such a device would provide protection
    against spikes, over-voltage and reverse voltage by shorting the
    power rail to ground and thereby triggering PSU shutdown.

    Devices in the BUX C*** series seem to be widely used, but I have
    not been able to find a datasheet or other detailed info. Can
    anybody shed some light on the matter?
  2. I know I'm not directly answering your question, but...

    The socket and power cords are polarized. It would be extremely difficulty
    to insert the plug the wrong way.
  3. pimpom

    pimpom Guest

    Thanks for the reply. Yes, I expect that reverse voltage
    protection would be a secondary consideration. Its primary
    function would be protection against over-voltage, transient or
    sustained. Something like a crowbar or TVS device, but without
    the need for very low capacitance. A datasheet or identifying the
    manufacturer would be a big help.
  4. whit3rd

    whit3rd Guest

    The most important reason a hard disk would need a protection
    device, is that the two power connections (motor power, 12V,
    and logic power, 5V) have a sequence condition requirement.

    So, the diode might clamp +12 to +5V so that a drop in the
    motor power (like, normal power turnoff) doesn't result in
    any short time during which the "+12" is actually at lower
    voltage than the "+5". Or, it might ensure that the +12
    supply never goes negative (which could happen if a
    motor is active when power is removed).

    Probably this diode is NOT a Zener type, which is
    relatively important: high current Zeners often fail short-circuit
    (which would halt the computer until disconnected).
  5. Arfa Daily

    Arfa Daily Guest

    BUX * devices are usually high voltage transistors

  6. Jim Yanik

    Jim Yanik Guest

    the PSU itself usually has overvoltage protection,where it really counts;on
    the +5 volt supply. The +12 is loosely regulated,and only runs the motor
    drive.some have "balance" nodes,that trigger SD if one or more of the
    supplies go too far outside a window.

    Then you don't need the expense of OV protection on every hard drive.
  7. Franc Zabkar

    Franc Zabkar Guest

    Is it a transient voltage suppression diode?

    - Franc Zabkar
  8. Franc Zabkar

    Franc Zabkar Guest

    It would require a very heavy hand, but I *have* seen it done.

    - Franc Zabkar
  9. pimpom

    pimpom Guest

    I rather expect that it is similar to a zener diode in action but
    more sophisticated than a simple discrete zener, perhaps with
    more precise breakdown voltage, lower dynamic resistance and more
    surge current capacity. And a shorted device is not uncommon.
  10. pimpom

    pimpom Guest

    In any case, reversing a 4-pin Molex connector will not reverse
    the polarity. It will just juxtapose the +5 and +12V lines.
  11. pimpom

    pimpom Guest

    Yes. But transistors have the BUX followed by numeric characters.
    These devices have a "C" before numerals.
  12. pimpom

    pimpom Guest

  13. Franc Zabkar

    Franc Zabkar Guest

    This forum discussion is about TVS devices in hard drives:

    The above discussion refers to this photo of a Seagate HDD with two
    TVS devices made by ON Semiconductor:

    The parts have the following markings:

    ON logo QE

    ON logo 620
    LEM .

    I can't find exactly the same parts in ON's datasheets, but here are a
    few similar looking devices:

    Unidirectional Zener TVS 13V 600W (marking = LEN):

    Zener Transient Voltage Suppressor 12V 600W (marking = LEK):

    400W Peak Power Zener Transient Voltage Suppressor 5V (marking = QA):

    600 Watt Peak Power Zener Transient Voltage Suppressor 5V (marking =

    Here is a Vishay patent that discusses TVS devices with particular
    reference to their application in hard drives:

    - Franc Zabkar
  14. Franc Zabkar

    Franc Zabkar Guest

    600 Watt Peak Power Zener Transient Voltage Suppressor, 12V,
    Unidirectional, p/n SMBJ12AON (marking = LEM):

    The "QE" marking belongs to a 1SMA5.0AT3, 400 Watt Peak Power Zener
    Transient Voltage Suppressor:

    ON TVS/Zener Device Data Book: Zener.pdf

    - Franc Zabkar
  15. Franc Zabkar

    Franc Zabkar Guest

    The BU? marking code appears to be used by ST Microelectronics in
    their SMBJ series 600W Transil (TVS) range.

    See page 2 of the datasheet:

    The BUX marking isn't listed, but it would appear that the "U"
    designates a unidirectional device.

    The "C" appears to be the manufacturing location, followed by a
    numeric YWW date code.

    - Franc Zabkar
  16. Guest

    I have come across these shorted protection diodes on 2 occasions now,
    both on Digital Audio Recorders powered by external wall warts. If
    the wrong polarity wall wart is used, you can get the wrong polarity
    12Volts to the Hard Drive. This is shorted out by the diode, which
    eventually burns to a dead short protecting the drive. On both
    occasions replacing the diode restored the unit to fully operational.
    Good job they are there IMHO.

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