Hard disk protection device

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I posted this question in sci.electronics.repair but none of the
well-meaning replies was of much help. I hope I have better luck
here. It's about the two-terminal SMD 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, but with
more capacity to absorb transient surges - perhaps a TVS or
polymer device. It would also provide protection against
sustained 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?

Re: Hard disk protection device

pimpom wrote:
Quoted text here. Click to load it

I have one on a disk drive here. It is an SGS Thompson (STMicro or st.com)
part. I can tell by the tiny logo printed on it. It sits across the
12V rails (I checked with a multimeter). Mine is labeled BUX C307,
but I am not able to find data.

There is an explanation here as to the function.


    "The protective diode originally designed using the "transil"
     technology at SGS Thomson is intended for protection of
     electronic circuitry from short power supply peaks not greater
     than 10 - 20 microseconds. But in that case their common
     failures demonstrate that HDD designers did not expect to
     encounter so poor quality of power supply units. Thus drive
     operation can be resumed after simple removal of that damaged
     element from its circuits but we cannot guarantee
     flawless HDD operation without that component."

So, that suggests it is a "transil".


This is a representative "transil". This isn't the BUX, but
you can see examples of the marking codes. The marking
code is not the part number. You need to translate from
one to the other.


It might be intended to suppress inductive transients, such as
might occur if power was removed while the drive was running.

It is not intended for sustained overvoltage operation.

It cannot provide reverse polarity protection. The many amperes
that the power supply can provide, would likely blow it away.

A series connected diode is how you'd provide polarity protection,
at the risk of destroying voltage regulation. The disk drive
connector is polarized, and unless the ATX supply is miswired,
there is no reason to suspect the environment would connect
the power up wrong.

So that part is most likely there to catch inductive transients,
with a secondary purpose of also clipping ESD if it happens.


Re: Hard disk protection device

Paul wrote:
Quoted text here. Click to load it

Thanks for the reply and the references. Regarding sustained
overvoltage and reverse polarity, it is fairly obvious that a
two-terminal shunt device would not be able to provide
non-destructive protection. I have seen several devices
permanently shorted, triggering PSU over-current shutdown, and
some were literally blown apart. Even that kind of protection is
certainly better than none.

Just for the record, someone in sci.electronics.repair says that
he knows of one instance where the Molex connector had been
forced the wrong way in. Even then, the +5V and +12V would be
interchanged without polarity reversal, and that is where
sustained overvoltage protection would be needed. So reverse
polarity protection would be very much a secondary consideration,
if at all.

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