Hard disk protection diode?

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.

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.

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).

BUX * devices are usually high voltage transistors

Arfa

"Arfa Daily" wrote in news:0S8fl.7569$ snipped-for-privacy@newsfe14.ams:

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.

--
Jim Yanik
jyanik
at
kua.net

On Mon, 26 Jan 2009 00:21:34 +0530, "pimpom" put finger to keyboard and composed:

Is it a transient voltage suppression diode?

- Franc Zabkar

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On Sun, 25 Jan 2009 11:07:31 -0800, "William Sommerwerck" put finger to keyboard and composed:

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

- Franc Zabkar

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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.

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

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

But there *are* such devices on every hard disk I've examined. At least, I can't think of any other purpose (counting inductive spikes as an OV condition). And not all PSUs are created equal. I have seen such devices burnt out on several HDDs, even to the extent of being literally blown apart.

Could be. In fact, I mentioned that in my second post. What I find surprising and frustrating is that I have not been able to find any data on the BUX C*** series.

On Mon, 26 Jan 2009 15:45:37 +0530, "pimpom" put finger to keyboard and composed:

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 R617

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 =

6QE):

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

- Franc Zabkar

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On Tue, 27 Jan 2009 06:13:23 +1100, Franc Zabkar put finger to keyboard and composed:

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:

- Franc Zabkar

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On Mon, 26 Jan 2009 15:19:08 +0530, "pimpom" put finger to keyboard and composed:

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

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y

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.

Gareth.