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I hope to repair a p.c.b. for a hard disk drive, which has a burnt out SMD component (by mistakenly connecting the supply back to front -- I had to guess and got it wrong), 2010 size, a connection at each end, with a polarity(?) line at one end . It has "ON" in a circle, and "QA" and "R 846 ." written on it.

I have searched on line with no success, so please can anyone identify this component?

enter image description here

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    \$\begingroup\$ A photo would give much more info than just the text on the part. \$\endgroup\$
    – brhans
    Commented Jul 5, 2016 at 18:15
  • \$\begingroup\$ ON in a circle usually means those guys, maybe you should search on their website. \$\endgroup\$
    – Dmitry Grigoryev
    Commented Jul 6, 2016 at 10:28
  • \$\begingroup\$ Not sure why this was closed. Component identification frequently comes up and is pretty much always on topic. \$\endgroup\$
    – Tom Carpenter
    Commented Jul 6, 2016 at 15:25
  • \$\begingroup\$ Indeed, the user doesn't ask for repair help. That's just to give some context to his question. \$\endgroup\$
    – pipe
    Commented Jul 13, 2016 at 8:01

1 Answer 1

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From your description of the component, and how it failed, it is this NSA5.0AT3G OnSemi 5 V TVS diode. Of course the photo from you will help confirm this.

These TVS diodes are common on hard drive PCBs. Common reasons for them to fail include sustained overvoltage (e.g. 12 V supply connected to 5 V input) or reverse polarity connection (as in your case).

Another way for you to confirm the component identification, is to reverse-engineer the function of that burned component on the PCB, by following the PCB traces from the drive's power connector. A TVS diode would be placed directly across the power input (with possibly a fuse or similar in series). Based on it being a 5 V TVS, it would be placed across the 5 V input of course (not the 12 V input, if yours is a 3.5" drive which uses that; 2.5" drives don't use 12 V power).

Beware that you might also have additional damage to that hard drive, including:

  • damaged PCB track, due to the high current flow through the diode while it was conducting (not all manufacturers include fuses or similar in series with the TVS diodes on these PCBs), and/or a blown fuse or similar (if fitted);
  • other damaged electronic component(s) on the PCB, especially if the full reverse voltage reached them for an extended period e.g. if the TVS failed open-circuit (or a fuse "opened" and disconnected the TVS) with the reverse voltage still applied;
  • damaged head(s), as they are very sensitive to reverse or excess voltage.

(The QA component marking is confirmed on page 1 of that linked OnSemi PDF, but I'm currently on a mobile so adding an image of that will be delayed until I'm back on a bigger screen & keyboard.)

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