Faulty HDD Power Supply

This external hard drive power supply was unreliable suggesting a bad solder joint was the cause.

It is rated to supply 12V DC at 2A and 5V DC at 2A.

Inside

Inside is a typical budget switch-mode power supply.

The through-hole components make it appear quite busy, but there are a couple of important parts missing; or at least highly recommended.

• mains input filtering
• better output filtering

Without these it is likely to be quite noisy; generating RF interference.

The Fault

The fault appears to be broken solder joints on the main switching transistor. There has not been enough solder applied and the 3 joints are supporting the small alloy heatsink.

The rest of the soldering looks to be reasonable, although some joints could do with a bit more solder.

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Repair

The repair involved tidying up the broken solder joints (and a few others for good measure) and replacing the 1KV 222 capacitor for a Y rated safety capacitor (see below).

Testing

The power supply is rated for 100-240VAC input so I tested it with a current limited 110VAC supply. This way any faults won’t blow fuses or do any additional damage.

With no load, the first time powered, the output LED came on the faded. There was then a heavy draw from the mains. Without the current limiting it would likely have blown a fuse or worse. But this was the only time it has done this. It’s now working correctly. What does this mean…. It may not start reliably with the current limit in circuit. If it was a previous problem, it would have almost certainly blown the fuse before now.

Load testing is difficult due to the output connector (I don’t have a free socket). So it’s easier to solder 3 wires to the board to connect some load. Loaded to about 1A on the 5V output and 0.4A on the 12 V output it operates correctly and does not get too hot during a 2 hour run.

It’s likely it will reliably supply close to it’s rated output but I wouldn’t want to overload test it. I’ve seen a few of these marginal power supplies fail; some at less than rated output current.

Other Issues

There are a few other criticisms.

The mains power connector has very rough pins. They have been stamped and not cleaned up. Possibly not a major problem if you leave it plugged in all the time. But this is for a portable hard drive that may be plugged and unplugged often. It will not be good for the contacts inside the plug on the mains lead.

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The second point is that the capacitor connected across the transformer, mains side to output side, is not “Y” rated. In theory this is a safety issue.

A “Y” rated capacitor is safety certified for use where failure could lead to the danger of electric shock. They have a high impulse voltage rating. Y2 to 5000 volts.

The third point applies here in New Zealand when used with a NZ 3-pin to IEC mains lead.

Although there is a fuse on the board on the mains input, it is in the Neutral line, not the Active (phase) line.

The fuse will effectively disconnect the mains supply if there is a fault within the power supply, but it does not isolate the phase in the unlikely event of a failure that also connects the output to the mains side of the supply. Probably very unlikely, but in my opinion not the best.