Avian’s Blog

I was recently doing some electrical work and had to turn off the power in the apartment. I don't even remember when I last did that - the highest uptime I saw when shutting down things was 617 days. As it tends to happen, not everything woke up when the power came back. Most were simple software problems common on systems that were not rebooted for a while. One thing that was not a software bug however was the Adafruit's tube clock, which refused to turn on when I plugged it back it.

I got the clock as a gift way back in 2010 and since then it held a prime position in my living room. I would be sad to see it go. Over the years its firmware also accumulated a few minor hacks: the clock's oscillator is quite inaccurate so I added a simple software drift correction. I was also slightly bothered by the way nines were displayed on the 7-segment display. You can find my modifications on GitHub.

Thankfully, the clock ran fine when I powered it from a lab power supply. The issue was obviously in the little 9 V, 660 mA switching power supply that came with it.

Checking the output of the power supply with an oscilloscope showed that it had excessive ripple and bad voltage regulation. Idle it had 5 V of ripple on top of the 9 V DC voltage. When loaded with 500 mA, the output DC level fell by almost 3 V.

I don't know what the output looked like when it was new, but these measurements looked suspicious. High ripple is also a typical symptom of a bad output capacitor in a switching power supply. Opening up the power supply was easy. It's always nice to see a plastic enclosure with screws and tabs instead of glue. Inside I found C9 - a 470 μF, 16 V electrolytic capacitor on the secondary side of the converter:

The original capacitor was a purple Nicon KME series rated for 105°C (the photograph above shows it already replaced). Visually it looked just fine. On a component tester it measured 406 μF with ESR 3.4 Ω. While capacitance looked acceptable, series resistance was around ten times higher than what is typical for a capacitor this size. I replaced it with a capacitor of an equivalent rating. Before soldering it to the circuit, the replacement measured 426 μF with ESR 170 mΩ.

After repair, the output of the power supply looked much cleaner on the oscilloscope:

The clock now runs fine on its own power supply and it's again happily ticking away back in its place.

I guess 9 years is a reasonable lifetime for an aluminum capacitor. I found this datasheet that seems to match the original part. It says lifetime is 2000-3000 h at 105°C. Of course, in this power supply it doesn't get nearly that hot. I would say it's not much more than 50°C most of the time. 9 years is around 80000 h, so I've seen a lifetime that was around 30 times longer than at the rated temperature. This figure seems to be in the right ballpark (see for example this discussion of the expected lifetime of electrolytic capacitors).