Replacing these capacitors?

Question

This is a controller board for a home ventilation system. It runs fine most of the time but sometimes the ventilation stops for one minute, then starts again and runs normally for anywhere between one minute and many hours.

Internet search results indicate that the two capacitors are at fault; the system works again for other users after they have replaced those two capacitors (1000uF25V, 1000uF50V,) so I am going to replace these two capacitors even if they visually look fine. I figure that even if they are not the cause of the failure, getting new ones won't do any harm.

1) Replacements are listed as e.g. 5000 hours lifespan.
a) That sounds awfully short for an always-on system. I don't want to fix this problem over and over.
b) Could I use solid-state equivalents instead; would that give a longer service time of the repaired board?

However, looking at the board it seems there is (was?) a heat problem nearby, but not right there. The reverse side shows that spot more clearly.

2) Could it be that the problem is not those capacitors at all, but rather another component? I can solder, but electronics is not my strong point so I am clueless how to identify and troubleshoot. From the discoloration on the reverse side, it looks like three component became hot (marked red in the third image): IC "78L05A", resistor "3321", and the transistor "BD438". Does this discoloration indicate a failure or is that expected with normal use?

Answer 1

I'm only going to answer (1) because (2) cannot be determined by guesswork so: -

Replacements are listed as e.g. 5000 hours lifespan

5,000 hours is about 7 months but, the 5,000 hours is at full temperature and full voltage rating.

If the actual working voltage is about half the full rated voltage then the 5,000 hours becomes 10,000 hours (every little helps of course). But the big guns are temperature...

If the actual operating temperature of that capacitor was 10 degrees lower than the rated temperature, then the operating life would double to 10,000 hours. If it were 20 degrees below then the operating life would be 20,000 hours. At 30 degrees below full rating the life would become 40,000 hours (about 4.5 years).

And if the voltage was at 50% of rating as well, the life would become 80,000 hours or about 9 years.

Try using this calculator and note the following : -

L1 is the 5,000 hours stated by the capacitor manufacturer and I assumed that it was a 105 degC rated component. Some capacitors are only rated at 85 degC and these may not be suitable for long life if the box which houses the electronics is quite warm internally.

Also note that capacitors of this type will self-heat due to ripple current and this would need to be determined in some applications in order to get a more precise feel for anticipated lifespan.

Remember also that the lifetime rating for an electrolytic capacitor is not a reliability number - it tells you about performance deterioration rather than predicting catastrophic failure. The MTTF for an electrolytic is still circa 1,000,000 hours in most cases.

Here's a graph that shows how a good electrolytic capacitor has very long life when operated within its ratings: -

This is a capacitor I specifically chose because I needed long working life. It was rated at 105 degC to 10,000 hours as depicted by the curve on the right hand side. But I wanted it to work at 72 degC max so I immediately gained an increase in lifespan from 10,000 hours to 200,000 hours. If I wanted to operate it close to 100% ripple current, by a little interpolation I might conclude I would get 170,000 hours lifespan at 72 degC.

Electrolytic capacitors that don't give this invaluable information in their data sheets (or don't have links to it) are just not worth buying IMHO.

Answer 2

The board is running hot. Install a small clipon heatsink on that "T7".

Let the air do some cooling.