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This commentator writes:

if the modem is many years old, the capacitors can leak electrolyte and go out of spec and instead of just blocking the 'low notes' of a telephone conversation, it might also be filtering out some of the carriers used for data transmission so the modem blacklists them as noisy (quieter signal means relatively speaking, the noise becomes louder) which results in a slower connection.

This commentator writes:

As with older computers, my suspicion would be that a "bulging" capacitor would be the most likely weakest link in hardware such as this.

This repair site shows a method for repairing bulging capacitors. They write:

Inside, he found some bulging capacitors. Unsheathing his iron and replacing the obviously faulty components, [psgarcha] plugged the router in and had everything work. Great. Until those caps failed again a few months later.

To me it seems that you should choose components that have a lifetime corresponding to the lifetime of the device.

My question is: Why do ADSL router capacitors bulge after a few months?

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  • \$\begingroup\$ Both answers are good. Electrolytic capacitors are rated for a specified lifetime at a specified temperature. Actual lifetime approximately doubles for every 10 degrees Celsius DECREASE in mean operating temperature. Caps are typically rated for 2000 hours at either 85C or 105C cap (but many other rating combinations are available.). A 105C cap will last ABOUT 8 times longer than an 85C cap in the same operating conditions as Trated is 30C higher and 2^(30/10)=8. A 1000 hour rated cap (less usual) will last 1/2 as long as a 2000 hr cap etc. So if a device runs at 65C internal (very hot) ... \$\endgroup\$
    – Russell McMahon
    Apr 16, 2017 at 23:18
  • \$\begingroup\$ ... a 1000 hour 85C cap will last ABOUT Hours_rated x 2^((Trated-Toperating)/10) or here = 1000 x 2 ^ ((85-65)/10) = 1000 x 4 = 4000 hours = ABOUT 6 months. That would be a very poor cap (and you can get ones that are worse in practice) and 65C is very hot BUT it can happen. |||| Some mix of: BAD caps, HOT environment, Voperating too high, Iripple very high -> short life. \$\endgroup\$
    – Russell McMahon
    Apr 16, 2017 at 23:21
  • \$\begingroup\$ @Russell mentions ones that are worse in practice. You can't do better for bad electrolytics than this charming variety: 1000uf.com/ipaware/img/fake%20capacitor.jpg \$\endgroup\$
    – user133493
    Apr 17, 2017 at 0:46

2 Answers 2

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I would not imply that ADSL router capacitors (always) bulge after a few months. I see no plausible cause specific to ADSL routers generally, regardless of what happens in such and such a model.

Electrolytic capacitors are a relative weak point in any device. With that said, they can last decades.

  • Appropriate manufacturer choice is important. The capacitors shown in the photo in your linked repair article are from a second-rate manufacturer.
  • Thermal design is also important. Even electrolytics from the best manufacturers (typically Japanese) will fail if they are allowed to get too hot. This can be due to bad original design, or user error (blocking ventillation, etc.)
  • ESR must also be considered. Some designs call for capacitors with especially low ESR. If average caps are substituted, there will be trouble. One single supply chain cockup and a whole batch of products can be vulnerable. Also, most ADSL routers are consumer products with aggressive cost saving in their design.

Read about the historic capacitor plague here: https://en.wikipedia.org/wiki/Capacitor_plague. This article provides plenty of information about failure modes for electrolytic capacitors.

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Very probably lazy/poor/apathetic design work. The capacitors might be very poorly made, or might be operating outside their specified temperature range due to thermal effects from other components (many routers run quite hot). Perhaps they are being run at or near their voltage limit or ripple current limit and are therefore overstressed.

It could be any number of things, all of which could be mitigated by proper design, but proper design implies paying someone to design a router properly and paying for the components necessary to build a router properly, which nobody wants to do. It's not like the capacitors that would last for longer than a few months don't exist, they just aren't being used, likely for reasons that are ultimately cost-related.

In short, low-margin consumer electronics failing due to short component lifetimes should not be a surprise: better designers and better components cuts that slim margin toward zero, and then the shareholders aren't happy.

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