How Do Capacitors Fail?

Capacitors are solid state, right? They’re just, little tin cans that do stuff to electricity, they shouldn’t really fail. How does something fail when it doesn’t have any moving parts or experience any stress? It’s not like the capacitor is burning, melted inside, or being changed every time the air conditioner kicks on, right?

Chemical Changes

It turns out, there actually is a great deal of stress on capacitors: heat. The hotter an environment is, the shorter that capacitors lifespan. Internally, most capacitors are not all solid. There is a (typically) liquid substance, an electrolyte, which plays a key roll in the movement of electricity through the capacitor. This liquid is sensitive to the temperature.

Overtime, the electrolyte can break down when it’s exposed to prolonged heat, outside it’s operating temperature or near it’s rated maximum. It’s just like leaving your cookies in the oven at 500 degrees for 5 days straight. You’ll come back to find not cookies, but a broken down, charred pile of dust that smells more like your grill than food. We can design the electrolytes for a variety of temperatures, but most are meant to peak at 140C or so.

It’s nowhere near 140C outside.

The trouble comes in when you put these capacitors next to hot equipment, without enough airflow. The mere act of using the capacitor will cause it to heat up. Energy moving through electronics will always create waste heat. If you have a 100 watt capacitor, it might lose 10-20 watts of that power to heat. Then there’s all the heat given off by the rest of the system. This is exasperated by dirty air conditioners or a lack of airflow. Eventually, the heat will cause the electrolyte to wear out and fail.

If the electrolyte doesn’t fail, the metal coil inside the capacitor would probably be next in line. With enough stress, the capacitor can short internally and just cease to function altogether. This is less likely than electrolyte failure, but it’snot impossible.

Rapid Failure

Most failures are gradual, a long process that plays out over months and years. Everything is headed to failure from day one, it’s only a question of how long until the laws of physics and entropy catch up with us. How long can the inevitable be held at bay? There are however, also rapid failures. Your capacitor has rated performances, it’s okay with maybe 440 volts, and 100 degrees C for operating. Push it outside that envelope and things get interesting (and expensive).

When we drive the capacitor over it’s rated tolerances, we start to build up heat. The farther we push, the more heat we get. The electrolyte will start taking in all that energy. Everything expands with energy. Eventually you’ll turn a solid to a liquid to a gas. We’ve got a tiny bomb. There’s pressure building inside the capacitor. Something has to give.

Capacitors today are designed to control exactly what gives out: the vent. Usually the top of the capacitor or a portion of it, is designed to take less pressure than the rest of it. When the pressure is too high, the top will rupture and everything that was in the capacitor will no longer be inside the capacitor. It’ll be sprayed out as a mixture of liquid and gas.

We were going to demo this, but the landlord, lawyers, and lead sales guy Scott weren’t pleased with the idea of tiny explosions and the smell of fresh, molten electronics in the morning. Instead, enjoy this youtube demonstration blowing up little capacitors (I wanted to blow up a big one!).



Recovering from Failure

When a failure like this occurs, the repair can vary. Under ideal conditions, you just replace the capacitor. Your HVAC contractor will shut off power to the unit, possibly do some soldering, and within an hour or two everything is back on track. There are however, some occasional complications. Some capacitor electrolytes are corrosive and may cause damage to other components, like circuit boards. If a board is damaged, it will probably need to be replaced. There are specialists who can repair these circuit boards, but the cost of those repairs generally outweighs the cost of a new replacement.

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