Pressure washer recall and aluminum electrolytic capacitors

I recently came across a disturbing piece of news about a recall of Ryobi pressurized washers on FOX Business (Figure 1). I got some pictures from there and elsewhere, which, with a little rearrangement and supplementation, point out a very real danger.

Figure 1 Screenshot of the news article on the pressure washer recall. Source: Fox Business

The so-called overheating capacitors can apparently be identified as shown in Figure 2.

Figure 2 The overheating motor starting capacitor under question. Source: Amazon.com

This component provides 300 µF, which is a magnitude of capacitance that can only be obtained in aluminum electrolytic capacitors. Such capacitors cannot be allowed to experience reverse voltage, though, so to achieve the 250 VAC capability, a pair of capacitors must be used in series as shown in Figure 3.

Figure 3 Capture of a schematic that combines two aluminum-electrolytic capacitors.

Each capacitor is paired with a diode that limits the reverse polarity that can appear across each capacitor to one forward diode voltage drop, call that 0.7 V. Supposedly, that voltage limit is still safe as reverse voltage across an aluminum electrolytic capacitor.

However!!!! If one diode fails as an open circuit, the reverse voltage that can be imposed on its associated capacitor can rise way above the diode limit, and that capacitor can fail.

Figure 4 A SPICE example of reverse voltage when one diode fails as an open circuit. Source: John Dunn

Such a failure can lead to a capacitor explosion.

When I was in college, I had a lab partner with whom I would perform each class experiment. One experiment involved a 22-µF 16-V electrolytic capacitor. It was a tiny little thing.

Unfortunately, my partner (It was NOT me!) put that capacitor in the circuit board backwards, and it was driven into reverse bias. It sat there for a while as the two of us were discussing the circuit under test when suddenly that capacitor exploded!!

That explosion was LOUD!! Everybody within fifty feet was looking in our direction. The aluminum shell of the capacitor had been torn open like a Tootsie Roll wrapper.

I suspect that the Ryobi capacitor issue was not from “overheating” as Figure 5 suggests, but that one of the diodes within the CD60 capacitor failed as an open circuit, which allowed excessive reverse bias to appear across its associated capacitor. (If a diode had failed as a short circuit, I doubt if the motor would start.)

Figure 5 An unwise reassurance that the capacitor may not blow when installed backwards. Source: LeftyMaker, YouTube

One would think that the CD60 capacitor would have a pressure release plug that would vent if internal pressure got too high. If there is such a mechanism, it seems that sometimes it is not working properly. The sheer physical size of the CD60 capacitor in the Ryobi product versus that little itty-bitty capacitor in my lab class makes me think of the CD60 capacitor as a potential hand grenade.

John Dunn is an electronics consultant and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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