One way selenium street

The oldest generator I have worked on

1969, its the year that man walked on the moon, the year the internet in its most primitive form was born and the year the generator that is the subject of this post was made. Unlike the lunar modules or the massive computers that were connected to the internet at that time, this generator is not in a museum and is still in use today.

The engine in question was aboard an old livestock carrier and is used to power the fans that kept the animals cool. The vessel was heading to the middle east so it was not something they could do without.

Walking into the control room was indeed like walking back in time with its analog gauges and incandescent indication lights, I would love to show you some photos but out of respect to my client I have decided not to, but I can show you the culprit that was preventing the generator from producing any voltage.

Selenium Rectifier

This is a Selenium Rectifier, selenium is the stuff they put in anti dandruff shampoo to stop the skin flaking, but also when strategically painted in a thin film between a metal plate and an tin cadium alloy spacer it allows electricity to flow in only one direction turning turning the plate into a diode. Stack the plates up and you have a rectifier for converting AC power to DC.

Their modern equivalents, the Silicon diode starting appearing around the same time this generator was made and due to their smaller size and higher efficiency made the Selenium diode all but redundant.

But not completely! Selenium diodes are still being manufactured today by a few specialist manufacturers around the world, their proponents point to their ability to self heal after a failure for there continued enthusiasm for this old technology. An ability their silicon cousins are lacking.

Silicon Diode Bridge

Due to the 3 week lead time on Selenium diodes, joining this club was not an option and I made the some may say sacreligious decision to replace this particular generator’s rectifier diodes with silicon ones. A unit pictured above is about a 10th the size of the originals and has a voltage rating around 40 times higher so I doubt failure will be an issue anytime soon, in which case self healing is probably not an important feature.

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