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What is a Selenium Rectifier?

By Andy Josiah
Updated May 16, 2024
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A selenium rectifier is a type of rectifier that uses selenium, a chemical element that is not a metal, as a semiconductor for electrical conductivity. For a certain period of time, the selenium rectifier enjoyed popularity as perhaps the most widely used rectifier. Silicon rectifiers eventually took over this title.

As early as the late 19th century, metal rectifiers were used to convert alternate current (AC) to direct current (DC). This process, known as rectification, was done to point electricity to one fixed direction, which DC offers, as opposed to AC, which reverses directions. The device uses a piece of metal as its semiconductor for its electrical conductivity. This means that such a component has to possess the ability to conduct an electric current, which is not as good as an conductor, but better than an insulator. The most common type of metal rectifiers included copper oxide and vacuum tube rectifiers.

Starting in the 1930s, some companies began manufacturing rectifiers using selenium, a chemical element that is actually not a metal. Usually, they are made to resemble stacks of thin round or square plates, which are then plated with aluminum or steel. The selenium rectifier rose to prominence when vacuum tube rectifiers failed to provide the sufficient amount of amperes of electrical current needed to charge the batteries on automobiles.

A notable advantage of the selenium rectifier was the ability of the user to stack more plates to increase the device's voltage endurance. Additionally, unlike vacuum tube rectifiers, they did not require a warm-up period, as they were capable of providing instant operation. Far from restricted to automobile batteries, selenium rectifiers were used for electronic devices as diverse as radios, televisions and photocopiers.

Use of the selenium rectifier reached its apex during the mid-1940s and mid-1970s, when they were commonly employed for radios and TVs. By this time, they had replaced DC generators. Before the advent of the selenium rectifier, DC generators were the only semiconductors used in battery charger applications that required high amounts of electrical current.

By the end of the 20th century, however, selenium rectifiers had been overtaken by silicon rectifiers, or silicone diodes. These devices were actually available in the mid-1950s, but it was not until the 1960s that they began to establish their dominance. These devices perform at higher voltages than selenium rectifiers, and are also less expensive and more reliable. The selenium rectifier, however, is still manufactured today as a replacement device.

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Discussion Comments
By Davey1000 — On Nov 20, 2013

In the 1960's I worked as a transmitter technician at a busy shortwave transmitting station. The transmitters used selenium rectifiers to produce 400 volts grid bias.

Once the transmitter was up and running however the vacuum tubes created their own bias. There was one man there who seemed to live on egg sandwiches and he was notorious for passing wind. One day, just as he walked through the transmitter hall, there was a foul eggy smell created. I thought no more of it but about half an hour later the shift leader said "Your grid bias has gone; didn't you notice the smell?" Well, actually I had but I'd assumed that the smell was from the egg sandwich eater! A lot of water has passed under the bridge since those days!

By miriam98 — On Feb 18, 2012

@Charred - I don’t know anything about crystal or antique radios, but I seem to have an abundance of DC power supplies laying around my office – I suppose they all use rectifiers although I can’t say that I’ve actually seen the component itself.

One thing I know is that it’s important not to get the power supplies mixed up. They don’t deliver the same output voltages and the power supply jack seems to be the same on all of them, so it’s easy to accidentally plug the wrong power supply into my laptop. Always read the labels!

By Charred — On Feb 17, 2012

@SkyWhisperer - I frequent the antique radio forums sometimes and they talk about rectifiers like you said. Sometimes they use silicon diodes as a selenium rectifier replacement. It works just as well.

Honestly I don’t think I’ve ever seen the rectifier but I have seen the silicon diodes – they are actually quite abundant nowadays. I don’t know which technology is better.

The diodes perform at higher voltages according to the article, but if you’re playing with antique radios you may not need that much voltage, in which case I suppose they work equally well.

By SkyWhisperer — On Feb 16, 2012

@MrMoody - Yeah, and a lot of things can serve as a power rectifier as well. I remember back when I used to play with electronics as a hobby. One of the first projects that I built was a crystal radio.

This thing had no power source and yet it could pick up radio waves. You could listen to radio stations, however faint, coming in through that crystal set.

How did it work? The “crystal” was a rectifier, converting the radio waves into sound and delivering the signal into your earpiece. It was awesome.

Again, since there’s no power boost the signals were faint but I was still delighted with my little crystal radio set.

By MrMoody — On Feb 15, 2012

I didn’t think you could get away with creating a conductive component that didn’t use metal. That’s news to me. Although I guess it shouldn’t be. Vacuum tubes use gas so I suppose it makes sense that you could use something besides metal.

Really a lot of things can conduct electricity. Water can conduct electricity but it would be difficult to make a component out of it. So in the end I think that it’s all about practical implementation.

My take from the article is that of all the chemical elements, selenium was deemed most effective at conducting and converting electricity, without needing metal.

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