Vacuum tubes used to be used in everything from TVs to radios until solid state devices took over decades ago. Vacuum tubes are still used extensively, however - for example, your microwave ovens has a big vacuum tube in it producing microwaves. Today I'm going to discuss the most basic vacuum tube, the vacuum rectifier. I hope this can make it simple to visualize how they work, because honestly they are pretty cool devices!

A typical vacuum tube rectifier
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Rectifiers

The vacuum rectifier tube essentially functions as a diode. If you don't know what a diode does, diodes are small solid-state devices that convert alternating current to direct current. If I put a signal through a diode, only the positive parts will come out, to simplify it. This makes diodes incredibly useful. A simple example is using a diode to prevent a battery from flowing back into a solar cell charger, but diodes are far more widespread - you couldn't read this on a computer without them.

A semiconductor diode
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Before semiconductor diodes were invented, vacuum tube rectifiers were used to rectify current. A vacuum rectifier basically does the same thing as a semiconductor diode: It forces current to flow in only one direction.

Thermionic Emission

Before we get into tube rectifiers, let's talk about thermionic emission. Thermionic emission is when a material emits free electrons when heated up. As you know, metals conduct electricity quite well. This is because of a band of lightly bound electrons in the metal which can carry current across the material. Apply enough energy to one these electrons and they can escape the metal. This is the basis of the photoelectric effect: Photons of sufficient energy can give enough energy to electrons to exceed the material's work function, or the minimum energy required to eject an electron. That's a post for another day, but it directly relates to thermionic emission. Heat up a metal to the point where the average thermal energy approaches the work function and more and more electrons will start to become ejected from the metal. Free electrons from heat!

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Vacuum Tube Rectifier

Now we're ready to discuss the tubes themselves. Put two metal wires in a vacuum and put a battery across them. Nothing happens - no current flows, the battery doesn't drain (technically if this was a very very high voltage battery, thousands of volts, we would see current due to cold cathode emission but that's not relevant here). This is because there are no charges to carry current across the empty vacuum gap. But, say we make one of the wires in the vacuum two wires connected to a chunk of metal. Now, hook up a second battery across the chunk of metal to heat it up.

If the metal heats up enough to where the average thermal energy (3kBT for metals) begins to approach a sizeable fraction of the work function of the metal, electrons start being ejected out into the vacuum. Now we have free charges to carry the current!

But there's one more trick, and this is what makes the vacuum tube actually useful as a rectifier. Let's call the heated-up chunk of metal the filament. Say we put the positive end of the battery on the filament and the negative end on the other wire, seperated by vacuum. Now we heat up the filament to the point where free electrons start being ejected from the metal. Does current flow? Does the battery drain?

The answer is no. Remember that electrons have a negative charge and thus are attracted to positive charges. This means that the electric field produced by the voltage setup I described above is attracting electrons back into the filament. No charges can cross the gap unless they are moving very fast, so no current flows. The battery doesn't drain.

But now switch the polarity: Say we put the positive terminal of the battery on the free wire and the negative terminal on the filament, while heating up the filament. Now electrons are ejected from the hot metal and the E-field pushes them to the second, positive terminal. Free charges are now crossing the vacuum gap, and the circuit is completed - the battery drains!

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Now you can see that this hypothetical vacuum tube only lets electric current flow in one direction - just like a diode. That's the vacuum tube rectifier. I think these are still used in guitar tube amps but I don't know much about them. I haven't personally used one as a rectifier, but I own a few to make XRays with.

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There are tons of other types of vacuum tubes, but rectifiers are the simplest.

Thanks for reading!