What is a vacuum diode?

Answer 1

A vacuum diode is an electron valve that supports current flow through it in only one direction. It is composed of two electrical "parts" or elements and the evacuated glass envelope. Additionally, there is usually a heater, a coil of special alloy wire that can be driven to red heat by a modest application of voltage. (A vacuum diode without a heater would be a cold cathode diode.) The heater is coiled inside a cylindrical element called the cathode. The cathode is composed of a special alloy, or, more properly is coated with an alloy, that has high thermal emission characteristics. That means that if it's heated sufficiently, electrons actually leave the metallic crystal structure of the metal (boil off) and hang around outside in the space nearby forming what is called a space charge. When we built the tube, we pumped all the air out, sealed the glass, and then fired a "getter" inside the tube to chemically bind the remaining bit of gas that was there. The tube really is highly evacuated. That's important because we don't want all those light weight electrons bumping into gas atoms and molecules when they are hanging around in the space charge, and especially when transiting the gap between the cathode and the other element inside, the anode or plate. If a voltage is applied across the cathode and plate, and the plate is negative, no current will flow in the tube. Electrons flow from negative to positive, but, darn it, they can't leave the plate to go to the cathode. That's called reverse biased. It's hooked up "backwards" like that. But if the cathode is connected to negative and the positive connected to the plate, electrons that were in the space charge will be driven away from the cathode and attracted to the plate. Current willflow in this direction. That's forward bias. There isn't a lot of internal resistance when the diode is forward biased, either. It conducts really well. The vacuum diode can be used to rectify an AC voltage and turn it into DC. In that application, it permits current flow during half the AC cycle (when it's forward biased), and does not allow it in the other (reverse biased) half. There aren't a lot of vacuum tubes around like back in the day. Modern electronics has seen innovations that have driven the vacuum diode away. Hey, wait a minute! They're still here! They sneaked back! But they're here in disguise!! I'll betcha that you've got one in your house! Yeah! You got a microwave oven at home? The heart of a microwave oven is a (specialized) vacuum diode that sits inside a magnetic field. It's called a magnetron, and it's the microwave generating device inside the appliance. Betcha didn't know that. The way the magnetron works is..., hey, wait. That's another question.