Direct and alternating voltage and the diode · why does an LED only work one way?

Two circuits, one lights up

0:00Two little circuits, exactly the same, with the same battery and the same LED. Only, in one of them the LED is the other way round. In which of the two does it light up? Don't think about why. Just choose.

Two kinds of voltage

0:17First the difference between two kinds of voltage, because everything else depends on it. A battery has a fixed positive and a fixed negative terminal. So the current always flows the same way. That's called direct voltage. The socket gives you something else.

0:33There the voltage keeps changing sign: one side becomes plus, then the other, and in Europe it swings fifty times a second back and forth. That's called alternating voltage, and the two hundred and thirty volts on it is a kind of average over that back and forth.

0:51For a filament or a heating element it makes no difference: they get warm whichever way the current flows. But a lot of electronics really needs direct voltage. And that's why every adapter has something that removes one of the halves.

One-way traffic

1:09That "removing one half" is done by a diode. A diode only lets current through one way. Connect it so that the current wants to go the right way, and it opens and behaves almost like an ordinary wire. That's called the forward direction. Turn it round, and next to nothing gets through.

1:29That's called the reverse direction. The symbol has that direction built in: a little triangle with a bar in front of it. The triangle is an arrow, and that arrow points the way the current is allowed through. Look again at those two circuits from the start. Which one lights up, and what do you look at to see that?

The LED

1:53An LED is a diode that lights up as soon as current flows through it. Those letters stand for light-emitting diode. So everything about the diode applies to it too: one way it works, the other way it stays dark. And that's the answer to the question from the start: exactly one of them lights up, the one whose arrow points along with the current.

2:15Two ways in which an LED differs from a filament lamp. It only switches on above a certain voltage, for a red one around two volts, and below that nothing happens. And it doesn't limit its own current: there must always be a resistor in the circuit, or it breaks straight away.

2:36That's also why it doesn't glow a little when you turn it round. It isn't weaker. It's off.

Three things to remember

2:43Three things. Direct voltage keeps its direction; alternating voltage changes sign, in Europe fifty times a second. A diode only lets current through one way: the forward direction works, the reverse direction doesn't, and the arrow in the symbol points the way in which it's open.

3:00And an LED is such a diode that lights up. The wrong way round isn't weaker, it's off, and it always needs a resistor.

Two questions

3:10Two questions. One. You have a circuit with an LED that doesn't light up. You measure, and there is voltage on the circuit. What's the first thing you check? Two. Why can't you just connect an LED straight to a battery?

The answers

3:29The first: whether it's the wrong way round. If there's voltage and nothing happens, the reverse direction is the first suspect. Look where the arrow in the symbol points and where the plus is. The second: because it doesn't limit its own current.

3:46Without a resistor next to it, the current rises far above what it can handle, and then it breaks within a second.

A component that does not follow Ohm's law

3:53A diode is the first component that doesn't obey Ohm's law. There's a separate video on the channel about what that law says exactly, and about which components do obey it.