Diodes

A diode is a part that lets current flow in one direction and blocks it in the other. It is the first part in this course that doesn't behave like a resistor, and understanding it makes LEDs and transistors much easier to follow. You should know Ohm's law before reading this page.

Anode and cathode

A diode has two ends with names. Current enters at the anode and leaves at the cathode. On a real diode the cathode end is marked with a band or stripe printed around the body. In the circuit symbol, the triangle points in the direction current can flow, and the line across its tip is the cathode.

When the anode is at a higher voltage than the cathode, the diode is said to be forward biased and it conducts. When it is the other way round, it is reverse biased and practically no current flows.

Forward voltage

A diode doesn't start conducting at the slightest voltage. It needs a certain minimum first, called the forward voltage. For an ordinary silicon diode, such as the 1N4148 or the 1N4001, it is about 0.6 to 0.7 V. Schottky diodes need less, about 0.2 to 0.4 V, which is why they are used where every bit of voltage counts. LEDs, which are diodes too, need much more: about 2 V for red and around 3 V for blue and white.

Once it conducts, the voltage across the diode stays close to its forward voltage, even as the current changes a lot. The rest of the supply voltage ends up across whatever else is in the circuit. In the tool, with 5 V and a silicon diode, about 0.7 V stays across the diode and about 4.3 V across the 1 kΩ resistor, so the current is 4.3 ÷ 1,000 = 4.3 mA. This is Ohm's law applied to the resistor, not to the diode.

That behaviour is also why a diode always needs something else in the circuit to limit the current. Connected straight across a battery in the forward direction, a diode would let through as much current as the battery can deliver and burn out, the same way a short circuit does.

Reverse direction

Turned around, a diode blocks the current and the whole supply voltage appears across it. This only works up to a limit, called the maximum reverse voltage, which is given in the datasheet. A 1N4001 can block up to 50 V, which is far more than anything in these guides.

Where diodes are used

• Reverse polarity protection. A diode in series with the supply means that a battery connected the wrong way round simply does nothing, instead of damaging the circuit. The cost is the forward voltage, which is lost.
• Motors and relays. When the current through a coil, such as a motor or a relay, is switched off, the coil produces a short voltage spike in the opposite direction. A diode connected across the coil, with its cathode towards plus, gives that spike a harmless path. This is called a flyback diode, and you will see it again in the transistor guide.
• Turning AC into DC. Phone chargers and power adapters use diodes to let only one direction of alternating current through. That is called rectifying, and it is outside the scope of this course.
• Light. An LED is a diode built so that it gives off light when it conducts. Everything on this page applies to it, and the next page is about using one properly.