Diodes
One-way valves for electricity, and the parts that protect your circuits.
The question
What part lets current through one way but blocks it the other, and why would you want that?
What you’ll discover: how diodes work, how to tell which way round they go, and how they protect circuits.
In simple terms
BeginnerA one-way valve
A diode lets current flow in one direction only, like a one-way valve in a water pipe. You already know one kind: the LED is a light-emitting diode. Ordinary diodes do the same one-way job without making light.
A diode has two ends: the anode (+) and the cathode (−). On most diodes, a painted stripe marks the cathode. Current flows from anode to cathode, toward the stripe. In a schematic, it’s the same triangle-and-bar symbol as an LED, without the light arrows.
What diodes are used for
- Reverse-polarity protection: one diode in the battery lead means that if someone puts the batteries in backwards, the diode blocks the current and nothing gets damaged.
- Turning AC into DC: household power in Canada is alternating current (AC) at 60 cycles per second, so it reverses direction 120 times every second. Diodes in a phone charger let current through in only one direction, the first step in turning AC into the DC that electronics need. (We never experiment with wall power; this happens safely inside a sealed charger.)
- Protecting against spikes: motors and relays produce a voltage spike when switched off. A diode across them, called a flyback diode, gives that energy a safe path (Lessons 11 and 13).
Go deeper
IntermediateThe forward voltage drop
A silicon diode doesn’t conduct until there’s about 0.6 to 0.7 V across it in the forward direction, and that small voltage stays “used up” by the diode while current flows. So a diode in series with a 4.5 V pack leaves about 3.8 V for the rest of the circuit.
Two common types: the 1N4148 is a small “signal” diode for low currents; the 1N4001 handles up to 1 amp and 50 V and is the usual choice for protection and flyback duty. Both have a black or coloured stripe on the cathode end.
AdvancedPN junctions and other diodes
A diode is a junction between two kinds of silicon: P-type, with a shortage of electrons (“holes”), and N-type, with extra electrons. Where they meet, a thin depletion region forms, with no free charge carriers. A forward voltage of about 0.6 V shrinks it and current flows; a reverse voltage widens it and blocks current, up to the diode’s breakdown voltage.
Some diodes use those effects on purpose: Zener diodes are designed to break down at a precise reverse voltage and are used as simple voltage references; Schottky diodes have a lower forward drop (about 0.2 to 0.4 V); and photodiodes produce a small current when light hits them, the principle behind solar cells.
Try it
In My First LED Circuit, replace the jumper wire between the red (+) rail and the resistor with a 1N4001 diode, stripe toward the resistor. The LED still lights (a little dimmer). Now switch off, swap the battery pack’s red and black leads on the rails, and switch on: nothing happens, and nothing is damaged. That’s reverse-polarity protection. (The LED would also block current here on its own; the diode matters most for parts that don’t block, like motors, chips, or the electrolytic capacitor in the Fading Light.)
Quick check
Which end of a diode does the stripe mark?
The cathode (−). Current flows toward the stripe.
About how much voltage does a silicon diode use up when conducting?
About 0.6 to 0.7 V.
Why put a diode across a relay coil or motor?
To give the voltage spike that occurs at switch-off a safe path, protecting the transistor or switch.
Words to know
- Diode
- A part that lets current flow one way only.
- Anode / cathode
- The + and − ends. A stripe marks the cathode.
- Forward voltage
- The voltage a diode needs before it conducts: about 0.6–0.7 V for silicon.
- Flyback diode
- A diode that protects against the spike from coils in motors and relays.
Learn more
- Science World (Vancouver): Current Electricity: eight hands-on circuit activities
- Lesson 5: LEDs and resistors
- Next: Transistors
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.