Electronics Course - Part 2 - First Circuit

Electronics Course - Part 2 - First Circuit

Skip to Next Tutorial

For this tutorial you will need some easily obtained parts;

• An LED
• A 9v Battery
• A Resistor (470 ohms)

 

Expect to pay about £1.00 for all the parts.

The circuit is very simple.
On the LED, one leg is longer than the other, twist one Resistor wire around it. Now put the remaining wires on the battery terminals.  One way round, the LED will light, the other it won't.

Congratulations, you have made your first circuit!

What did I just do?
You have created a SERIES circuit

The LED and Resistor are connected "in series", which means one after the other. The battery is our EMF, and it has 9 volts. This is pushing the electrons from the negative side of the battery, through the LED, then the resistor and back to the battery positive.

Each component has a resistance, the resistor is 470ohms, and the LED is almost zero. There is also some internal resistance in the battery, but for now, we can ignore that.

The LED has a special junction inside it, that only allows the electrons to pass through one way. (its called a PN junction, we will look at that later in the course). As the electrons go through, they bash a different particle, called a photon, which is the particle that creates Light.

When you look at the specification of LED's, you will see reference to Vf and If. These are the ideal values of voltage and current the LED would like.

In our example, Vf = 2v and If = 15mA (0.015 Amps)

The battery has 9v, the LED wants 2v of this, which leaves 7v spare. We want 15mA to flow, so we use ohms law to see how big the resistor should be ( R= V/I) which works out at 466 ohms. We choose the nearest resistor that is bigger than this, which is why we get 470 ohms.

The formula to work out the resistor size is R=(Vs-Vf)/If

Without the resistor, the circuit would have a very high current. V/R = A LOT! The LED is a greedy little component, it will allow all those electrons to flow, but it will eat so many it will pop! Like a 5 year old with ice-cream. The resistor limits the current in the circuit to a level the LED can comfortably take.
DON'T TRY THIS AT HOME! We did, see it in this video, at 3:10

Although electrons flow from - to +  when experiments with electricity started, they believed flow was from + to -, they got it wrong, but the convention has stuck. It will confuse you all the time until you learn a simple rule. "Ignore it, stick with convention"

The voltage around this simple circuit is 9v at the start, and 0v at the end, between the two components its 7v. Each component takes some of the force to get the electrons through. The current stays the same at each location in the circuit. This diagram might help you see what I mean.

Next Tutorial