Circuits Lecture 3

Refresh We’re learning this so we can built electronic and mechanical do-dads that are part of installation art pieces.

Circuits In the previous example with two charged objects connected by wire, a current actually flowed for a short time when the connection was made. After the changes balance, there’s no more pressure to force current to flow and it stops.

Circuits Now imagine that we have a source of current that lasts a long time (a cell). One end or side of the cell emits electrons (the –ve side). Current will not flow unless there is a loop so that the electrons used from one end are returned to the other. This ‘loop’ is always needed for current to flow, and is called a circuit.

Circuits Here is a drawing of a simple light bulb and battery. Light won’t go on because the circuit is open.

Circuits Now electrons can flow through the wire, repelled by the –ve battery terminal and attracted to the +ve. Light is on.

Circuits A switch is simply a way to break the circuit so that current cannot flow. By the way, engineers are taught that current flows +ve to –ve while electrons flow the other way. Makes no real difference.

Circuits The same circuit would explode a bomb. When the switch is thrown the bomb explodes. Instead of a simple switch we have a fuse, which is a timed switch connected to a primer explosive. Question coming up …

Circuits Here’s a standard bomb defusing scene from a film. “Hogan’s Heros” from the 1960s, but there are many others. Question coming up…

Circuits So which wire is this one? Which one should be cut?

Circuits Answer? Does not matter. The circuit will be broken in either case. The bomb will not explode.

More Complicated Circuits We could have more than one light bulb in the circuit at the same time. All will light up if the conditions are right. There are two basic ways to do this. Parallel and series, the ways are called.

Series Circuits Let’s use Christmas lights. If connected in series the out side of one bulb is connected to the in side of the next.

Series Circuits If one bulb is burned out then the circuit is broken and they all go out! Also, imagine that the battery Is 5 volts. That voltage occurs across the whole string of bulbs. The voltage across any one should be 1 volt – the voltage gets split. However, the electron flow has to pass through all of the bulbs.

Parallel Circuits If one bulb is burned out then it goes out but the others stay lit. Also, imagine that the battery Is 5 volts. That voltage occurs across the whole string of Bulbs, which is to say each bulb. The electron flow splits into parts to flow through each bulb. Thus, 1/5 of the total current flows through each bulb. When a bulb burns out more current Flows through the remaining ones.

Parallel and Series Series and parallel circuits can be mixed in a circuit that does something useful. What’s the current in each part? What voltage across each bulb?

Parallel and Series For example, here is a radio…

Components The radio circuit diagram had some odd symbols. Each represents a device with specific electrical characteristics that are useful in building electronic circuits. The diagrams themselves are a fundamental way to communicate between designers and builder, like a blueprint. Such a diagram represents the circuit in one way, but is not the circuit and is not the only way to represent it. It’s like musical notation.

Components Real devices are represented by specific symbols, and connecting them is done by drawing lines between connected things. Like the drawings of light bulbs and batteries we did a few slides ago. But we don’t usually use light bulbs anymore. We use Light Emitting Diodes.

Light Emitting Diode A Light Emitting Diode (LED) is a modern device that is seen everywhere electronic equipment is found.

Light Emitting Diode A Light Emitting Diode (LED) is a modern device that is seen everywhere electronic equipment is found.

Light Emitting Diode LEDs emit light when an electric current passes through them. They have to be connected the right-way around (+/-) +ve side. Longer wire, smaller inside part. -ve side. Shorter wide, wide flat part inside.

Light Emitting Diode Let’s make it light up. Connect it to a battery:

Light Emitting Diode OK, science is an experimental subject. What happened? Electronic devices have a range of current and voltage they can tolerate. Exceed that and the heat generated by the excess will burn out the component. To light up the LED we need to limit the current somehow,

Resistor A component called a resistor gives resistance to current. They are often used to limit the current to a sensitive device like an LED. Electrical resistance is like friction is to mechanical ones. In fact, sometimes we have to introduce friction to a mechanical device so that it does not run too fast and break apart. Same idea.

Resistor Electrical resistance is measured in units named Ohms. Georg Simon Ohm (17 March 1789 – 6 July 1854) was a German physicist.

Resistor Resistors vary greatly in value from a fraction of an Ohm to many million.

Resistor Value is coded as a sequence of coloured bands The Resistor Colour Code Colour Number Black 0 Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Grey 8 White 9 What value is this resistor?

Resistor This is orange orange brown = 3 3 x 1 or 33 ohms The gold band tells us how accurate this is. Gold = 5%

Resistor Let’s use a resistor to fix the exploding LED problem. We can use various values of the resistor in this circuit

Resistor If the resistor is too small (too few ohms) the LED burns If the resistor is too large then the LED will be dim.

Resistor A sensible value for the resistor is 1000 ohms. X = 1000 We could calculate this, but we don’t know enough yet. Maybe later.

Resistor What does the resistor actually do? Well, friction, remember. That makes heat. So the resistor converts current flow into heat If we flow too much current through a resistor it can burn too!

Labs: Use the meter to measure current and voltage. Build LED circuit.