Powering a circuit: electricity

Site: ΕΛ/ΛΑΚ Moodle
Course: 3D printing with circuits and Arduino
Book: Powering a circuit: electricity
Printed by: Guest user
Date: Tuesday, 6 December 2022, 6:44 PM

Description

Tame electrical waves and currents.

1. Power vs. ground

As we mentioned before, the battery has a positive and a negative end. The convention has it that the positive side produces power (+) and the negative one is ground (-).

Just as all paths in the circuit must begin with the power side, they must all end at the ground side if you trace them along the entire length. Ground can be thought of as the “zero” side, the place where all power has been used up.

2. Debugging

In the technology world, and especially in computer systems, we use the word 'debugging' to describe the process of finding errors and fixing them.

In case your LED didn't light up, we need to debug it and figure out what went wrong.

Power properly connected?
If we do not get any output (no LED light), first thing to check is if we have power, as in + to + and - to -.
LED properly connected?
As we mentioned before, LEDs have polarity, so we need to double-check that the positive lead (long leg) and the negative lead (short leg) are connected correctly.
Resistor?
Next up, if both the battery and the LED are OK, we are checking the resistor. If by mistake you used a different resistor, perhaps:
  • a much smaller resistor did not limit enough the flow of electricity and you burned your LED (when that happens, dispose it!)
  • a much larger resistor limited too much the flow of electricity so not enough power to light up the LED
Connections
The most common error in circuits that we build on breadboards are bad connections. As we said before, circuits are closed loops, so if one connection fails, we do not have a loop. You should also know that breadboards are used for temporary circuits and testing and of course are not 100% reliable. Apart from looking very carefully, the best way to check the connections, is by using a multimeter.


3. The multimeter

A multimeter is a tool used for a wide range of electrical measurements.

A typical multimeter will measure voltage (voltmeter), current (amperemeter), resistance (ohmmeter), and continuity.

To set up the multimeter, we plug the black probe into the ground / common port and the red probe into the voltage terminal.

A multimeter also typically has a display that shows the measurements.

For now, we will only use the multimeter to test for continuity, so we will turn it to the beeper/continuity mode; there should be a sign similar to this one:


Once we are ready, first we touch one probe with the other to see if it works. You should hear a beeping sound which indicates a connection between the two points the probes are touching.

Now check the connections in your breadboard between each component to see if they are in the proper tie points.


You may learn more about using the multimeter with this video:


4. Conductors vs. insulators

As we have mentioned when explaining lines and wires, conductor materials are the ones that allow electricity flow.

Insulators on the other hand, prevent the flow of electricity through it. Typically insulators are materials like rubber, wood, and plastic.

In the Glow and Move circuits, the 3D printed parts were insulating enclosures for the circuits, which were connected through their metallic parts.

5. AC vs. DC


Apart from the rock legends, AC/DC refers to the two main types of electrical flow, direct current (DC) and alternating current (AC).

You might have noticed in the multimeter symbols, that there are two measurement modes for voltage, one that looks like V~ and one close to V---.

V~ refers to alternating current, which is the type of current we find coming out of a wall socket.

Try to measure the socket voltage in the lab!


It is called alternating current because the electrical signal alternates above and below the electrical ground (0V), constantly fluctuating between a positive and a negative voltage.



The other symbol represents direct current. It is so named because it travels in a direct line above ground. This type of electricity consists of a steady positive voltage, set apart from a ground plane.

DC electricity is the type of current that comes from batteries.

In DC voltage, we refer to ground (-) as the absence of voltage (0V) and to power (+) as the existence of voltage (the steady positive voltage).

"Ground"

Ground really is another convention in electrical circuits. Our breadboard and battery grounds do not really go to the earth's ground and, unless they are connected, two different circuits' grounds maybe be on different voltage levels. So naming a certain level as 0V or ground is arbitrary!

On the other hand though, the AC power system that brings electricity to houses uses the actual ground as ground to close the circuit. This means the electrical cables and pillars only bring the power to us. Shocking?


The reason why AC voltage is used for this is  because back then when energy systems were built AC electricity was easier to transmit over long distances.
AC to DC
As consumer electronics mostly use DC power, we are filled with AC/DC converters, the most common one for you being a smartphone or laptop charger. If you look closely on a charger label, you will see exactly the specification for AC input and DC ouptut.
Image result for charger label
Safety
Whereas AC is dangerous for the human body, DC is completely safe. However you should still avoid touching the circuits directly with your conductive hands, as you may damage the equipment by making a connection through your body.

6. Voltage and current

We have mentioned these terms a few times already, without having them explained properly. 

Imagine a ball being thrown through the air.

  • Voltage is the speed the ball is travelling. This is measured in Volts. It's symbol is a capital V.
  • Current is the size of the ball being thrown. This is measured in Amperes (or Amps for short). Its symbol is a capital A.

All DC electricity can be thought of as having a voltage and a current. In order to determine what these are, you can use a multimeter.

Measure the voltage of a battery

Place the red probe on the positive terminal of the battery. Place the black probe on ground (or "minus") terminal of the battery. You should get a reading close to its nominal value, e.g. 9V.

If you reverse the probes of the multimeter you will notice that the meter will give you a negative voltage reading. The reason for this is that DC electricity has a positive voltage and a ground voltage. The reading on the multimeter is actually the black probe's voltage level subtracted from the level of the red probe.



7. Ohm's law

Let's imagine the ball again. If you have a small ball travelling at a very high speed, it could potentially have as much or more power as a large ball travelling at a very low speed. In this way, you could say there is a direct relationship between the speed a ball is travelling, the size of the ball, and the potential power of the ball.


Of course though, we are actually not really talking about balls, but electricity. When dealing with electricity, voltage and current are in a direct relationship with power. In a circuit, power is expressed in terms of Watts. The symbol for this is a W.

Watts = Voltage * Current

There is also another factor we have yet to talk about that also plays a role, and that is resistance. In our analogy, resistance is the headwind that the ball must fight against to move forwards. On a calm day, there might be little resistance to its flight, but on a windy day, it might have to fight against the wind pretty hard. Again, we are actually talking about electrical resistance in a circuit and not throwing a ball.


Resistance pushes against the flow of electricity. As such, it is also in direct relationship with Watts, Voltage and Current. Resistance is expressed in Ohms (after it's discoverer). This mathematical relationship between Watts, Voltage, Current and Resistance is unsurprisingly called Ohm's Law.

Ohm's Law is not something you must memorize, but it will play an important role later when determining how much resistance a circuit must have. Thanks to this law, a circuit having a minimum amount of resistance is not optional, but necessary. The energy in the circuit must encounter resistance in order to expend itself. The thing in the circuit which uses energy is considered the Load. If an electrical supply is connected to ground without a load to use up the energy, bad things will happen.

You should NEVER connect your positive voltage source directly to ground.

One of the other fundamental concepts of Ohm's Law is that electricity must encounter a minimum amount of resistance in a circuit and be able to expend itself. If you connect power and ground directly together, there will be a lot of energy that has no way of expending itself. Your circuit will then try to release this unused energy in highly antisocial ways. Basically, the energy will turn into heat. However, having nothing in particular to warm, either your power source or wire will start to dramatically heat up. This can potentially result in a damaged power supply, melted wire, a fire, or potentially an explosion.

Another name of this phenomena is a "short circuit." You likely have heard this term before.


8. Resistors

A resistor is an electronic component that limits the flow of electrons. In doing so, it dissipates energy in the form of heat.

The amount of resistance that a resistor offers is measured in Ohms. The symbol for ohms is the Greek omega symbol - Ω.

In terms of electronics, the resistor reduces electrical current by a precise amount. If you consider that in a circuit you typically have a fixed input voltage, and resistors offer a fixed amount of resistance, you can then use Ohm's Law to determine how much a resistor will limit current. This is useful in a number of scenarios, including working with LEDs.


To measure ohms with a multimeter, turn the dial to the Ω symbol and selecting the proper range.

Resistor values
Determining how much resistance a resistor offers is a little trickier and can be established by deciphering the coloured stripes from left to right. You will typically see four stripes, but you may also encounter resistors with five.

When reading a resistor with four stripes, the first two stripes are combined together to form a number between 1 and 99. The third marking is the multiplier. The last marking determines the tolerance.

Image result for resistor colour coding

A resistor with a gold band has a resistance with a +/- 5% tolerance or - you could say - margin of error. What this means is that the resistance can be over or under its value by 5%. So, if you had a 100K resistor and measured with a multimeter, it could read anywhere from 95K to 105K. This will be alright for the purposes of this course.

Variable Resistors

A variable resistor is a resistor whose value varies within a set range.


The simplest and most common variable resistor is the potentiometer. Every time you use a slider or knob on an electrical device, you are using a potentiometer. For instance, every single mechanical light dimmer you have ever used is a potentiometer.

Potentiometers come in a wide range of resistances, and have a physical actuator that sweeps from 0 ohms resistance to whatever value is marked upon it.

By connecting a wire to either one of its outer terminals, and another to the centre terminal, we can wire a potentiometer as a variable resistor.

We can always check which pin is which with a multimeter.