Voltage, Current, and resistance. And don’t forget Ohm’s law!
Trying to write this in a simple term to get you started is as easy as it looks.
However, this tutorial may skirt round information that at this point tou really don’t need to know about.
Even half truths will be told here to get the idea across as simple as possible.
The concepts that we’ll discuss briefly here take up entire books to describe in depth but let’s just get an idea of what’sgoing on first.
At some point, if you are working with a Raspberry Pi or any other developer board, you may want to delve into the world we call Electronics.
You may want to try a simple electronics equivalent of the “hellow world” project in the IT world, better known as a blinking led, to get you started or you may have aspirations for way more in depth circuits.
If so you are going to need to know something about voltage, current, and resistance.
And how they are linked together.
Wrapping your head around these and how we use them is vital to making a difference between a working circuit and one which falls flat and simply refuses to work, or possibly worse goes bang.
And trust me, I know from bitter experience what can happen when you make a mistake in your calculations and the latter happens.
More about my personal mishaps as we go on!
So what are they and how are they all linked together?
You’ll find that out in the following minits.
Voltage
Voltage, also in some circumstances called electromotive force (EMF) is the difference of electric potential between two places. Yes OK I know that doesn’t help much right? Imagine a standard power cell – A battery (maybe – see below). It has a Plus side and a Minus side. There are a load of negatively charged objects (we’ll call them that for now) sitting at the ‘-’ terminal and they want to get to the ‘+’ terminal.
Now imagine these objects are a tank full of water under a certain pressure. The amount of pressure dictates how much that water wants to get into an empty tank somewhere else. There’s your voltage.
Why do I keep saying cell instead of battery?
Many of those little things you call batteries are not batteries in real electronics talk they are cells. You will know just what an AA cell looks like, or a AAA, or D cell. They’re cells because they only have one core.
The first proper battery you are likely to come across would be one of those square 9V offerings. These are actually made up of several cells that together add up to 9V.
Not that it matters but interestingly the energy flow when a cell is connected (or any other power source for that matter) doesn’t flow out of the + side towards the – side.
It’s the other way round!
Alessandro Giuseppe Antonio Anastasio Volta, an Italian physicist and chemist came up with what’s going on, which is why we use the word Volts, named after him. He also discovered Methane and invented the first electric battery. A very clever man!
Current
When a circuit becomes ‘closed’ it simply means that the – and + sides have been joined in a way that allows power to flow. How fast it is flowing is called current.
Join two the two sides of a battery together with a piece of wire and current will flow very rapidly between the poles. Put some other components in between and the force of the current will make those components do our bidding. Maybe a light (lamp in electronic terms) will come on, a motor will turn, your Raspberry Pi will boot up.
Current is measured in Amperes and attributed to another genius, this time a Frenchman named named Andre-Marie Ampere. Remember the point that he was French as it will help explain something later.
Resistance
Resistance is how we control how much power is flowing between two points. Go back to the water tank analogy and put a hose pipe between the full tank and the empty tank. Water start flowing between the two as fast as it can.
Now squeeze the pipe – The water coming out the other end slows down. We’ve reduced the current by adding resistance.
Why would we want to do this? Well components need just the right amount of current and voltage to work correctly. As such we will often need to adjust things within our circuits to suit the components we are using.
Remember I said about my own mishaps? I wanted to light up an LED (Light Emitting Diode) in a circuit I was working on. An LED has almost no resistance (just like a small piece of wire – although near enough everything has at least some resistance). As such it will suck in and spit out all the power it is presented with.
Too much power though and the LED will go bang. Sometimes with a little pop and sometimes with greater force. I miscalculated how much power was about to hit this LED and it went pop loud enough to make me jump and I near knocked myself out on a shelf just above my soldering station.
Resistance is measured in Ohms after genius German physicist Georg Simon Ohm who first described the relationship between voltage, current and resistance. Interestingly resistance itself and its relationship with voltage and current was discovered by Henry Cavendish, an English physicist 45 years earlier but he didn’t publish his work. If he had then we would probably describe resistance in ‘Cavens’ or something similar.
This sort of thing happens often – even without the internet and modern communications (which all the above mentioned people effectively had a part in developing) they were all chasing similar goals and their published papers were used by many other physicists of their era to further develop ideas.
Introducing Ohm’s Law
No matter who discovered it, Ohm’s law is one of the fundamental concepts that is vital to know.
In its simplest form it looks like this….
$V = I*R$
Voltage = Current x Resistance.
Why on earth is current shown with an I rather than a C? Well it’s a big world and I said earlier about remembering who discovered current? A French chap called André-Marie Ampere? Well the word he originally used to describe current was intensité or intensity in English.
Hence the I.
Ohm’s law is incredibly useful because it means we can calculate all sorts of handy things. If we know just two parts of the story, say current and resistance, we can calculate voltage.
Going back to your school days and doing a bit of mathematical trickery we can swap that equation round to become either
$I = {V \over R}$
or
$R = {V \over I}$.
This brings me on to one of the things that confused me most when I first started out in electronics. It would also have saved me a crack on the head if I’d used it correctly.
From following tutorials to get an LED to light up I knew I needed a resistor before the LED but I didn’t really understand why.
As I said earlier, an LED will suck in all the power you can give it but that may well be too much and it will go pop. We need that resistor there to limit the amount of power getting to the LED, so we can use Ohm’s law to calculate the resistor value given that we’ll know some other factors.
As you go on through your electronics leaning you’ll spend countless hours pouring over information about components, and LED’s are no different. These essential papers are called datasheets and literally everything has one.
The following may seem a little complex at first but it will become clearer with practice.
Looking at a datasheet for the little red LED I have in front of me right now (in this case it’s a standard 3504 Red, 5mm LED) I find that it needs 20 mA (milli-Amps) of current and has a 25mA maximum current. It also has a forward voltage drop (denoted by $V_f$) of 2.1V so we are going to need to supply at least that much to get things lighting up. Forward voltage drop is a characteristic of all LED’s. (We’ll go into such things as well as lots of other components elsewhere).
Mainly though we now know a value for the resistance – ‘I’.
I = 20mA or better for calculating I = 0.02A
We know how much voltage we’re supplying – just take a look at your battery. Let’s assume we are using a 9V battery to power this circuit. So we now have V = 9v but we should subtract the forward voltage drop so we finally get V = 6.9V – This is effectively the extra power we need to protect our LED from. Using Ohm’s law we feed in our values and the equation looks like….
So what do we have?
- $I = 20mA$ or $ I = 0.02A$
- ${V_f} = 6.9V$
So Now we can calculate the required resistance:
$R = {V \over I} = {6.9 \over 0.02}$
Which gives a final resistance value of 345 Ohms.
That wasn’t so bad was it?
You’re not actually likely to find a single 345 Ohm resistor but that’s fine as we have quite a bit of tolerance in the LED – Remember that it’s upper limit is 25mA so feeding that figure into our equation gives a resistor value of 276 Ohms. Anything in between those values should be fine and you’ll find it easy enough to find 340 Ohm resistors or even a 300.
It’s worth noting that you can use multiple resistors together to increase or decrease their overall resistance so if you don’t have a single resistor with the value you want then you can put several together to get your magic number but more on that in another tutorial.
Finally
Have a go yourself – what if your battery is supplying 12V? What are the upper and lower resistance values on our practice red LED? What if you change your LED to a 2542 standard 5mm white LED which has a forward voltage drop of 3.1V but the same current requirements?
Incidentally, you don’t have to do the mathematics yourself there are several online resistance calculators you’ll find with a quick search. We’ll probably add such a calculator and others here on the site as we go on.
Let that sink in – It’s enough to be thinking about for now! Later we’ll use resistors in conjunction with capacitors to produce an RC circuit and use it to control a timer IC (Integrated Circuit) which will help you understand things even more in a practical way.
