these are leds or light emitting diodes if we pass a current through one it produces light if we exceed the voltage and current limit it will instantly be destroyed the led has a tiny wire inside this can only handle a certain amount of current passing through it when we look at an led being destroyed under a microscope we can see the tiny wire exploding inside so how do we connect leds how do we reduce the current to keep the leds safe and how long will a battery power our circuit for that's what we'll be covering in detail in this video to protect our leds we use a resistor the resistor is going to make it harder for electrons to pass through the electrons are going to collide and this will generate heat the resistor will become hot and we can see that with a thermal imaging camera for example this one is over 150 degrees celsius at just 12 volts with a current of 6 milliamps so we definitely do not want to touch this the resistor can be placed on either side of the led although we traditionally install this on the positive side the reason it can be installed on either side is because the resistor is restricting how many electrons will flow in this simple series circuit the resistor acts something like a traffic jam reducing how many electrons can flow most people incorrectly assume it acts like a speed bump and that the electrons must be slowing down just before the resistor and then speeding up again the speed of the electrons remains constant the number of electrons flowing is what changes the higher the resistor value used the lower the current will be and so the dimmer the led will shine we need to remember that leds will only allow current to flow in one direction with the positive connected to the long lead and the negative connected to the short lead if we connect the led the other way around it will simply block the current and the led will not turn on you can test this circuit yourself take a red led a 9 volt battery a resistor of between 360 and 390 ohms another higher value resistor of between 3 kilo ohms and 9. 1 kilo ohms and a multimeter connect the low value resistor and the led to the battery in series and the led will illuminate i'm using a breadboard for this which makes it very quick and easy to test electrical circuits but you can also just twist the wires together you could solder them or you can use some connectors and it will all work fine for this simple experiment notice that if we turn the led around we see it blocks the current so it will not illuminate it only works in one direction if we replace the resistor with the high value 9. 1 kiloohm resistor we see the led is very dim we can also connect them in parallel to compare the brightness so now with the 360 ohm resistor and the led in series we can connect our multimeter into the circuit making sure to place the multimeter into the current reading mode we should see somewhere between 17 and 20 milliamps depending on which led and resistor you've used we can switch the position of the led and the resistor it will work fine and give us the same current reading now remove the multimeter from the circuit and place the multimeter into the dc voltage mode measure across the two far ends of the circuit and we should see around 9 volts this is what the battery is providing to our circuit and it is also equal to the total voltage drop of the circuit now measure across the led and we should see around 2 volts this is the voltage drop of the led it's removing 2 volts from our circuit now measure across the resistor and we should see a voltage drop for the remaining 7 volts so 2 volts plus 7 volts is 9 volts which is the same as our battery you might have noticed that the values measured weren't exactly 2 volts 7 volts or even 9 volts there's always going to be a difference between the design and the actual measurement for example this resistor is rated for 390 ohms but when we measure it it's actually 386 ohms each component including your multimeter will have an error tolerance it will be close to the design value but never exactly this value for most circuits like the simple ones it doesn't matter we can assume the design values are correct just remember that the values we calculate will always be slightly different to our actual measurements we also need to know about the forward voltage this is basically just the voltage drop we measured earlier the manufacturer will provide a chart like this one which shows the forward current at a given forward voltage so if we connect a voltage source across the leads and apply 2 volts we should see 20 milliamps of current flowing if we supplied 1.
6 volts we should see zero milliamps because the led will be off the chart for this led begins at around 1. 7 volts so we know that we need to provide a minimum of 1. 7 volts for this led to start illuminating we can test our led's minimum opening voltage using a multimeter if you select the diode mode on your multimeter and then connect the red lead to the long anode and then the black lead to the short cathode of a red led we should see something like 1.
7 volts so this is the minimum voltage required to turn the led on most standard leds are rated for a current of 20 milliamps or 0. 02 amps we want to try and stick to this value if we go below this then the led will be dim if we go too far above this then the led will be destroyed we can go above 20 milliamps but the lifespan of the led will reduce the higher we go we'll see how to calculate that a little later in the video the red led typically has a voltage drop or forward voltage of 2 volts and this will result in a 20 milliamps of current in our circuit we can test that with a dc power supply when i set the voltage at a constant 2 volts we see 20 milliamps of current but not all leds are created equally this one doesn't reach 20 milliamps until 2. 1 volts are supplied and this one doesn't reach 20 milliamps until 3.
7 volts is applied this variance is due to the materials used and also the manufacturing process undertaken so you should always try to use leds from the same batch and also from reliable manufacturers leds also come in many different colors and each color has a different voltage drop also so you will need to look these values up from the manufacturer's data or you can also test them yourself or you can use these typical values from these standard charts but they might not match the led you actually have okay that's the basics covered so let's move on and make some example circuits let's say we have a 3 volt supply and we want to connect a single red led what resistor do we need well we know this wire is 3 volts and this one is our ground wire which will be 0 volts the led has a voltage drop of around 2 volts and so our resistor needs to remove the remaining voltage so 3 volts subtract 2 volts equals 1 volt we know the led needs a current of around 20 milliamps so 1 volt divided by 0. 02 amps equals 50 ohms of resistance make sure you convert your milliamps to amps for this calculation to make it easier we do have a calculator on our website where you can just input your values i'll leave a link in the video description down below for that okay now you try to solve this one before i do let me know your answers in the comment section down below let's say we have a 9-volt battery and we want to connect a yellow led which has a voltage drop of 2 volts and requires 20 milliamps of current so what size resistor is required well we have a 9 volt supply so subtract 2 volts for the led and that leaves us with a 7 volt drop for the resistor the current is 20 milliamps so 7 divided by 0. 02 amps equals 350 ohms of resistance now the problem is that we don't have a 350 ohm resistor we only have a 330 ohm or a 390 ohm so which one should we use as we saw earlier we need to ensure the current doesn't exceed 20 milliamps so we will have to calculate which resistor suits us best to do that we just divide the required voltage drop of 7 volts by the resistor value of 330 ohms to get 0.
021 amps and then if we do the same for the 390 ohm resistor we will get 0. 018 amps both of these values are very close and they both will work but to be safe we will choose a 390 ohm resistor as our led will therefore last longer we can also combine resistors to get the exact value we need and i'll explain that a little later in this video we will also need to choose the resistor power rating we can calculate this using the formula power equals current squared multiplied by resistance so 0. 018 amps squared multiplied by 390 ohms gives us 0.
126 watts so a one quarter watt rated resistor will be fine for this circuit how long will this battery power our circuit for let's say this battery is rated for a typical 500 milliamp hours we simply divide this by our total circuit current which in this case is 18 milliamps so 500 milliamp hours divided by 18 milliamps will give us around 27 hours although this is the very maximum it would power our circuit for in reality it probably will not achieve this okay so what if we wanted multiple leds one option is to connect them in series in this design the voltage drop of each led will add together so the total voltage drop in the circuit shouldn't exceed the battery therefore a 3 volt battery can only sufficiently power one led at 20 milliamps and a 9v battery can sufficiently power four leds if we connect four leds and connect this to our dc bench power supply we see they will not turn on until their total combined minimum forward voltage is reached at around 6. 3 volts however the optimal 20 milliamps of current will not be reached until around 8. 6 volts at 9 volts the current is around 35 milliamps which is obviously too high so we will need a resistor if we connect 5 leds they won't turn on until around 8.
3 volts at 9 volts they are all on but the current is very low so the leds are dim that's because the voltage isn't enough to fully power the leds the optimal 20 milliamps isn't reached until 10. 7 volts in this example so we can use this method but we are limited by the voltage of the battery what if we want more leds well we need to connect them in parallel we can either place a resistor on each led or we can use one resistor to feed all the leds let's start with the first example this design lets us use different color leds although it's easier to calculate if they are all the same color let's say we want to connect six leds to this nine volt battery each led has a voltage drop of two volts and requires 20 milliamps this entire rail is 9 volts and this entire rail is 0 volts so each led will get 9 volts across it that's obviously too much so we will need to place a resistor against each led so we have 9 volts subtract 2 volts for the led which leaves us with 7 volts so we need to drop 7 volts on each branch we calculate the resistor value by 7 volts divided by 0. 02 amps which equals 350 ohms and then we find the power rating so 0.
02 amps squared multiplied by 350 ohms gives us 0. 14 watts so a one quarter watt resistor will be used then we need to add up all the currents in each branch so 0. 02 amps multiplied by 6 leds gives us 0.
12 amps a 9 volt battery has a capacity of around 500 milliamp hours and our circuit is using 120 milliamps so 500 divided by 120 gives us around four hours of runtime we can see that there is still enough voltage on each branch to connect more leds let's say we place three leds on each branch so each branch has a reduction of the six volts therefore nine volts subtract six volts equals a three volt drop by the resistor so three volts divided by 0. 02 amps gives us 150 ohm resistor notice the total current in each branch didn't increase so we can add more leds until the maximum voltage is reached if we want to use different color leds then we place the different leds on the different branches and we find the suitable resistor for example we might have a red blue and green led each led has the same current requirement of 20 milliamps but the red led has a voltage drop of 2 volts the blue has 3. 4 volts and the green has 3 volts the resistor for the red led is therefore 9 volt subtract 2 volts which gives us 7 volts 7 volts divided by 0.
02 amps will lead us to a 350 ohm resistor the blue led is 9 volts subtract 3. 4 volts which leaves us with 5. 6 volts so 5.
6 volts divided by the current of 0. 02 amps leaves us with a 280 ohm resistor and the green led will be 9 volts subtract 3 volts which leaves us with 6 volts 6 volts divided by the current gives us a 300 ohm resistor the total current is therefore 60 milliamps so the battery will last around 8 hours the other way we can connect leds is by connecting them in parallel and then using a single resistor to limit the total current for this design you should only use the same color or the same rating leds we'll see why that is shortly in this video let's say we have a 9 volt battery and 3 red leds all with the same voltage drop of around 2 volts and they each require 20 milliamps of current so we just add the currents together to get 60 milliamps and that current has to flow through this one resistor now as they are connected in parallel they will all have the same voltage difference across them therefore we calculate the resistor by nine volts subtract just two volts equals seven volts then as all the current is flowing through this one resistor we will need to divide the 7 volts by the 60 milliamps and that will give us a 116 ohm resistor the power calculation comes out at 0. 49 watts so a half watt resistor will be used the reason we need to use the same rating leds is because the voltage difference across here is just 2 volts so if we use the same rating leds they will all illuminate but if we place a blue led in the circuit this requires a higher voltage which you will not be able to get so this led will not turn on now when we deal with these circuits we often find that the resistor value we have calculated doesn't exist or we simply don't have it in stock so we can combine resistors to get the value we need for example if we wanted a 200 ohm resistor we could place two 100 ohm resistors in series or we could place two 50 ohm resistors and a 100 ohm resistor the resistor values will just add together in series which makes it very easy to increase the resistor value to reduce the resistor value we simply place them in parallel then we do some maths to find the equivalent resistance let's say we have two 10 ohm resistors we can calculate that using this formula this is much easier than it looks we just enter this into our calculator and we see it gives us 5 ohms of equivalent resistance so two five ohm resistors will give us 2.
5 ohms of total resistance a 200 ohm and a 50 ohm resistor will give us 40 ohms of resistance and three 10 ohm resistors would give us 3. 33 ohms of resistance how can we tell the value of a resistor well these colored stripes on the body will tell us the value but we will need to look it up on a chart we can get four or five band resistors typically so let's look at some examples of these with the full stripe type the first two stripes are the digits which we combine the third stripe is the multiplier and the fourth stripe is the tolerance for example this four band resistor is brown black brown and gold band one is equal to one band two is equal to zero giving us band 3 is the multiplier which is 10 so 10 multiplied by 10 is 100 ohms then the gold is the tolerance of 5 so it could be as low as 95 ohms or it could be as high as 105 ohms when i measured this one with a multimeter we can see it was reading 98. 2 ohms which is within the tolerance so we saw that the previous resistor wasn't very precise if we need more precision then we will need to use a smaller tolerance like this one percent tolerance five band type with this type the first three stripes are digits the fourth is the multiplier the fifth is the tolerance this one is orange orange black black brown so this is a three this is a three this is a zero with a multiplier of 1 giving us just 330 ohms the tolerance is 1 percent so it could be between 327 ohms and 333 ohms but when i measured this one with a multimeter we can see it was reading 329.
9 ohms so it's perfect okay that's it for this video but to continue learning about electronics and electrical engineering check out one of the videos on screen now and i'll catch you there for the next lesson don't forget to follow us on facebook twitter instagram linkedin and of course the engineeringmindset.
