Electronic Basics #8: Everything about LEDs and current limiting resistors

if you watch my videos then you know I love LEDs like every second video involves them but there's been some questions and comments on how to use them probably and sometimes I don't even handle them to correct way so today I will tell you how easy it can be and how difficult it can get to handle a LED probably so let's get started first of all the beginners tutorial for example you buy your LEDs on Amazon or Ebay or elsewhere often there exists no datasheet for them which is a shame but there are two important parameters which are always given the forward voltage which is three point two volts and the current they need to light up ideal which is 20 milliamps now you've got your power source for example a 12 volt acid battery or button cells or in my case a 9 volt battery the simplified circuit to light up a led looks like this if we would not use a resistor the LED would die faster than you can say well so what value does the resistor need Kirchhoff's voltage law gives the ansan the voltage some in a closed network must be 0 on the left side is the voltage of our power source and the right side is the voltage of our load and I think everyone sees that the voltage of the resistor must be 5.

8 volts to solve the equation then you can just calculate the value for the resistor by Ohm's law resistance equals voltage divided by current and since the LED and resistor are connected in series the 20 milliamps will also flow through the resistor so five point eight volts divided by zero point zero two MS equals two hundred and ninety ohms two of those 150 ohm resistors in series will work fine when you don't have the correct value then use a bigger one like I did with 300 ohm another rating for the resistor is power those are a quarter watt resistors we heat them up with a power of five point eight volts multiplied by 0.

02 amps which equals zero point one one six watts since zero point one one six watts is smaller than zero point two five watts everything is fine if you overload your resistor then it might look somehow like this now you want to light up two of your LEDs you could build the same resistor LED combination in parallel but that is a waste of power just put the two LEDs in series and repeat the same calculation process this time we only have a voltage drop of two point six volts across the resistor a resistor value of 130 ohms and the power loss of 0.

05 two watts we got twice the light and half of the wasted power that is awesome but we do not have enough voltage to put three and serious the LEDs will be darker this way which brings us to the advanced territory first of all never trust the manufacturer it says three point two volts but the LED draw us way more current than 20 milliamps three volt as a forward voltage is much more precise let's imagine you have a 3.

3 volt power source and the blue LED with a forward voltage of 3.

3 volts so you don't need a resistor right well it does work but it is not a good style when we record the characteristic line of the LED you can see that the current consumption ascends exponential which means when you have a small voltage change in your power source it can destroy all your LEDs in that case try using a small resistor to linearize the current consumption this way voltage changes do not affect your LEDs that much let's take a look at this circuit I used in my moped mod video you can see I have soldered a lot of LEDs in parallel and just used one power resistor to limit the current and this guy right here criticized this a lot and he is right maybe a bit overdramatic but still right and here's why even though the forward voltage should always be 3 volts with those LEDs it is not every LED is different and here in my case the forward voltage varies between two point nine volts and 3.

1 volts with this many LEDs in parallel it is not possible to use one big resistor because all of those on the different voltage level so I calculated with the average and always went a bit bigger with the resistor value that means in this example that those with less forward voltage will get more current than the others which means they will die sooner and the others have to end you a more current which make things just more terrible utter my defense it is always effort versus benefits and since I was using so many LEDs was just the easier solution which will probably last long enough let's go back to the two LEDs in series since they exists the variation in the forward voltage this gets more complex as well the easiest solution for this is to just measure when 20 milliamps flow because the voltage of those always varies a bit but they all want those 20 milliamps and that is actually the best way to drive LEDs not in a constant voltage mode by the constant current mode you can build a simple constant current source with the lm317 and a resistor here's the schematic even though this works the efficiency is quite horrible DTL z 5 94 zero is also a popular example of a constant current driver but this will be a subject in another video for now this should be enough I hope you liked the subject please support my videos by sharing them and please don't forget to like stay creative and I will see you next time.

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