Why use Current Limiting Resistors | AddOhms #8

every project is made better by adding LEDs but you can't add an LED without something to limit the current like a current limiting resistor in this round of atoms we're going to talk about how resistors and LEDs work and why you need to use them to get caution viewer discretion is advised components were harmed in the making of this video when a voltage is applied to a resistor a certain amount of current will flow through it the amount is defined by Ohm's law which is going to be covered in more detail with another video for now let's look at this example if you put 10 volts across a 10 ohm resistor one amp of current will flow so remember the amount of current flowing through a resistor is calculated as voltage divided by resistance LEDs on the other hand work a little bit different a key parameter of all diodes including LEDs is their forward voltage this is the voltage required for the LED to become conductive basically what this means is that no current will flow through this LED until at least that much voltage is present here are three things to know about LEDs and diodes in general number one the forward voltage is the maximum voltage the LED will drop number two if less than the forward voltage is applied no current will flow through it and number three when the forward voltage is applied the LED effectively becomes a short circuit for example if we look at the data sheet for this blue LED we can see that it's forward voltage is around three point two volts let's put that led into a circuit with a voltage source and a 470 ohm resistor as well as a couple of multimeters from you know what this would be so much easier if I just show you real actual hardware instead of try to draw at all so let's go to that okay much better these fancy boxes are called benchtop multimeters they perform a variety of measurements but today we're interested in current and voltage the top meter will monitor the voltage drop across the LED while the bottom is measuring the current through the LED instead of a battery we're using this bench power supply to provide the voltage and current to the circuit as we take the voltage from zero volts up to one volt nothing much exciting is happening the current is practically zero while the voltage drop across the LED is about the same as the voltage of the supply it's basically acting as an open circuit right now when the supply gets close to three volts the LED suddenly turns on with some blue light because it's letting current flow if we keep going to five volts the voltage across the LED changes a little bit but not a whole lot it's still about three volts well the LED is dropping as forward voltage of about 3 volts while the resistor gets the rest of the supplied voltage Ohm's law is taking over the circuit and the resistor is acting as a current limiter for the LED all right maybe you don't believe me right now so let's take a look at what happens when the resistor is removed we took the LED out and now our going up to 1 volt like we did before nothing is going on and no current is being drawn as we go up to 2 volts still nothing's happening yet but as we get close to three bolts which is near the forward voltage of the LED it turns on and draws about five milliamps which sounds like a small amount right well let's keep going to five volts at this point we see it's starting to draw almost 140 million is where we have to be careful because without a current limit the LED is drawing lots of current let's see what happens when oh wait never mind the LED died in fact it's melted internally and now is a dead short before ending I do need to mention that current limiting resistors aren't the only options for LEDs another method is to use a constant current source this is a type of source that will change its voltage to achieve a preset current output examples include this modified lm317 circuit and another example is the chip from ti called the TLC five nine four zero which is a constant current PWM driver which is very commonly used to drive LEDs in a future video we'll take a look at how these constant current sources work but for now check out the show notes for some more information or tool is related to calculating current limiting resistors links mentioned in this episode and downloadable versions of this video visit the show notes at Adams comm slash APA you can also send us questions via Twitter our Facebook page or leave comments with this video if you like this video please share it with your friends and subscribe to this channel

Channel: AddOhms

Views: 239,654

Keywords: Electronics, AddOhms, Light-emitting Diode (Invention), LED, Ohm's Law (Namesake), Tutorial, Resistor, ohms law, resistor (invention), light emitting diode, electronic basics, current limiting, light-emitting diode (invention), electronics (field of study), electronics 101, resistors in series, light emitting diode project, light emitting diode working animation, light emitting diodes how do they work, types of resistor and their functions, current limiting diode

Id: 81zNcctopBI

Length: 5min 10sec (310 seconds)

Published: Sun Feb 09 2014