How to Turn an LED On

hi today i'd like to talk about how to drive an led what so you know what an led is that it's used in a toaster to the backlight of a lcd tv that a single led has two legs and it comes in through hole and smd packages and a variety of different colors and it's basically a diode that the current can flow in one direction through yeah you think you know everything then tell me what is event horizon and its relationship to an led i thought so now sit down and listen good by the way video is sponsored by audible where you can listen to anything you like new members can try audible for free for 30 days from the link in the description more at the end now yell your answers at the screen can we drive diodes with dc or ac you're all wrong the answer is maybe of course you can drive it with both dc and ac led or light emitting diode is a diode which means it sends the electric current through in one direction and blocks the current from going back if you'd like to know how a diode works i made a full video about it go watch that oh you're back okay so leds when the current passes through them emit light that's why they are light emitting diode now you might say current going in one direction only sounds like dc so i have to show you that leds actually run on ac i just need an ac source of course everyone has an ac source at home i just tied an led to the end of my power line and i plug it in but it's not gonna not all components blow in spectacular ways just because you don't see the sign doesn't mean the high voltage is not there so the led blew for two reasons the current was above its limit and the reverse voltage was above its limit so if you look at the data sheet of an led you see that for this one the continuous forward current is 30 milliamps and the reverse voltage is 5 volts you don't put any limiting components on your leds they are sure to die although not in spectacular ways another bit of information looking at the data sheet is that when the led is on there is typically some forward voltage across it which for white and blue leds is around three to four volts and for the other colors it's around two volts so knowing that looking at the voltage current curve like any other diode when the forward voltage of an led passes some threshold the current through it exponentially spikes up so with a little bit of voltage change you get massive current change so basically the voltage across the led is pretty much fixed for a large range of current through it and putting a large reverse voltage across the led larger than the breakdown voltage will also result in the diode conducting large amounts of currents and both cases can overpower the led and burn it which is what happened when i place 120 volt ac across it to limit the current through the led we can put a resistor series to it so for example if you have a source voltage of 5 volts and led has a forward voltage of say 3 volts and we want 10 milliamps through the led the resistor would be 200 ohms fine let's drive our led from dc first such a weak bull here's my led series with 220 ohms and a 5 volt supply doesn't even turn on probably connect it backwards there you go it's on yay is this what you want your life to be an uninteresting boring what's the use of all this knowledge if we can't directly drive them from the ac 120 volt so let's do that here we go i put 5.6 kilo ohms series with the led for around 20 milliamps and brighter light here we go the brightness is around the same if not worse and the resistor is smoking why is it not brighter c the rms or effective voltage for a full sine wave is its peak voltage divided by square root of two but we have an led diode which passed led diode led led only passes the positive cycle through and the rms or effective voltage for that is the peak voltage divided by two so if a 120 volt ac which has 170 volt peak is half rectified the rms for that is 85 volts which means if we still want a continuous 20 milliamp rms car and through the led we would need a resistor around 4.25 kilo ohms well all i have is a 3.9 kilo ohm so a bit more current and we plug in there you go now we have a brighter led uh the resistor is smoking too much though [Music] see that's the issue with driving leds with the resistors especially at high voltage while the led itself was consuming around 60 milliwatts of power the resistor was consuming 1.7 watts and burned so especially at higher power application having a serious resistor to control the current just wastes a ton of power like for example if you want to take down a terrorist that's living in a tent in the middle of a desert you don't nuke the entire desert you don't nuke the entire desert no matter how fun it might be but especially in low voltage low milliwatt power applications who cares about a bit of power waste go ahead use resistors for those which reminds me we can use a capacitor to limit the current through the led as we know capacitors and inductors limit the current through them due to their impedances and they don't consume real power which means they don't get hot when they limit the splash unless of course the voltage exceeds their limits let me see if i have a capacitor that can handle 120 volts hey i found a ceramic one micro for a 250 volt capacitor that should limit the current of the led to around 32 milliamps which is a bit over the and the led is already dead next led you're a new led are they dying let me see if i can extract an led string from one of these led light bulbs don't break in my hands no injury and here's what's inside a cheap led bulb a string led and a series resistor to control the current something i said we shouldn't do and it's broken i wonder why and the string led is basically a bunch of tiny leds in series so it would require a higher forward voltage to turn on hopefully this string is not broken and my capacitor can limit the current through it here we are it is working now it burned yeah it overheated so i would need either a smaller capacitor for higher impedance and lower current or a longer led string so that there will be much larger voltage across the leds so less current but anyway you get the point this is actually one of the ways they drive led strings in some of the light bulbs and the good thing about capacitor is again it doesn't consume power now what do we use to turn an led on and off a switch of course and what do we use for a switch a transistor that can be a mosfet or a bjt we just send the on off signal to the input of the transistor and the led turns on and off here's my five volt supply and the circuit here don't mind the burn breadboard i'm using a mosfet so if i hold the gate and touch the fireball thing the led turns on and touch the ground it turns off of course we can use a microcontroller to drive the led but here i'm using a function generator now let's see what happens when i increase the frequency somewhere above 60 hertz we stop seeing it flickering and it becomes continuous but as you may have noticed it's half bright now this is because same as when i was driving the led with the ac line although we don't see it the led is only on half the time so the average brightness is dimmer we can use this technique we can use this technique to adjust the brightness of the led see if i decrease the duty cycle so it's off most of the time it's dimmer and if i increase the duty cycle it's brighter this is the simplest way to adjust the brightness which almost everyone uses now you might ask why don't we just tune the voltage across the led to adjust the brightness didn't i already tell you a little bit of voltage change across the led results in huge current change and so brightness it's much harder to do it's much easier actually to adjust the current through the led which translates in voltage across the led so some could say you're actually tuning the voltage across whatever here's a simple example of a current source circuit with a bjt npn transistor if you provide a fixed voltage to the base of the transistor minus the 0.6 volt drop on the base emitter there will be a fixed voltage on the resistor at the emitter so there is a fixed current running through it which is almost equal to the collector current of the transistor so almost no matter what load you place at the collector of the transistor even if the supply voltage changes the current through the load remains around the same so we can place an led as the load to make sure its brightness doesn't change with the change of the supply voltage and we can tune its brightness by adjusting the base voltage here you go i'm changing the base voltage of the transistor using a potentiometer and the current and so the brightness changes all good and dandy for low power applications but for high voltage high power applications we want something more efficient so we rely on capacitors and inductors or a switching power supply let me harvest another led string first these led strings require tens of volts to turn on because there are so many leds in series in them but let's see if you can turn it on using 5 volts here is a simple voltage boost circuit with a mosfet transistor as the switch the switch being fully on or off minimizes the power consumption over the driving circuits when the switch turns on the inductor falls between the power supply rails and the current through it rises charging the inductor when the switch turns off the current through the charged inductor has momentum and wants to continue going so the voltage across it shoots up so high it turns on the led string it pushes its current through the leds dissipating its energy quickly until the next cycle so the led pulses on fast so fast we can't see it blinking here i threw the circuit together with whatever component i had the leds are here a 47 micro henry inductor transistor my 5 volt supply and a 20 kilohertz pwm at low duty cycle and if i turn on the supply the leds don't turn on but there is some noise looking at the output though i see every time i turn the transistor off i have spikes of 100 volts which is apparently not enough to turn on the led but increasing the duty cycle and the on time i should be able to increase this voltage there you go and my led string is on what is that 250 volts so i guess there might be spikes of high current through the leds but at a very low pulse width that's why it doesn't blow up remember although you can send a certain amount of maximum current continuously through the led you can send maybe 10 times of that through the led at short intervals as long as the component doesn't heat up above its burning threshold you can increase the current through it to much higher than the maximum continuous levels say by pulsing the current of course we can increase the pwm frequency so that the inductor current never drops to zero for a more continuous operation for such switching circuits we can control the output current by adjusting the pwm duty cycle so we can place a fancy feedback and controller to accurately adjust the output good good but who cares about 5 volts let's see if i can run it from the 120 volt ac for this circuit first we rectified 120 volt ac with a full bridge rectifier and a capacitor to get around 170 volt dc when the switch is on the current flows through the leds into the inductor charging it when the switch turns off the inductor pushes its current through the freewheeling diode back into the leds and again we can adjust the current by changing the duty cycle here you go well the components i'm using are much larger than they need to be but that's all i have available every single time i forget that the ground of the scope is connected to earth and i end up shorting it to the live you know what i'm going to connect the live and neutral of the scope only to the same power lines are that are powering the circuit no earth connected simple as that ah it's not good to leave the transistor in an unknown state when you are powering up the circuit c if we leave the gate of a mosfet floating its voltage could accidentally be high turning the transistor on shorting stuff when we don't want to so in general it's a good idea to have a large resistor between gate and source to make sure its voltage is zero when it's not controlled trying again please don't blow we have what 160 volt dc 20k signal confirmed at very low on time connecting the signal very slowly increasing the duty cycle oh i can't see the leds blinking i'm doing it i'm doing it my god i'm sweating like a pig here it's getting brighter let me lower the frequency it works turn it off now we did it guys that was nerve-wracking i was using a flash capacitor to clean up my rectified voltage i guess it's a good idea to discharge it now oh geez well all these circuits were just examples to give you ideas there are a ton of different driving mechanisms out there you can search and find and regarding the relationship between event horizon and leds thanks to my sponsor audible you can start listening to exclusive audible originals from top celebrities experts and exciting new voices on top of an incredible selection of audiobook titles across any genre in existence for example as someone who likes to listen to stories when i'm drawing through daily chores i like to listen to the new audiobook i'm glad my mom died by janet mccurdy in her own voice you know just to make sure my daughter doesn't feel the same way about me after i die and as a new member you can get it for free signing up from audible.com electroboom or in the us text electroboom to 500 500 to sign up and try the first 30 days of audible for free and with your membership you also get one free title every month as well as access to the ever growing list of titles and new favorites and formats like the exclusive awards music series or podcasts just slap the audible app on your phone and keep your brain busy with your favorite title anywhere you want at your 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Channel: ElectroBOOM

Views: 1,222,315

Keywords: educational, electrical, ElectroBOOM, science, electronics, engineering, entertainment, equipment, measurement, experiment, mehdi, mehdi sadaghdar, arc, mishap, physics, Sadaghdar, test, tools, circuit, funny, learn, shock, spark, discharge, LED, Driving LEDs, boost circuit, switching supply

Id: zGsQbOjXeEs

Length: 17min 0sec (1020 seconds)

Published: Fri Aug 26 2022

Reddit Comments

[u/ExplosionsAndFire]

The UV LEDs are working a lot better now!! I just gotta use them to convert some of our monoketal into the cage and we have a videoooo

[u/Mozdog04]

This guy's a legend, some of his vids are hectic

[u/Bavarianscience]

Yeah absolutely. At least mehdi knows how to limit led current. I literally screamed resistors Tom, resistors! at the screen while watching that led video.

[u/Plylyfe]

I love this guy. Man's a mad lad