Why use Current Limiting Resistors | AddOhms #8
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- Опубліковано 8 лют 2014
- LEDs add magic to every electronics project.
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Often not understand is what else you need to make that LED work correctly. Learn what current limiting resistors do for LEDs and watch what happens when you don't use one.
For this video we go the lab and run so much current through a LED, it destroys itself!
#LEDs #Electronics #Resistors - Наука та технологія
Great video.
For those concerned with the components harmed in this video, keep in mind that many more components were likely saved because of this video.
Touché :)
About time somebody explained the resistor before every diode. Thank you broseph!
excellent way of demonstrating it!
Dude I'm learning about electronics in college right now and I find these videos very enlightening
Thanks!
This was great, been searching for "what affects the tolerance of a resistor?" for a while now, and I think this has helped. Ever heard of - Henucas Panrooklyn Equalizer - (just google it )?
It is a smashing exclusive product for discovering how to repair electronic devices like a pro without the headache. Ive heard some extraordinary things about it and my friend got cool results with it.
WOW! This is by far the best explanation of how the current limiting resistor protects the LED! great work!
These videos are the best for learning electronics. It would be nice if you could do more and go more in depth.
Standing ovation. Thanks!
These videos are really good, thank you.
Awesome!!! Exactly what I was looking for!! Great youtube channel buddy! 👍👍👍👍
amazing videos, so much enlightening
You always describe in easy way to understand.
I will subscribe your channel.
i am a complete noob and this helped clarify a lot of info on resistors. Thanks
Super informative video :) Thanks for posting
Thaaanks so much for making these FANTASTIC educational videos!!! This is helping a LOT of people out there!!! Ohhhh yeaaahhh =]
Yeah this and so many other same videos combine is still blowing my "whatever" lol
as basic as it gets it's really well explained.
This intro gets me totally pumped for resistors bro
Thanks for the Video! Forgive me for chiming in, I would appreciate your thoughts. Have you considered - Runayslarn Survival Manifestation (erm, check it on google should be there)? It is a great one of a kind product for discovering how to repair your electronic items without the hard work. Ive heard some great things about it and my mate at very last got amazing results with it.
@@panedole fk off
Thank you... Finally someone explained what is going on with LEDs and Resistors.... I'm Happy :-)
excellent demonstration. I always struggled to visualize current vs voltage through components JUST by looking at diagrams and long formulae! We need more teachers like you in electronics classes. Heck youtube is probably enough for learning LoL
There won't be alternative media without mainstream, no UA-camrs without teacbera
@@theguy4u7773 this alternate media is now transforming to “mainstream”
There’s no cable in my house, kids learn thru UA-cam, online courses, no CNN, BBC, news is Twitter, online articles, TV is Netflix, Hulu, Disney,....the lines are blurred now. It’s a transition phase.
Hi may I ask when using super small nano 3 Volt LEDs, can I adjust brightness using a variable resister if so what range.
Good explanation...thanks
You are awesome... please keep on creating more videos...
keep it up i love waching this
Can you use a buzzer as the current limiter? For example: I use a 9V battery and put 2 LEDs with a voltage drop of about 3V in series and attach a mechanical buzzer (operating voltage 2-4 V at 20mA) in series to that. So there is about 3V left for the buzzer which will draw 20 mA, which means 20 mA will go through the LEDs. Is this correct? If no: how do you get the same effect?
Why are your videos sooo short- youre great keep it up!
sir i like ur videos very much ,,, thanks for such awesome videos
i love to watch ur videos
i also show ur videos to my friends and juniors
ur videos are very interesting , short , simple and easy to understand
thanks for uploading such videos
hats off to you sir
good to learn
At 3:22 you said we took the LED out... but the text said LED with resistor disconnected. Small slip up but a very informative and interesting video.
+fatherof8kids Occasionally I slip and don't catch it.
In the "Analog vs Digital" video I said "digital" once when I meant to say analog. No one has caught that one yet. ;)
yes i notice that also, but now i understand about the current and voltage mutch more
I find that in many cases, I can use leds without resistors, like when op amps are used to drive them. Op amps rarely can deliver more than a few 10's of milliamps! Sometimes I will use power supplies that have 3V outputs and can naturally drive blue and white LEDs without issues.
So what if the forward voltage of your LED is 3v. Let's say you power it with 2 AA batteries but no resistor. Is it still going to draw enough current to blow the LED. Since the higher the value of the resistor the dimmer the LED will be, is there a lower limit to the value of the resistor where it would no longer protect the LED?
Connecting two or more resistors in series what does it mean
At a certain value of current,a variable power supply stars keeping down the output voltage to limit the current on a certain value. please explain how does it happen?please...................
Thanks!
Good question Norm. Even if you use 2 AA batteries, the problem is that the LED will draw as much current as it can. It's "current limiting resistor" will become the internal resistance of the battery.
When new, the batteries will provide a LOT of current at first. Over time, they'll drop rapidly.
I have 60 volt transformer I want to be able to vary the voltage from 0 to 60 volts dc I can't use an lm317 because I would burn it out, what do I do?
Owen Chase science channel What do you mean by transformer?
Transformers only work with AC. Are you trying to convert AC to DC and then create a variable power supply? Because no matter what you do, you won't be able to take the 60V AC output of a transformer and output 60V DC, without some loss.
AddOhms ya I know how to convert it to dc I just want to make it variable.
Owen Chase science channel I certainly wouldn't use a linear regulator. When your output voltage is very low, it would be dropping the entire difference between Vin and Vout, which means very little output current.
So, you probably need to look at a switching (buck) converter. The TPS-series from TI has a high input voltage and they offer variable versions.
You could get pretty close to 0 and pretty close to 50V with a single switching circuit, but it would be horribly inefficient and relatively difficult to select the passive components.
Instead, I'd recommend a couple of different stages or ranges, similar to how a bench power supply works. Of course, then you need to build a front-end to manage that.
Thanks
Hello,
Can you guide me for limiting the output current to 350mA of a power supply (with output 250vdc, max output 15Amp). Should I simply add 0.22 Ohm 10W (9 Resisters) in Parallel before load?
What is the load? Why are you working with a power supply capable of outputting 3.750 kilowatts, but only want 350 milliamps?
Nicely done. So it doesn't matter if the resistor is in the circuit before of after the LED? If it is good design to have the resistor after the LED then is it good design to have one resistor after 3 LEDs as long as the resistor value is correct for each of the LEDs and as long as only one of the three LEDs is lit at one time? Like as in a stop light?
+Volthaus Lab Since it is a series circuit, it doesn't really matter where you place the resistor. A good design practice is to place the resistor between the source and the LED.
As for your 3 LEDs in parallel--or a RGB LED, I've seen designs that save space doing it with one resistor. However, I personally wouldn't. Also if they are different colors, you're going to want different resistor values. Red LEDs are generally dimmer than Green and have different forward voltages.
I know it's a bit late but, if I use 3.3V instead of 5V in my circuits, and blue or white LED's with forward voltage of between 3.3-3.7V, would you still recommend a resistor? Thanks...
You have to be extremely careful. Around 3:40 you can see this LED draws about 5 mA at 3.0 V and 18 mA at 3.5 V. Not much change in voltage, but a huge change in current. If this is a one-off project, I would make careful measurements to make sure the LEDs aren't drawing more than 20 mA -- or a current higher than the supply can handle (which may be less than 20 mA.)
That said, if it were my circuit, I probably wouldn't use LEDs with such a high forward voltage. But, I also RARELY drive LEDs to their maximum forward current. 20 mA is too bright for most of my uses.
Good point. 20mA is a bit bright on pretty much any of the newer LED's. Cheers Mate.
Hello, nice video ! is it possible to have a power supply that limits the current to 1A, 2A, 3A... independently from the voltage. I mean i would like to have a 12vdc where I can set the output to max 1A, or 2A..., but keeping 12v constant. Thanks.
You would need to design a circuit to limit the current. Most current limiting circuits will change the voltage to reduce the current. A resistor would not be enough.
What would be the best amount of ohms for a current limiting resistor for and LED?
+Aura Dominator Depends on how bright you want it
Cool.
If you install a latching switch just downstream from the power supply, would this circuit generate any inrush current? Power supply is on, switch is open, then the switch is closed for several seconds, then the switch is opened again. Would the resistor limit the transients caused by opening and closing the switch? What if the circuit had 7 LEDs, an adequately sized resistor in series and was powered off 24vdc? Am I right to be worried about inrush currents? Or am I totally mixed up and inrush is entirely a function of the power supply design since the LED circuit is passive? Thanks for the video!
I think you are confused on what is "in-rush." In-rush is caused by capacitive and inductive elements. LEDs are mostly resistive. (Though they, like all components, have some C and L parasitics.) The amount of current LEDs draw at the instant voltage is applied compared to their steady state is nearly the same.
Thanks!
One more question. If PWM is applied for dimming purposes to this same circuit, is it still accurate to treat the LED like a resistor? Or now is it more like a switching transistor? Would it create rf emissions? I think no, because the diode use-case is passive in nature on this simple DC circuit (dissipating energy only, not generating it) that only contains LEDs and a current limiting resistor. What are your thoughts?
The changing electromagnetic field can radiate, creating "RF" or just "plain" emissions. Some components are better conductors of this energy. Traces and wire are excellent transmitters and antenna. Resistive elements, not as much.
Also, the frequency that PWM runs out tends to be very low, far from the traditional "RF" range. Specifically to limit the ElectroMagnetic (EM) emissions or at least limit the emissions at frequencies nobody cares about.
i know the standard resistor value to be used with LED is 330 ohm. How was this value derived?
The value of the current limiting resistor is determined by the LED's forward voltage and the forward current you want to set. Using those two pieces of data, you use ohm's law to calculate the resistor's value.
(Most people set the forward current to the LED's Max, which is generally around 20mA. This isn't necessary and shortens the life of the LED.)
I have a written tutorial on it here: www.baldengineer.com/led-basics.html
cool video , I'm surprise you didnt put the formula to calculate the current limiting resistor,
+Andre Bella This one was intended to be "why" and not "how" to use them. I had planned an Ohms Law episode to cover it, but decided to do something else for that.
Excellent video - thanks. But LEDs still confuse me. In the opening you said that when an LED turns on it effectively becomes a short. So I'm confused how it can be dropping 3v across it when the supply is 5v. It would seem to me that its effective resistance is higher than the resistor (which is only dropping 2v). What am I getting wrong?
"A short" doesn't mean 0 ohms. A piece of wire can create a "short circuit" which means it provides a low impedance (or resistance) path. Even that piece of wire has a resistance associated with it. Just like the diode. There's *some* resistance associated with it.
Thanks for the answer. I know you can't treat LEDs as pure resistors but would it be correct to say that an LED dropping 3v at 20mA ( at any Vcc, assuming resistor drops the rest) has an *effective* resistance of 150Ω under those particular conditions?
+vinnerism You would need to know what current flows through the LED at 3 volts, with no current limiter. Then ohms law could tell you the resistance--at that voltage.
Great video!
The LED flashes red @ 4:05, I wonder what causes this
Pure heat. The LED _briefly_ turned into an incandescent light bulb.
Do I still need a resister if the LEDs are in a 4*2 style powered by a 12 battery?
4 in series with 2 "branches" in parallel?
Check out Video Q&A #1 (video #9). I addressed this question there.
awesommeeee!:D
quick question. In some extension cords, and plug adapters, there is an LED indicator, when the cord or adapter is connected to the AC mains. I've opened up one and was surprised that the LED is connected directly to the AC mains, with just a 100 K ohms resistor. How is this possible?
It's perfectly fine. The 100K resistor is the current limiter. The LED is a diode, so it only conducts in the forward direction. The LED is actually flashing/pulsing since it is only on for half the A/C cycle. Without looking at it closely, it appears to be on steady.
Another common method is to use a "capacitive dropper." That's where you use a capacitor to AC-Couple a lower voltage circuit. But in the case of just an indicator light, that's usually more expensive so just a resistor and LED gets used.
Thanks for the reply. I've just started learning electronics, and trying to understand the capabilities/uses of these components. Most of the electronic circuits are in DC, that's why I was surprised with this setup. I've googled about this circuit and most of the responses in forums, did not believe that this setup is possible. As I understand in your video, the voltage will have no direct effect on the LED (even at 220v) because it is dropping or just using it's forward voltage (e.g 3.2 v) and the rest goes to the resistor. Only the current has a direct effect on the LED, that's why there is a need for the current limiting resistor. I've also came across Kirchoff's Law while browsing that somehow explained the voltage drop across the resistor together with a load like an LED, based on this the voltage drop on the resistor will be 220V-3.2V (Vf of LEd)= 2168v (AC or DC?) is this correct? Forgive me for not trying this myself since I'm still new and a bit scared to test it out on live mains.
This became too detailed to respond here. So I wrote up an explanation here. I don't mean to pick apart your question. I just feel it is better to break down each point to be more clear. discuss.addohms.com/t/leds-in-ac-extension-cords/678
Hi, Thanks again for the reply, and I actually appreciate that you've explained every part of my question. I believe that the most "unbelievable" on this circuit was that the resistor did not "burn" out, and I was not able to convey the right question.But it was rather simple by using Ohm's Law and calculating the power, as I found out later. The resistor voltage drop is 216.8V and current is 0.002A which when power is calculated is only 0.4336W, so if a resistor with a 1/2-1 Watt rating is placed on the circuit it will not burn up. Thanks again!!
5 years late but..
1) are we sure it's a LED and not a neon lamp? Neons are often used as mains indicators and have a large resistor in series with them.
2) it's wrong to say the LED will only conduct in one direction. LEDs have a reverse breakdown voltage of ~5Volts, so you can't rely on them to be non-conductive at mains voltage
Hi. . AddOhms.. i have 50 pcs of
5mm LED UV Light :-
-Input Voltage: 3-3.4V-Input Ampere: 18-20mA-Wavelength: 395nm -Colour: UV
i want to built a UV Light box.. and i want to know
1. which wiring is good to use series or parallel ?2. how much volt power do i should use to power up 50 LED3. how much resistors do i need?
there some guide for me to follow :) Thanks!
1. Use Parallel arrangement. Bcoz in "Series" if one LED will fuse then all remaining LEDS will not work.
2. 5 Voltage Source will be enough for all 50 LEDs
3. 220 Ω to 650 Ω (Depends on how Bright you want). My suggestion is to use 1K Potentiometer(Variable Resistor).
Note: If you use LED in Series then you need 3 x 50 = 150 Voltage which is difficult to produce.
Excellent video. I have a problem with x2 12V electromagnets getting really hot, like boiling point hot. They are running off a 12V car battery and after 20 minutes you can cook with them. I tried a tiny resistor on the + side but smoke came out of it very quickly. Do I need to use a current limiting buck converter? Or are cheap Chinese electromagnets just going to get hot?
Thanks
Electromagnets are coils, which is just a lot of wire. You have to measure the resistance of the coil to know how much current it will pull at 12 volt. Then you can use Ohm's Law and the power law to how much power that series limiting resistor needs to be rated at.
@@AddOhms Hi, thanks for replying. Unfortunately these electromagnets are shop bought and are sealed. I have no idea how they are made, or how to measure the resistance of the coil. I'm just passing current though them, this is the only thing I can control.
Am I basically screwed because of how they are made? I tried lowering voltage down to 9V and lower, which keeps them cool, but then they are not quite powerful enough. I'm now thinking about buying a constant current buck converter to keep the volts at 12 but reduce the amp flow, but I'm wondering if that has the same effect? Or maybe I could use a PWM signal somehow to keep current down, but thinking about it maybe this has the same effect too.
Thanks
@@AddOhms By the way what do you mean series limiting resistor? Is this something I should have in my circuit? I tried adding a small resistor of 47ohms but it started to smoke very quickly. I'm now running a buck converter to drop the voltage. One magnet seems to get hotter than the other, but they are both the same specification. I am also trying out reversing the polarity on the hot one to see if that has any effect. It seems to want to draw more current than the other one. Is this a result of "made in China"?
The 47 ohm resistor you put in series was a series limiting resistor. If it was a 1/4watt resistor, it couldn't handle more than 144 mA before it would start to burn up.You need to measure the current the magnets are drawing so you can calculate a suitable limiting resistor.
Then measure their resistance with a multimeter.
did you make the constant current video. great explanation btw
Sadly, no. Still on my list. Thanks for reminding me though.
No probs you provide a service and you are good at it. keep it up! maybe one day the world will recognise knowledge as the most important thing ;) and you can get the billion subs you deserve lol
Please make a led driver using ics
Do adding resistor consume additional battery power?
A LED without a resistor will burn a lot of power until the LED dies.
How would I limit current in a phone charging circuit? Say I have a 2.1 amp charger but I don’t want to charge it that fast to extend the battery life
The charging circuit is in the phone. So there isn’t much you could do externally, except, get a supply that is only capable of delivering lower current. Phones do use pull-up resistors on the data lines to indicate to the supply whether they are capable of accepting 500mA, 1A, or 2A. So an option would be to build an adapter that disconnects the data lines between the phone and usb power supply.
AddOhms I was hoping I could just build something that went in between the charger and the phone to reduce the amps
Build an adapter that disconnects the data lines on the device side, but provides pull-down resistors on the host side. For 500mA that would be 15kOhm on D+ and D-. More information here: www.maximintegrated.com/en/app-notes/index.mvp/id/4803. Skip down to "An Array of Power Sources."
Ok, so you picked 470 ohms, why, exactly? That's what I find is missing from this discussion.
Using the LED Calculator Wizard you link to on the notes page, I can get 470 ohms if I specify a 12v source, 3.2v LED Forward voltage, and (looking back at the data sheet, I assume the "forward current", which I don't think you mentioned, is 20 mA), 20 mA for the forward current.
However, the voltage you use in the demonstration seems to want to be 5v.
What say you? :)
Thanks.
+lahattec Pick the current you want to go through the LED. Most people look at the data sheet and see "20mA." That's the max. Less current, less bright. Subtract the LEDs forward voltage from the supply for the resistor's voltage drop. Use ohms law and divide that remaining voltage drop by the current value you picked. That is the resistance value to use.
Ok. Thanks very much for that.
So, assuming a 5v source, and a desired forward current of 5mA, then the voltage drop 'left over' for the rest of the circuit (the resistor), after taking into account the LED's forward voltage of 3.2v is 1.8v.
So then dividing the 1.8v by .005A equals 360 Ohms.
One weird thing is that the LED calculator comes up with 390 Ohms. Any idea why the difference? Did I miss something?
Perhaps the difference is that the LED calculator, as it says, "rounds up to the next standard resistor value". Maybe that's the difference.
I actually made a tutorial on my channel about voltage, current, and resistance, and showed the application of all the theory!
AddOhms yes.
How about if you use PWM yo limit the current, taking into account the voltage drop and the voltage you are using?
+Victor Rivarola Nope. PWM isn't analog. It's the full voltage on, when it is on. You still need a resistor.
+AddOhms Are you sure of that?
According to my tests... it works!
drive.google.com/file/d/0B1GqzCtvayD3RzN0TlUxR0Q1MGM/view?usp=drivesdk
+Victor Rivarola I could even post the code if requested.
You know what, here is the code. Just copy it into an Arduino IDE and upload it to the device... THEN hook up the LED to it.
-----------------------------------
// LedWithoutResistor - This lights an LED connected to a PWM chip without
// a power wasting resistor. WARNING: You need to check your LED's vorward
// voltage drop and the arduino's output voltage for this to work,
// otherwise you risk burning it.
// (c) Star Labs Informatica N.C.V.V., Modified BSD lisence
// directory.fsf.org/wiki/License:BSD_3Clause
/*
fLedsForwardVoltageDrop nLedsHighLevel
------------------------- = ----------------
fArduinosVoltage 255
*/
const float fLedsForwardVoltageDrop = 3.0;
const float fArduinosVoltage = 5.0;
const byte nLedsHighLevel = fLedsForwardVoltageDrop * 255 / fArduinosVoltage;
// The LED on nLed is supposedto have no resistor, while the other one obviously does.
const byte nLed = 11;
const byte nBuiltinLed = 13;
// setup -- All of the program is done at the begining
void setup() {
pinMode (nLed, OUTPUT);
pinMode (nBuiltinLed, OUTPUT);
analogWrite (nLed, nLedsHighLevel);
digitalWrite (nBuiltinLed, HIGH);
}
// loop -- There is nothing here, just fullfills Arduino's API.
void loop() {}
+Victor Rivarola "It works" is a bad result. "It works", for now... You need the current limiting resistor. When the PWM is on, the LED is drawing the max current it can draw. It is damaging the LED and the IO pin. It will take long for the circuit to fail, but it will fail.
Its comfusin when you say the led drops 3 volts cause you make it sound like the voltage decrease by 3..
It does. In a series circuit, voltage drops add up to the source. So the LED drops its forward voltage and the resistor drops the rest. The two voltage drops combined is the same as the battery (or whatever the source is.)
as we know V=IR
if R is not connected in the circuit (means R= 0 ohms)
then will the Voltage be zero?
Resistance is never 0. There is some resistance in the LEDs and some resistance in the wires and usually some resistance in the power source (e.g. battery).
These small resistances will drop whatever voltage the LED doesn't, creating unnecessary heat and potentially damage things. I sort of talk about this in #9, where I address putting 4 LEDs in Series: AO #8 Current Limiting Resistors
If R is removed then its value is infinite (not "0"), and you are left with pure capacitance. Enter imaginary numbers: i=√(-1).
I have a project that requires 13 LED's 3.4 each at 20mA in series. 3, AA batteries. I try the net calculator for resistance and gave me -1985. can you tell me what am doing wrong and what resistance cand handle 13 LED's? thanks.
Putting 13 LEDs in series, each with a forward voltage of 3.4 volts means you need at least a 44 volt supply. You’re not going to get that out of a few AAs, even if you built a boost converter. Putting them in parallel and limiting the current to 20mA each means 2.6 amps out of the batteries. That is asking a lot of AA cells.
Excellent confusing video.
Fantastic pointless comment.
Smokin’ 😂
HEY I NEED A 120mA CURRENT LIMITING DIODS FO MY POCKET LED TORCH WITH INNER POWER SOURCE OF 3.7V . ANY IDEAs NOW ???
hello i need help i have a battery with 2amp 7.2v and i need to get 220mah what resistor i need
220 mAh is a measure of energy stored. If you know the current being drawn, then you can calculate the time it'll operate. Batteries supply whatever the load draws. So it depends on your load. It's highly doubtful you need a resistor.
@@AddOhms I don't know, I mostly have a fpv receiver and when I connect it to the battery, it heats up a lot
thank you for your reply
you can limit current with a capacitor to
+prototype9000 No, you can't.
If put in series, it will act as a blocking cap, preventing any current flow. If in parallel, it'll significantly increase the current draw while it charges up.
Neither acts as a current limiter.
+AddOhms it allways works for me changing the capacitance changes the amount of current being passed try it and see
+prototype9000 That is not current limiting.
That practice is damaging the LED and possibly the driver (e.g. the I/O pin.) The charge and discharge of the capacitor will be pushing far too much current. Granted it'll be in peaks.
While it might "work" in a demo, it is not a good circuit design practice.
try taking a capacitor and placing it on a dimmer switch with a light bulb measure voltage and current then take another capacitor same voltage rating but a smaller capacitance and measure the current and tell me the amperage has not dropped. throw away the dimmer if you want and leave the capacitor in the circuit for the same effect it wouldnt work on a dc circuit at all unless the input is pulsed but it will limit current on the ac circuit
If it's not DC then then capacitor is smoothing out the changes. You are seeing the RMS voltage and current change, not a current limit.
Do the same measurements with an oscilloscope, instead of a multimeter, and you'll see the true wave shape. You aren't limiting current, you're just smoothing out the peaks and valleys.
My LED died soon after the voltage gets 5V. While I was wondering why the LED keeps lighting in 5V, and then died too...
I thought you decreased the voltage, not the amps.
They are related.
Great video, Now as a friendly advise in symbol for resistance is three pekes both sides more than this it's a Heater symbol
its a shame that we cant get the old incandescent grain of wheat bulbs to keep vintage audio gear more authentic and have to resort to using leds and resistors
I just love the intro music, can Somebody name the music please?
where is that video of how the constant current sources work please?
After so many years, this is only the 2nd request. So, not high on the priority list.
But doesn't the resistor act also as a voltage limiter.
Zero clue what this means.
@@AddOhms fixed it lol
You don't "limit" voltage, you "drop" it.
@@AddOhms but doesn't it though. If you have a 9v battery and only 3 goes through them you effectively limit the amount of voltage an led recieves?
I have no clue what you are on about.
LEDs are a bit confusing to me because they have nonlinear resistance and can change resistance as it heats up.
Resistors also change resistance with temperature, as do almost all components.