LEDs without the use of current limiting resistors

I am driving LEDs with a constant current driver, and wanted to increase the current ( pulsed ) to increase the brightness.

I asked my LED manufacurer what the safe pulsed current is for the particular LED, and they said 30mA - its rated at 25mA normally.

So its definately not safe to start thumping huge current pulses through the LED, ( or from the Arduio pin )

GMikes tutorial sums it up nicely, but as he says, different LEDs have different pulse tolerances, so check with the manufacturer.

Unless you have a constant current drive, include the 2c resistor.

I dont know where he is getting his figures from, but He seems to be using a 6.25% duty cycle, so maybe is is using a calculation based on duty cycle and the parts forward voltage rating. Lets say your LED is fully lit with 20ma at 3.5, and you are giving it 5V, at 5v lets say you are using 100ma, but with a duty cycle of 6.25 % its using 6.25ma.

Those numbers are made up just to try to make sense of what he is probably doing, Its not too clear from his documentation.

I can tell you that I have run LEDs without resistors, and right now I have a couple sets of 192 LEDs running with out a single resistor, and they have been running for months now without issue. The really crazy thing is that no matter how angry mike gets, my LEDs still shine brightly.

Heres 192 LEDs, 0 resistors, 1 arduino, 1 4-pack AA batteries (6v).

You are saying its OK to pulse it at 100mA, my manufacturer says 30mA - hmm .

Some TV remote controls use the internal resistance of the battery to limit the current through the IREDs, perhaps your running 192 LEDs from a couple of AA batteries is doing the same, how many LEDs are on at the same time ?

Whichever, its bad practice.

I run my 25mA rated LEDs at 18mA, and out of the last 25,000 LEDs since October, I have had only a couple of failures, so I stick to that current.

I am pulsing the latest project ( 50% ) at 21 mA, still well inside of the recommended current.

Boffin1:
You are saying its OK to pulse it at 100mA, my manufacturer says 30mA - hmm .

Some TV remote controls use the internal resistance of the battery to limit the current through the IREDs, perhaps your running 192 LEDs from a couple of AA batteries is doing the same, how many LEDs are on at the same time ?

Whichever, its bad practice.

I run my 25mA rated LEDs at 18mA, and out of the last 25,000 LEDs since October, I have had only a couple of failures, so I stick to that current.

I am pulsing the latest project ( 50% ) at 21 mA, still well inside of the recommended current.

I never said it was ok to pulse 100ma to anything, I described what I think the author of the document that the OP posted was trying to elaborate, but was unclear on how they made their calculations, I gave an example of made up numbers, and i clearly stated that I made the numbers up.

Your manufacturer is talking about "safe limits", most of the datasheets that I've looked over show LEDs at a 20ma max. I dont know how much current the LEDs Im using are running at, but they are running at 5V from wall wart USB battery chargers, one is 500ma, the other is 800ma. When both are hooked up to a kill-a-watt, it shows 1 watt running both (it floats between 0 and 1) so my guess is it draws about 1/2 amp. I do know that the red LEDs are 1.8-2.2 VF) so 5v should be way too much for it.

I have another set that I have just completed, and I've run it from a 6 volt lantern battery, and from 4 AA batteries.

I have read that you can safely double the safe limit when using a duty cycle less than 50%, but I havnt done any testing with that. I have run 74hc595s way over their 70ma limit, for a few days without issue.

I never said it was ok to pulse 100ma to anything,

So have you measured what you supposed good LEDs are actually taking?
There are two things getting damaged the LEDs and the electronics driving them.

I dont know how much current the LEDs Im using are running at,

So how do you know you are not damaging things?

I have read that you can safely double the safe limit when using a duty cycle less than 50%,

Safely doubling safe limits is an oxymoron. You can read all sorts of rubbish on the internet, it doesn't make it true.

I can only tell you what will work 100% of the time for 100% of the people. I know this because I have spent my whole life designing reliable electronics. Any idiot can design unreliable electronics that works for maybe two or three years. Or that works and significantly shortens the life of parts.

You can of course choose to ignore this, I can not stop you from being a total idiot. What I can do is point out that you are an idiot and point out why you are an idiot and why you are suffering from self delusions. Hopefully I can stop sensible people from following your idiotic total lack of logic.

  1. You do not have enough equipment or expertise to know if you are damaging a component.
  2. You appear not to know about peak current and power dissipation and how that relates to average current and average power dissipation.
  3. You think that any activity that doesn't immediately kill you is a safe activity.
  1. You think that any activity that doesn't immediately kill you is a safe activity.

LOL, who first coined the phrase "That which does not kill us only makes us stronger" ?

Lefty

Grumpy_Mike:

I never said it was ok to pulse 100ma to anything,

So have you measured what you supposed good LEDs are actually taking?
There are two things getting damaged the LEDs and the electronics driving them.

I dont know how much current the LEDs Im using are running at,

So how do you know you are not damaging things?

I have read that you can safely double the safe limit when using a duty cycle less than 50%,

Safely doubling safe limits is an oxymoron. You can read all sorts of rubbish on the internet, it doesn't make it true.

I can only tell you what will work 100% of the time for 100% of the people. I know this because I have spent my whole lave you measured what you supposed good LEDs are actually taking?
There are two things gife designing reliable electronics. Any idiot can design unreliable electronics that works for maybe two or three years. Or that works and significantly shortens the life of parts.

You can of course choose to ignore this, I can not stop you from being a total idiot. What I can do is point out that you are an idiot and point out why you are an idiot and why you are suffering from self delusions. Hopefully I can stop sensible people from following your idiotic total lack of logic.

  1. You do not have enough equipment or expertise to know if you are damaging a component.
  2. You appear not to know about peak current and power dissipation and how that relates to average current and average power dissipation.
  3. You think that any activity that doesn't immediately kill you is a safe activity.

I havnt measured the current, but Im fairly confident that the arduino is delivering 5v, and I know that 5V is higher than the forward voltage, and technically should be doing damage. It should also be noted that LEDs wear, even when using them at the recommended specifications, they will eventually fail. so your statement about 100% of the people 100% of the time is 100% false.

  1. false.
  2. false.
  3. false.

What is true, is that you want to call me an idiot, and make a lot of false statements to support your claims that im an idiot. Shame on you mike, you should not behave that way.

I hope you get over your cold or whatever.

What is true, is that you want to call me an idiot,

I don't want to call you an idiot, it saddens me that when faced with the truth you can't see it.
That is my definition of an idiot.

So let's just look at one claim I made:-

  1. You do not have enough equipment or expertise to know if you are damaging a component.

You said this was false.
So you do have equipment and expertise to assess a damaged component!
If you truly have a decapsulating machine and a scanning electron microscope then please post the pictures of your driver output stage after 6 months of over driving an LED with out any form of current control.
If you do not have this equipment then what equipment do you have that can assess internal component damage?

This is starting to look like my threads.... :grin:

popcorn.gif

The thing is that I would love to know wich rules/relations/equations does this guy use to get the current average just with the amount of time that the led is on and off, in this part of the code:

From the datasheet for the mcu and the led.

Leds, especially high brightness leds, behave far more like resistors (at high current levels) than like diodes, in that their voltage drops go up significantly with the current through them - as a north-eastern-ish V-I curve.

That, coupled with diodes surprising resistance to pulsed current, makes them fairly resistant to current shocks.

Together with the mcu's output resistance, this "no resistor" approach works.

Engineering isn't about "you cannot do that", or "that's wrong". Engineering is about knowing why you cannot do it so you know when / where you can do it.

People holding a simplistic view just lack common sense to understand that.

Engineering isn't about "you cannot do that", or "that's wrong". Engineering is about knowing why you cannot do it so you know when / where you can do it.

People holding a simplistic view just lack common sense to understand that.

You know dhenry that is probably one of the few bits of wisdom from you that I can agree with. But even then such wisdom when responding to beginners to electronics is bound to do more harm the good. Too much cargo electronics going around these days as it is.

Lefty

Mike, I dont have that equipment, nor the expertise to use them, but i have used a microscope before to examine dies. You can probably just use a regular microscope for this scale, and The LED even has a clear lens to see right into it. I still wont be bothering to do that. Instead I built the thing, and have been running it as a test to see how long it will take to fail, and it has shown no signs of failure yet. Its been months with no sign of dimming, no smoke... it just keeps working.

Dhenry, thats some sound theorizing. I remember before when you mentioned batteries having an internal resistance, and shift registers possibly working as current limiting devices. Your theories have merit, but I have no idea how to test the theory.

its also possible that parts are under rated or materials or manufacturing have changed, and made improvements that never got re-factored in or tested.

If you have steady state current, open a LED wire connection up and put a meter in series, see what the current flow is.

If its multiplexed, put a shunt in series, and with a scope see the voltage is across the shunt, calculate the current.

I like where your heads at. When in doubt, measure it.

I dont have a scope, I do have a couple cheapo digital multimeters.
Not only is it multiplexed, but its charliplexed. 192 LEDs on 16 arduino pins.

I have a spare arduino, is there a way I could make a scope out of an arduino?

See if this works for you.
http://www.negtronics.com/simplot

but I have no idea how to test the theory.

You don't need to: the datasheet lays it out clearly for you.

The typical diodes have a log-linear v-i curve, and the typical leds have a logn-linear v-i curve at low current levels and a linear curve at high current levels.

The resistor really is there to provide some negative current feedback to prevent a thermal runaway on the diode.

I take this as an implicit "I'm happy to field all future "my Arduino and/or LED array is knackered because I don't understand current limiting or datasheets because I'm an artist/hobbyist" type questions" from now on, dhenry.

Well volunteered.

You use a lot of words that dont make a lot of sense, are you intentionally being confusing?

It sounds like you thing that someone here hasnt read a datasheet or heard of a current limiting resistor being used with an LED, but im pretty sure everyone here knows about that stuff, so this just sounds like an insult with no merit, why even post it?

Thanks for the link, im going to read up on simplot today.

Hippynerd:
I dont have that equipment, nor the expertise to use them, but i have used a microscope before to examine dies. You can probably just use a regular microscope for this scale, and The LED even has a clear lens to see right into it. I still wont be bothering to do that. Instead I built the thing, and have been running it as a test to see how long it will take to fail, and it has shown no signs of failure yet.

This is not something you can do with an optical microscope. You need a scanning electron microscope to find out about the conductivity of the material. The effect of excessive current in an LED is that there is a depletion of charge carriers which reduces the light output over time. Waiting for a total failure with one sample is going to tell you absolutely nothing about your design. Most people do not understand about statistics and probabilities which is why the Casino owners are so rich and poor people continue to buy lottery tickets.

Its been months with no sign of dimming, ......

OK so lets see how you have tested that statement. I assume you have simply looked at it, I assume you have not taken any measurements of the light output. The eye's response is non linear, it is almost impossible to tell the difference in brightness of 25% over a month. That is why we have instruments to measure things. So I am not impressed with your test. If however you have conducted a real test I would be most interested.

Now dear henry:-

Engineering isn't about "you cannot do that", or "that's wrong". Engineering is about knowing why you cannot do it so you know when / where you can do it.

You may be surprised to here that I absolutely agree with that statement. However there are several points that you keep forgetting.
The biggest error you make is this forum is for beginners, they need to keep things as simple as possible because they know little and they are easily confused with esoteric arguments especially when you leave out all the caveats that your arguments normally involve to make them work.
Lets look at one of your off trotted out remarks:-

Leds, especially high brightness leds, behave far more like resistors (at high current levels) than like diodes,

First off here we are not talking about high brightness LEDs, and second any curve if you look at a small enough section of it looks like a straight line. It is not a matter of common sense it is a matter of knowledge of electronics, you seem to have a bit of the latter but non what so ever of the former.

the resistor really is there to provide some negative current feedback

Nearly right. In fact the current produces a negative voltage feedback to in effect reduce the applied voltage.

the datasheet lays it out clearly for you.

Indeed it does, things like internal impedance of the driver will limit the current to a non infinite value, but it is odd that you are willing to believe the data sheet over some things and totally reject the information over others.

Engineering is about being in control of what ever it is you are engineering, be it a jumbo jet, the world's highest building or a humble LED. It is about making decisions and trade offs.
With today's planned obsolescence culture then you indeed might want to design a circuit that only has a life slightly longer than the warranty you give it. You might however want to design a circuit that has a life significantly longer. In those cases you would not even drive the component at the rating given in the data sheet you would derate the device by 80% or even more.
If you want UL approval on anything then the capacitors in the power supply have to run at less that 80% of their voltage ratings even though you might make the "engineering decision" to run it at the maximum operating voltage.

Engineering is about making those informed decisions. So yes by all means run an LED with no current limiting resistor after all they do cost $0.001 each. The LED and driver will be in land fill sooner but hey the economy ticks over faster because people have to buy more stuff. But people have to know this stuff. Like simple Hippynerd, he has absolutely no idea what he is doing and is happy if his one sample hasn't stopped working after a year, but you can't say he is making an engineering decision, he is being fooled into a simplistic way of thinking. Someone has done a good job on him too as he clings to his misguided beliefs.

So dear henry, while you like to think you are much cleverer than most people here confusion and obfuscation are no substitute for real engineering.

This link tells about the fundamentals of reliability:-
http://www.ece.cmu.edu/~koopman/des_s99/electronic_electrical/

This tells a little about how LEDs fail:-

but im pretty sure everyone here knows about that stuff,

I wish.