Arduino Forum

Using Arduino => LEDs and Multiplexing => Topic started by: myownway on Jan 07, 2013, 02:41 pm

Title: LEDs without the use of current limiting resistors
Post by: myownway on Jan 07, 2013, 02:41 pm
Hi, this is my first post and I'm sad to introduce myself with a question. But I'd really appreciate an answer.

Well, I'm in a project and I was searching some info on the internet when I found this code: http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/LED/_7Seg_Example.pde

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:

Code: [Select]
//Display brightness
//Each digit is on for a certain amount of microseconds
//Then it is off until we have reached a total of 20ms for the function call
//Let's assume each digit is on for 1000us
//If each digit is on for 1ms, there are 4 digits, so the display is off for 16ms.
//That's a ratio of 1ms to 16ms or 6.25% on time (PWM).
//Let's define a variable called brightness that varies from:
//5000 blindingly bright (15.7mA current draw per digit)
//2000 shockingly bright (11.4mA current draw per digit)
//1000 pretty bright (5.9mA)
//500 normal (3mA)
//200 dim but readable (1.4mA)
//50 dim but readable (0.56mA)
//5 dim but readable (0.31mA)
//1 dim but readable in dark (0.28mA)


I wonder that this may be related to joule/second = w, but I'm not sure.

Thanks in advance.
Title: Re: LEDs without the use of current limiting resistors
Post by: Grumpy_Mike on Jan 07, 2013, 02:49 pm
The rules he is using is pig ignorance coupled with a lot of stupidity.
This is crap.

See this:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html (http://www.thebox.myzen.co.uk/Tutorial/LEDs.html)
Title: Re: LEDs without the use of current limiting resistors
Post by: John_Smith on Jan 07, 2013, 03:52 pm
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.
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 07, 2013, 05:17 pm
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).
https://www.youtube.com/watch?v=1FElbyUbTHM
Title: Re: LEDs without the use of current limiting resistors
Post by: John_Smith on Jan 07, 2013, 06:06 pm
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.
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 07, 2013, 06:47 pm

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.
Title: Re: LEDs without the use of current limiting resistors
Post by: Grumpy_Mike on Jan 07, 2013, 06:59 pm
Quote
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.

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

So how do you know you are not damaging things?

Quote
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.
Title: Re: LEDs without the use of current limiting resistors
Post by: retrolefty on Jan 07, 2013, 07:12 pm
Quote
3) 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
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 07, 2013, 07:15 pm

Quote
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.

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

So how do you know you are not damaging things?

Quote
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.
Title: Re: LEDs without the use of current limiting resistors
Post by: Grumpy_Mike on Jan 07, 2013, 07:37 pm
Quote
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:-
Quote
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?
Title: Re: LEDs without the use of current limiting resistors
Post by: Hexadec on Jan 07, 2013, 08:34 pm
This is starting to look like my threads.... :smiley-mr-green:


Title: Re: LEDs without the use of current limiting resistors
Post by: dhenry on Jan 07, 2013, 11:47 pm
Quote
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.
Title: Re: LEDs without the use of current limiting resistors
Post by: retrolefty on Jan 08, 2013, 02:27 am
Quote
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
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 08, 2013, 05:36 am
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.
Title: Re: LEDs without the use of current limiting resistors
Post by: CrossRoads on Jan 08, 2013, 05:42 am
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.
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 08, 2013, 06:55 am
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?
Title: Re: LEDs without the use of current limiting resistors
Post by: CrossRoads on Jan 08, 2013, 07:21 am
See if this works for you.
http://www.negtronics.com/simplot
Title: Re: LEDs without the use of current limiting resistors
Post by: dhenry on Jan 08, 2013, 01:13 pm
Quote
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.
Title: Re: LEDs without the use of current limiting resistors
Post by: AWOL on Jan 08, 2013, 02:03 pm
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.
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 08, 2013, 05:16 pm
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.
Title: Re: LEDs without the use of current limiting resistors
Post by: Grumpy_Mike on Jan 08, 2013, 05:44 pm

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.

Quote
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:-
Quote
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:-
 
Quote
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.
Quote
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.
Quote
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/ (http://www.ece.cmu.edu/~koopman/des_s99/electronic_electrical/)

This tells a little about how LEDs fail:-
http://en.wikipedia.org/wiki/List_of_LED_failure_modes (http://en.wikipedia.org/wiki/List_of_LED_failure_modes)
Title: Re: LEDs without the use of current limiting resistors
Post by: AWOL on Jan 08, 2013, 06:18 pm
Quote
but im pretty sure everyone here knows about that stuff,

I wish.
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 08, 2013, 06:19 pm
Oh poor misguided mike, he just doesnt understand the real world, and clings to his tired old beliefs, insulting people that disagree with him :(  (see how lame and unproductive those kinds statements like that are?)

I guess you forgot(or you chose to ignore that...) that I had mentioned that Im running 2 cubes 24/7 and I have a 3rd that I run occasionally, and that they have been running for months (since the beginning of november I think.) If you dont like the 2 as a small sample size, consider that each cube has 192 LEDs maybe that size is big enough for you?

This cube isnt really my design, I found it on the internet while struggling to get my other cubes working (which by the way, are still unfinished, and not working, but yet these cubes are running strong, so one advantage to this design is that its working now, not some mysterious point in the future. This design has some advantages over other designs, its easier to build, has less parts which means less places for mistakes. Those may be unimportant to you, but to someone that wants to program an LED cube, it is important.

It may turn out that it takes 5 years before this cube fails, a cube with resistors may only last 1 year more, or it may last 10 years more, but one thing we know, they will all fail eventually, resistors will likely delay it, but can not prevent it.

Regarding dimming, clearly there is no way I will have expensive equipment to try to view any damage, the closest I can get is optical magnification, which may not be good enough, but what I do have is a fat sack of LEDs and I can compare the brightness of a new LED to one that has been running for months.

Where do you get resistors for less than one cent each? I get them cheap on the internet, and they are 2-5 cents each, in packs of 100 or more. LEDs arent very expensive either. My whole cube and arduino costs about $25 each, its a fairly inexpensive test.

It may turn out that I can extend the life of the cube with a few resistors, but as it is, its looking pretty good without them.

The reason casino owners and lotteries make money is because people are greedy, not uninformed.
Title: Re: LEDs without the use of current limiting resistors
Post by: AWOL on Jan 08, 2013, 06:53 pm
Quote
The reason casino owners and lotteries make money is because people are greedy, not uninformed.
I disagree - greedy AND misinformed.
Title: Re: LEDs without the use of current limiting resistors
Post by: Grumpy_Mike on Jan 08, 2013, 07:03 pm
Hippynerd - I am truly sorry for knowing stuff that you don't know. I will try and forget it all so I will be as good as you. Please forgive me. I will now try and get drunk in an attempt to kill of more brain cells in order to stop me being old, but I might be some time.
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 08, 2013, 07:33 pm
Ha ha, good luck with that. I never asked for an apology, but its nice that you offer one. I forgive you, but please do try to make valid points without resorting to insults.
Title: Re: LEDs without the use of current limiting resistors
Post by: retrolefty on Jan 08, 2013, 07:37 pm
Quote
The reason casino owners and lotteries make money is because people are greedy, not uninformed.


Lets see.
You can't win if you don't play.
You lose if you play.

I guess there is no way out of this delima.  :D
Title: Re: LEDs without the use of current limiting resistors
Post by: cmagagna on Jan 08, 2013, 08:03 pm

Where do you get resistors for less than one cent each? I get them cheap on the internet, and they are 2-5 cents each, in packs of 100 or more.


Pretty much anywhere. Here's some 10K resistors from DigiKey for 0.2 cents each at qty 100:

http://www.digikey.com/product-detail/en/1622826-4/A106045CT-ND/3477682

As an aside, this is easily my favorite thread of the new year.
Title: Re: LEDs without the use of current limiting resistors
Post by: Chagrin on Jan 08, 2013, 11:36 pm

As an aside, this is easily my favorite thread of the new year.


Not one mention of Sparkfun's place in all of this yet (OP's example (http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/LED/_7Seg_Example.pde) was written by a Sparkfun employee).

Sparkfun did sell a no-resistor-multiplexed-led display, their 7 segment serial display (https://www.sparkfun.com/products/9766). They updated (https://www.sparkfun.com/products/11441) it. Spot the difference ;)
Title: Re: LEDs without the use of current limiting resistors
Post by: dhenry on Jan 09, 2013, 12:03 am
Quote
Oh poor misguided


Your goal is noble.

However, at some point, you have to recognize an individual's right to remained challenged.

It makes you happier, and it makes the people who refused your help happier too.

Win-win.
Title: Re: LEDs without the use of current limiting resistors
Post by: dhenry on Jan 09, 2013, 12:04 am
Quote
Sparkfun did sell a no-resistor-multiplexed-led display,


How "misinformed" they are, :)
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 09, 2013, 01:42 am


Where do you get resistors for less than one cent each? I get them cheap on the internet, and they are 2-5 cents each, in packs of 100 or more.


Pretty much anywhere. Here's some 10K resistors from DigiKey for 0.2 cents each at qty 100:

http://www.digikey.com/product-detail/en/1622826-4/A106045CT-ND/3477682

As an aside, this is easily my favorite thread of the new year.


Those are 1/16w 0402 smt parts, I actually have some of those, have you ever tried soldering them?

I should look over digikey and mouser more often, but I find them those sites a bit overwhelming.
Title: Re: LEDs without the use of current limiting resistors
Post by: CrossRoads on Jan 09, 2013, 02:44 am
How about regular 1/4W carbon resistors then? $0.01 each for 10-lots.
http://www.taydaelectronics.com/resistors/1-4w-carbon-film-resistors/10-x-resistor-220-ohm-1-4w-5-carbon-film-pkg-of-10.html

I don't like what Tayda did to their website - the menu shows up on the left, while anything selected now shows up underneath it with a big empty space next to the menu.
My screen is set to 1280 x 800 - anyone else seeing the same?

Looks to be IE9 thing - don't see the layout issue with Google Chrome.
IE9 didn't used to do to that, this is fairly recent.
Title: Re: LEDs without the use of current limiting resistors
Post by: Osgeld on Jan 09, 2013, 03:08 am
works fine for me

edit: firefox
Title: Re: LEDs without the use of current limiting resistors
Post by: retrolefty on Jan 09, 2013, 03:38 am

How about regular 1/4W carbon resistors then? $0.01 each for 10-lots.
http://www.taydaelectronics.com/resistors/1-4w-carbon-film-resistors/10-x-resistor-220-ohm-1-4w-5-carbon-film-pkg-of-10.html

I don't like what Tayda did to their website - the menu shows up on the left, while anything selected now shows up underneath it with a big empty space next to the menu.
My screen is set to 1280 x 800 - anyone else seeing the same?

Looks to be IE9 thing - don't see the layout issue with Google Chrome.
IE9 didn't used to do to that, this is fairly recent.


On IE 8 it works ok, with chrome when you mouse over a menu category on the left the submenu displayed is 95% hidden behind whatever picture is displayed in the middle of the screen, just that very edge of the submenu can be seem. I just sent them an email about it. I'm at 1280x960 display.

Lefty

Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 09, 2013, 04:33 am

How about regular 1/4W carbon resistors then? $0.01 each for 10-lots.
http://www.taydaelectronics.com/resistors/1-4w-carbon-film-resistors/10-x-resistor-220-ohm-1-4w-5-carbon-film-pkg-of-10.html

I don't like what Tayda did to their website - the menu shows up on the left, while anything selected now shows up underneath it with a big empty space next to the menu.
My screen is set to 1280 x 800 - anyone else seeing the same?

Looks to be IE9 thing - don't see the layout issue with Google Chrome.
IE9 didn't used to do to that, this is fairly recent.


I think thats where i got the ones am currently using. after shipping their costs go up 2-5 times depending on your patience.
Standard 7 - 16 Days $1.08
Economic Express 6 - 9 Days $4.08
DHL Express 2 - 4 Days $16.00

I can also get them from thaishine, for a few bucks with 4 day shipping. Its much better than the local prices (about a quarter each), but its no 10 for a penny.
Title: Re: LEDs without the use of current limiting resistors
Post by: CrossRoads on Jan 09, 2013, 04:41 am
I've only used the slow/cheap shipping. I stock up when I buy, $30-40 worth of stuff at a time, so shipping becomes a small part of the total price.
Title: Re: LEDs without the use of current limiting resistors
Post by: cmagagna on Jan 09, 2013, 06:21 am

I should look over digikey and mouser more often, but I find them those sites a bit overwhelming.


I agree they can be daunting at first but they're manageable. I much prefer DigiKey's interface; I find it the easiest to use. Many times if I get something from Mouser, Newark, etc. I'll find it on DigiKey then cut & paste the part # at the other sites.

I start by searching for as close to what I need as possible, e.g. "atmega328p-au" or "1/4 watt 10k resistor". This generally does a good job of getting close.

Next I check the "In Stock", "Lead Free", and "RoHS Compliant" boxes and search again; there's usually no point in searching if they don't have it in stock and the other two seem to help whittle things down further.

Next I start going through the search boxes like "packaging" or "height", etc. for things I know I want, e.g. a specific size or footprint and click "apply filters" each time.

When the result list is a reasonable size I sort by "unit price" and buy the cheapest one (unless I know I'll eventually need a lot of the part, in which case I give "quantity available" some weight).

Having said all that, I try to do as CrossRoads says and batch up purchases at one of the Chinese companies; this is usually the overall least expensive option.
Title: Re: LEDs without the use of current limiting resistors
Post by: Osgeld on Jan 09, 2013, 06:36 am
mousers interface is down right frustrating to me, but its better than it was?

digikey is overwhelming until you get it, then once you do you can take an idea like "i need a shift register" and filter it down to the cheapest one in the quantity range you want, while meeting your requirements with two - three filter passes, its almost an art
Title: Re: LEDs without the use of current limiting resistors
Post by: CrossRoads on Jan 09, 2013, 06:40 am
I don't buy Chinese or e-bay generally (WS2803 is one exception), except for iteadstudio.
dipmicro.com in Canada/Western NY, taydaelectronics in Thailand
and Digikey, Mouser, Avnet, Pololu.
Sometimes Newark. They have eagle symbols for some parts too.
Phoenixent.com for strip sockets & headers & wirewrap stuff.

I use these sources for their repeatability.

Digikey/Mouser both irritating digging down trying to find things like header strips.  I buy that kind of mechanical stuff from pololu and phoenixent instead.
For resistors, caps, transistors, diodes, surface mount, digikey is the best.
Mouser has best prices on Atmel parts.
Avnet for TPIC6x595 shift registers.
Title: Re: LEDs without the use of current limiting resistors
Post by: Osgeld on Jan 09, 2013, 06:59 am
I usually stick to digikey and mouser for small quantities

I got to give props to arrow and avnet, they are 2 of our suppliers at work, and hey, they buy me nice lunches hehe, and both have good support even if they dont know who you are

DK on the other hand sent me 250 .1uF 805 caps, which were suposta be 5Amp rectifier diodes in a SMC package (for those not familiar with surface mount packages its a grain of rice vs a jellybean, never mind its totally the wrong component), and I got a deadline on (next)Tuesday ... I had to get bitchy with them and reorder somewhere else cause a couple weeks later and people still cant find info on the mix up is not cool, so they are a bit on my list right now
Title: Re: LEDs without the use of current limiting resistors
Post by: dhenry on Jan 09, 2013, 01:14 pm
Quote
they buy me nice lunches hehe


Someone at work got a drawer full of atmel's USBKEY, as  free samples. That would have been worth some serious money during the height of ps3 jailbreak.
Title: Re: LEDs without the use of current limiting resistors
Post by: John_Smith on Jan 09, 2013, 03:10 pm
Just an analogy here,   my neighbour , when he decides to drive to the casino, never wears his seat belt.

Both of these choices are his ( although the seat belt is compulsory )  I am just trying to show his mentality here.

He has never been killed yet in 50 years of driving without a seat belt.  If asked about it he might well say " I heard of someone who had a crash and was thrown out of the burning car ,  and survived because he wasn't wearing a seat belt "  There are a few variations of this.

The point is,  if there was a forum for newcomers to learn about driving, should we be saying " don't worry about wearing a seat belt, I know someone who has got away with it for 50 years ! "  ?

Another point is, wouldnt at least one LED manufacturer jump on the bandwagon and say " you don't need a resistor with our LEDs " and sell more ?     Perhaps they know something, being manufacturers.

Title: Re: LEDs without the use of current limiting resistors
Post by: dhenry on Jan 09, 2013, 04:37 pm
Quote
wouldnt at least one LED manufacturer jump on the bandwagon and say " you don't need a resistor with our LEDs " and sell more ?     Perhaps they know something, being manufacturers.


They do, that's why you never hear an LEd manufacturer (reputable ones) who says "you have to use a resistor with LEDs".

Because both statements ("you have to use a resistor" and "you cannot use resistors") are wrong. Whether you should use a resistor or not is application specific and needs to be evaluated specifically.

Just common sense.
Title: Re: LEDs without the use of current limiting resistors
Post by: fungus on Jan 09, 2013, 04:53 pm

Quote
wouldnt at least one LED manufacturer jump on the bandwagon and say " you don't need a resistor with our LEDs " and sell more ?     Perhaps they know something, being manufacturers.


They do, that's why you never hear an LEd manufacturer (reputable ones) who says "you have to use a resistor with LEDs".

Because both statements ("you have to use a resistor" and "you cannot use resistors") are wrong. Whether you should use a resistor or not is application specific and needs to be evaluated specifically.

Just common sense.



Wrong.

Manufacturers supply things called "datasheets" with their parts. In the datasheet there's always a section titled "Absolute Maximum Rating". The values in there are determined by people who know stuff about their product and know what power is likely to damage it.

Title: Re: LEDs without the use of current limiting resistors
Post by: John_Smith on Jan 09, 2013, 05:42 pm
Quote
Because both statements ("you have to use a resistor" and "you cannot use resistors") are wrong. Whether you should use a resistor or not is application specific and needs to be evaluated specifically.

Just common sense.


exactly,  you dont need a resistor with a constant current driver for example, but something should limit the current to the manufacturers recommendations, common sense.
Title: Re: LEDs without the use of current limiting resistors
Post by: Headroom on Jan 09, 2013, 06:33 pm
Interesting thread and a very good example why accidental success does not result in true knowledge.

An LED needs a constant current supply, there is no way around that. Thats based on not too complicated and widely published physics. Perhaps that currnt limit can be achieved by some implicit resistance of the components involves, e.g the internal resistance of a battery etc.

However, the question is whether one would want to base a sound design on something that is more a byproduct of a statistically distributed manufacturing process thats only monitored within rlatively wide limits or a design goal kept within close tolerance. A battery for example is a supplier of electrical energy with the usual design goals  being a stable voltage over time and mAh etc. The internel resiststance is what you get with it and it may not even be stated on a data sheet.

A resistors design goal, however, is to have a specified resistance and can be bought at different tolerance ratings. Using components for their intended purpose is obviously more a sound approach. That does not mean that you can't get away with not using that approach occasionally. Using that approach, however for a reliable solution is dangerous.
Title: Re: LEDs without the use of current limiting resistors
Post by: retrolefty on Jan 09, 2013, 06:51 pm

Interesting thread and a very good example why accidental success does not result in true knowledge.

An LED needs a constant current supply, there is no way around that. Thats based on not too complicated and widely published physics. Perhaps that currnt limit can be achieved by some implicit resistance of the components involves, e.g the internal resistance of a battery etc.

However, the question is whether one would want to base a sound design on something that is more a byproduct of a statistically distributed manufacturing process thats only monitored within rlatively wide limits or a design goal kept within close tolerance. A battery for example is a supplier of electrical energy with the usual design goals  being a stable voltage over time and mAh etc. The internel resiststance is what you get with it and it may not even be stated on a data sheet.

A resistors design goal, however, is to have a specified resistance and can be bought at different tolerance ratings. Using components for their intended purpose is obviously more a sound approach. That does not mean that you can't get away with not using that approach occasionally. Using that approach, however for a reliable solution is dangerous.


Yes Sir, you state the case well, no further explanation required.

Lefty
Title: Re: LEDs without the use of current limiting resistors
Post by: John_Smith on Jan 09, 2013, 08:41 pm
Quote
That does not mean that you can't get away with not using that approach occasionally. Using that approach, however for a reliable solution is dangerous.


Right, and the whole point of Arduino is to simplfy things for beginners, no direct port manipulation for example, so lets stick to the basics on the hardware side when newbies are asking if a resistor is required  for an LED.
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 09, 2013, 10:14 pm

Quote
That does not mean that you can't get away with not using that approach occasionally. Using that approach, however for a reliable solution is dangerous.


Right, and the whole point of Arduino is to simplfy things for beginners, no direct port manipulation for example, so lets stick to the basics on the hardware side when newbies are asking if a resistor is required  for an LED.


Now I know what the ugly stepchild feels like.  Lets hide him in the closet when company comes to visit. sigh.

So, the next time someone specifically asks:
"LEDs without the use of current limiting resistors"
And gives a very specific example, we are supposed to tell them that its impossible to do that, and they should never think such dangerous thoughts?
Title: Re: LEDs without the use of current limiting resistors
Post by: retrolefty on Jan 09, 2013, 10:34 pm


Quote
That does not mean that you can't get away with not using that approach occasionally. Using that approach, however for a reliable solution is dangerous.


Right, and the whole point of Arduino is to simplfy things for beginners, no direct port manipulation for example, so lets stick to the basics on the hardware side when newbies are asking if a resistor is required  for an LED.


Now I know what the ugly stepchild feels like.  Lets hide him in the closet when company comes to visit. sigh.

So, the next time someone specifically asks:
"LEDs without the use of current limiting resistors"
And gives a very specific example, we are supposed to tell them that its impossible to do that, and they should never think such dangerous thoughts?


You sir are free to do whatever you wish to your arduino on your projects. However if you post information to people asking for help, some people will probably continue to warn against poor advice that sometimes get posted around here.

So unless your looking for some kind of validation for your methods and advice I don't see where you have a valid complaint to share with us.

Lefty
Title: Re: LEDs without the use of current limiting resistors
Post by: dhenry on Jan 09, 2013, 10:51 pm
Quote
lets stick to the basics


Essentially you are saying that some members are too stupid to understand the truth so we should tell them falsehood for their own good.
Title: Re: LEDs without the use of current limiting resistors
Post by: Grumpy_Mike on Jan 09, 2013, 10:54 pm

Quote
lets stick to the basics


Essentially you are saying that some members are too stupid to understand the truth so we should tell them falsehood for their own good.


No we are not saying that and well you know it.
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 09, 2013, 10:57 pm



Quote
That does not mean that you can't get away with not using that approach occasionally. Using that approach, however for a reliable solution is dangerous.


Right, and the whole point of Arduino is to simplfy things for beginners, no direct port manipulation for example, so lets stick to the basics on the hardware side when newbies are asking if a resistor is required  for an LED.


Now I know what the ugly stepchild feels like.  Lets hide him in the closet when company comes to visit. sigh.

So, the next time someone specifically asks:
"LEDs without the use of current limiting resistors"
And gives a very specific example, we are supposed to tell them that its impossible to do that, and they should never think such dangerous thoughts?


You sir are free to do whatever you wish to your arduino on your projects. However if you post information to people asking for help, some people will probably continue to warn against poor advice that sometimes get posted around here.

So unless your looking for some kind of validation for your methods and advice I don't see where you have a valid complaint to share with us.

Lefty

Im not looking for validation here buddy. Answer the question, what are you suppose to tell someone that asks a valid question, and provides a documented example.

I think the best answer to the question is, yes, that is possible under certain circumstances.
It may be that they need the display to fit in a tiny space, but it only needs to work for 3 days and never used again.

Also, I cant take credit for the design, I just found it on the interwebs, and built one. I can only take credit for experimenting with it, talking to people about it.

Sincerly, Danger McGee.
Title: Re: LEDs without the use of current limiting resistors
Post by: Grumpy_Mike on Jan 09, 2013, 11:08 pm
Quote
Answer the question, what are you suppose to tell someone that asks a valid question, and provides a documented example

You tell them the truth. That it is a crap design made by someone who didn't understand what they were doing or understand the damage it is causing.

You point them at your own web site where you did experiments and prove you are drawing current that is over five times the level that the data sheet says damages the arduino.

In the Internet age on the web documents are not worth the paper they are written on. Which you might say includes my site. So do your own experiment and see for yourself.
If you disagree with the results then publish your findings.
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 10, 2013, 12:53 am
showing some tests is one way, but calling it a crap design by someone that doesnt understand what they are doing is totally subjective, thats just your feelings, not the truth.

The truth seems to be that you may reduce the lifespan of some or all of the parts. If you dont protect your LEDs with resistors or some form of current limiting device, they will probably fail sooner than if you had.

Findings posted. testing ongoing.

I dont absolutely disagree with you, i disagree that things are as absolute as you say.
Title: Re: LEDs without the use of current limiting resistors
Post by: smeezekitty on Jan 10, 2013, 01:35 am

showing some tests is one way, but calling it a crap design by someone that doesnt understand what they are doing is totally subjective, thats just your feelings, not the truth.

The truth seems to be that you may reduce the lifespan of some or all of the parts. If you dont protect your LEDs with resistors or some form of current limiting device, they will probably fail sooner than if you had.

Why willingly reduce the lifespan of components if it could easily and cheaply avoided?
It makes absolutely no sense to me. Datasheets do not lie. There is reason they are only rated at 20-40ma.
Title: Re: LEDs without the use of current limiting resistors
Post by: CrossRoads on Jan 10, 2013, 01:44 am
Avoiding failure also means avoiding messy rework. Especially LEDs in a cube, with leads formed around other leads, or perhaps inaccessible.
Title: Re: LEDs without the use of current limiting resistors
Post by: Grumpy_Mike on Jan 10, 2013, 09:38 am
Quote
but calling it a crap design by someone that doesnt understand what they are doing is totally subjective, thats just your feelings, not the truth.

The original post, oh what a long time ago it was, asked what assumptions the original designer had made. He asked if it was some consideration over the total thermal energy or something. When you looked at it there was no such consideration, in fact all that had happened was he had looked at the piceved brightness of the LEDs for differing pulse widths. I consider that a crap way to design anything. It shows no understanding of what is actually going on. It is common to see such crap designs at places like instructables. It is a result of either ignorance or a missunderstanding about peak verses continuous current.
Now you might want to replace the word crap with some more politically correct phrase like differently correct but that is up to you. I feel there is little subjectivity with the words I used.
Title: Re: LEDs without the use of current limiting resistors
Post by: John_Smith on Jan 10, 2013, 10:17 am
Hey Hippynerd,

I like your your profile   "  Damnit Jim, Im a technician, not a engineer!  "





Title: Re: LEDs without the use of current limiting resistors
Post by: fungus on Jan 10, 2013, 11:01 am

... differently correct ...


I like that phrase. I'm going to start using it.
Title: Re: LEDs without the use of current limiting resistors
Post by: fungus on Jan 10, 2013, 12:08 pm

And gives a very specific example, we are supposed to tell them that its impossible to do that, and they should never think such dangerous thoughts?


No, we tell him to read the datasheet.

Except he won't have it, therefore we err on the side of not killing his components.

Yes, some LEDs will take more current when pulsed, but that doesn't mean all of them will. We know that some of them definitely won't.

Handing out general "advice" to newbies based on something you managed to do by pure dumb luck is bad. If you can't see that then you're an idiot (so don't be surprised when people call you one in forums).

Title: Re: LEDs without the use of current limiting resistors
Post by: myownway on Jan 10, 2013, 03:30 pm
So, is this a valid way to drive a display of 4 digits, 7 segments common anode?

a 74HC595 conected to the A...G with resistors on every segment to limit the current and a direct pinout from arduino to every digit 1...4?

or should I put a transistor on every digit? I think it is an obvious question but I'm not sure if the output current of  every pin on arduino is limited to 40 mA or it is just a limit that it shouldn't reach though security.

PS: In the case of using transistors to drive the digits, should I use resistors too at the gates?

Thanks for all the answers.
Title: Re: LEDs without the use of current limiting resistors
Post by: Hippynerd on Jan 10, 2013, 05:50 pm


showing some tests is one way, but calling it a crap design by someone that doesnt understand what they are doing is totally subjective, thats just your feelings, not the truth.

The truth seems to be that you may reduce the lifespan of some or all of the parts. If you dont protect your LEDs with resistors or some form of current limiting device, they will probably fail sooner than if you had.

Why willingly reduce the lifespan of components if it could easily and cheaply avoided?
It makes absolutely no sense to me. Datasheets do not lie. There is reason they are only rated at 20-40ma.

it doesnt have make sense to you, thats ok. If someone asked you to build 100 fixtures, that only need to work for 3 days, then never again, and the money for the parts was coming out of you pocket would it make sense?
It isnt really up to you or me, its up to each individual what they want to do, and under some curcumstances that you may never have considered that it could be useful. Like when its not cheaply or easily avoided.

Myownway:
You could use a transistor on common anodes, and resistors on the cathodes.

Fungus, how is building something that someone else designed and built dumb luck. I have repeated the test 3 time, and had the exact same results, its not dumb luck that these cubes havnt failed.

You have convinced me only that you are angry, and want to call me names, that is not a reflection of me, that is your business. I wont resort to calling you names, because that isnt helpful, its counter productive, and just mean.

My being able to do something that you can not do, does not make me an idiot, it just makes you mean. If you want to get on your high horse, and talk about whats bad, then lets talk about calling people names, and justifying other people calling names, is that good or bad fungus?

Mike, I dont remember anything about perceived brightness, it looked to me he was doing a duty cycle calculation. The code in the original post lists mentions a variable called brightness, but I dont see anything about perceived brightness. Myownway does ask "I wonder that this may be related to joule/second = w, but I'm not sure."

I do like the expression differently correct, but I dont see that as appropriate either. "It will work, but it will likely fail sooner than if you use resistors or other current limiting device." states all the important information, without injecting feelings.

Crossroads:
Yes, the rework can be quite difficult in a cube, i've had to replace a couple LEDs in my first cube, and it was pretty difficult, even on a small single color cube. The I have repaired a few spires from this charlieplexed cube, and its actually a lot easier than a single color cube. I think its the lack of gridded wires that makes it easier to get into the LEDs on the spires. Have you looked at any of the pictures that took? Its is way easier to repair a spire before you solder it onto a board, but if one LED is messed up, its not too hard to replace.

boffin: Im glad you like it, you can use it if you like, i dont own any rights to anything, and I really dont care either way.
Title: Re: LEDs without the use of current limiting resistors
Post by: fungus on Jan 10, 2013, 06:23 pm

Fungus, how is building something that someone else designed and built dumb luck. I have repeated the test 3 time, and had the exact same results, its not dumb luck that these cubes havnt failed.


Did you use a different source of components each time...? I've got datasheets for LEDs that say they can take 200mA pulses, I've got datasheets for LEDs that say they can only take 35mA pulses. Dumb luck is dumb luck until you try it with the 35mA LEDs.



...that is your business


The only problem is that your taking your business and telling noobs that that's the way to do it. You're not paying for their components, don't give them advice that might destroy them.

Title: Re: LEDs without the use of current limiting resistors
Post by: fungus on Jan 10, 2013, 06:43 pm

It isnt really up to you or me, its up to each individual what they want to do, and under some curcumstances that you may never have considered that it could be useful. Like when its not cheaply or easily avoided.


Well, OK, so long as you add a disclaimer to all your posts:

"Warning, this exceeds the LEDs specifications and might destroy your LEDs".

Except you don't, do you? Noooo...you brag about how you do it all the time and it's perfectly fine to do so.
Title: Re: LEDs without the use of current limiting resistors
Post by: AWOL on Jan 10, 2013, 07:01 pm
OK, enough.
Thread locked
Title: Re: LEDs without the use of current limiting resistors
Post by: CrossRoads on Jan 10, 2013, 09:00 pm
@myownway,
Where you put the resistors and if you use a transistor is all a matter of how you plan to multiplex.

Say you had common anode display:
If you were driving 1 segment on at a time, and multiplexing by cycyling thru all 28 segments, then you could get by with 1 resistor per common anode, with current sourced from an arduino pin and 1 cathode at a time pulled by an arduino pin.
If your multiplexing is going to be by digit, then you would need a PNP transistor sourcing current to the anode, and a resister per segment with the cathode all pulled low at the same time by ardiuno pins.  The PNP would control the 20mA x 7 segments when an 8 is displayed at full brightness.
(check your datasheet, 20mA may be too much for your display).

In place of the arduino pins, you could use a TPIC6B595 shift register with open drain outputs. You would shift in the font pattern you wanted and turn on the PNP transistor to turn a digit. Cycle thru the 4 digits that way.
The PNP needs a High to turn off and Low to turn on. You could use a 2nd TPIC6B595 to turn on the PNPs, with pullup resistors to turn them off.  Thus 3 IO pins could control the 4 anodes/8 cathodes (if using decimal point).
If you had a Large display, the kind with 3-4 LEDs/segment and that needs 12V, then the PNP base needs to be pulled to 12V to turn it off, and the TPIC6B595 provides the 5V to 12V buffer from Arduino pin to PNP base.

A P-channel MOSFET could be used also. A Logic Level part is needed for sourcing 5V, a standard level part can be used to source 10+V with TPIC6B595 controlling the gate.

If you didn't use the TPIC6B595, then NPN transistor between arduino and 12V transistor is another option for buffering 5V to 12V.