Min or Typ?

[EngIntoHW]

All the values I've suggested will - as I said it's EXTREMELY non-critical.

Frist thing you need to think about is how bright you want it - this is obviously related to the current, but you've no way of knowing how bright a particular current will make it. So chuck a resistor in and see how bright it is, if you then want it brighter then reduce the value, if it's too bright, then increase the value. Calculating doesn't really come in to it, as even if you knew how bright a specific current would be, I suspect the brightness value would be totally meaningless to you (it certainly would to me).

Thank you again for the support

As each LED is different than other (even when they own the same MFG #), it won't help much to test a R(LED)'s value on a certain LED.

You said here, always take MIN values.
Here, what value of VF would you take to calculate R(LED):
Min, Typ or Max? (say that Min was mentioned as well).

 

[Nigel Goodwin]

Thank you again for the support

As each LED is different than other (even when they own the same MFG #), it won't help much to test a R(LED)'s value on a certain LED.

You said here, always take MIN values.
Here, what value of VF would you take to calculate R(LED):
Min, Typ or Max? (say that Min was mentioned as well).

As I mentioned, for an LED it doesn't matter it's so non-critical.

If you specifically wanted to try and pass an exact current (although it would be pretty pointless), use the minimum value, as that would err on the safe side.

 

[EngIntoHW]

As I mentioned, for an LED it doesn't matter it's so non-critical.

You mean that there's weak relation between IF and Brightness?

 

[Nigel Goodwin]

You mean that there's weak relation between IF and Brightness?

No, but there's a very weak relationship between current and perceived brightness - as you've no way to quantify how bright you want it, or what current that might take, why bother about the exact current?.

 

[EngIntoHW]

Well, I want it to be as brighter as possible.

Low IF necessarily leads to lower brightness.

So taking MIN value of VF will necessarily lead to lower brightness than if we picked TYP value.

 

[Nigel Goodwin]

Well, I want it to be as brighter as possible.

Low IF necessarily leads to lower brightness.

So taking MIN value of VF will necessarily lead to lower brightness than if we picked TYP value.

Why do you want it as bright as possible? - what is the application?.

Taking the min value will mean it's lower brightness than taking the typ value - but taking the typ value will lead to a much higher failure rate. Assuming you're running the LED at the maximum current to get maximum brightness, any LED less than typ will be run past it's specifications.

 

[EngIntoHW]

Hey again,
Thanks for keeping this thread going and answering my questions
I appreciate it.

Why do you want it as bright as possible? - what is the application?.

The LED will be inside a plastic enclosure of a product.
A Light Tunnel will allow the user to see the LED's color.
The LED should indiciate on different events, such as a button-pressing (and other events).

but taking the typ value will lead to a much higher failure rate. Assuming you're running the LED at the maximum current to get maximum brightness, any LED less than typ will be run past it's specifications.

Say that:
1. VF(TYP) = 3.3V @ 20mA
2. VF(MIN) = 3V @ 20mA
3. IF(MAX_RATING) = 25mA

So R(LED)(TYP) = (5V - 3.3V) / 20mA = 85Ω.
Hence, for a MIN VF value, we get: I(LED) = (5V - 3V) / 85Ω = 23.5mA < IF(MAX RATING)

So where's the failure?

 

[Nigel Goodwin]

Hey again,
Thanks for keeping this thread going and answering my questions
I appreciate it.


The LED will be inside a plastic enclosure of a product.
A Light Tunnel will allow the user to see the LED's color.
The LED should indiciate on different events, such as a button-pressing (and other events).


Say that VF(TYP) = 3.3V @ 20mA and VF(MIN) = 3V @ 30mA
and that IF(MAX_RATING) = 25mA

So R(LED)(TYP) = (5V - 3.3V) / 20mA = 85Ω.
Hence, for a MIN VF value, we get: I(LED) = (5V - 3V) / 85Ω = 23.5mA < IF(MAX RATING)

So where's the failure?

You said you wanted it as bright as possible - then you run it at 20mA, which is a LONG way from been as bright as possible.

You really need to bung an LED in place, and try different resistors to see how bright you want it to be - generally too bright an LED is really annoying. But from your description, it doesn't need to be particularly bright - you will be amazed how bright LED's are even at quite low currents, try it and see.

 

[EngIntoHW]

You said you wanted it as bright as possible - then you run it at 20mA, which is a LONG way from been as bright as possible.

You're probably correct.
How would you suggest to run it as bright as possible?

 

[Nigel Goodwin]

You're probably correct.
How would you suggest to run it as bright as possible?

Read the maximum permitted current off the datasheet, and set it to that current - use the minimum voltage drop to calculate the resistor. But I'm still dubious you want it that bright - what you've mentioned is a bit vague, but doesn't suggest it needs to be exceptionally bright.

 

[EngIntoHW]

Read the maximum permitted current off the datasheet, and set it to that current - use the minimum voltage drop to calculate the resistor

Got you.
Would it significantly decrease the lifetime of the LED?
The LED will usually be ON for 100ms and OFF for 100ms.
The max time it'd be continuously turned on is 3sec - but it'd happen rarely.

But I'm still dubious you want it that bright - what you've mentioned is a bit vague, but doesn't suggest it needs to be exceptionally bright.

I use SMD LEDs, as through LED woud take too much space which I can't afford.
It's very important that the user will be able to clearly see the LED's color when it's turned ON.

 

[Nigel Goodwin]

Got you.
Would it significantly decrease the lifetime of the LED?

Exceeding it's specs will shorten it's life, you see failed LED's everywhere.

The LED will usually be ON for 100ms and OFF for 100ms.
The max time it'd be continuously turned on is 3sec - but it'd happen rarely.


I use SMD LEDs, as through LED woud take too much space which I can't afford.
It's very important that the user will be able to clearly see the LED's color when it's turned ON.

Then test it and see! - it's hardly difficult - or are you planning sending it off to be manufactured without ever trying it first?.

 

[EngIntoHW]

Hey
Thank you very much again.

Exceeding it's specs will shorten it's life, you see failed LED's everywhere.

If you stay below the maximum ratings, will it shorten it's life?

Then test it and see! - it's hardly difficult - or are you planning sending it off to be manufactured without ever trying it first?.

You're right,
I hope to do these tests soon.
Thank you.

 

[Nigel Goodwin]

Hey
Thank you very much again.

If you stay below the maximum ratings, will it shorten it's life?

I would imagine the lower the current the longer it's life is likely to be, but if you stick below the manufacturers maximum recommended current you should meet (or exceed) their predicted life. LED's tend to fail where they are driven hard, close to or exceeding the recommendations. You see loads of rear lights in cars where some of the LED's have failed, I rebuilt a Mercedes third brake light last year - I've still got most of the 50 SM LED's I bought to do the job (they came in a pack of fifty - at about 1/50th the cost of a replacement light).

 

[BrownOut]

The actual reason for these distinctions is the variability in the manufacturing process. It is a matter of the statistical variation of die across a wafer in a lot. The manufacturer bases the minimum, typical and maximum on specific target criteria adjusting the process to meet the criteria for maximum die yield. A number of manufactures will place the three sigma curves on their datasheets for more complex IC's.

Thanks you for your comment, Mr Cecil. I always enjoy getting your perspective. Could you elaborate on which value is the most useful? In my experience, the designs I've worked on are usually too complicated to hand analyze, and so we use software tools to tell us if they will work within parameters or not. The software give us the options to analyze the designs using "pessimistic" or "typical" estimates of device performance. I've had success designing systems using the typical parameter, and have never needed the pessimistic analysis. But I've not completed that many designs ( I"m usually a contributor, not the design lead )

I imagine the decision depends factors other than miminim and typical performance. There are, for instance, the cost of rework, the availiblity of high performing devices, etc. I can tell you this much, when I was in charge of grading parts ( for the device manufacturer ), the devices that made the grade did so in the MINIMUM criteria, so take that for what's it's worth. It might be best to buy parts the meet the design goals with their minimum specs.

 

[EngIntoHW]

I would imagine the lower the current the longer it's life is likely to be, but if you stick below the manufacturers maximum recommended current you should meet (or exceed) their predicted life. LED's tend to fail where they are driven hard, close to or exceeding the recommendations. You see loads of rear lights in cars where some of the LED's have failed, I rebuilt a Mercedes third brake light last year - I've still got most of the 50 SM LED's I bought to do the job (they came in a pack of fifty - at about 1/50th the cost of a replacement light).

Thank you Nigel
I learned a lot from you in this thread.

I very appreciate your help.

 

[MRCecil]

Could you elaborate on which value is the most useful?

As far as older discretes and IC's I will ALWAYS use the typical parameter in the datasheet. The reason is that these devices were developed 20-40 years ago before the time of even the state-of-the-art, now outmoded, class 10 clean rooms of the late 1980's, fully uncontaminated gases, pure and flat silicon wafers, consistent resist, accurate process tools with consistent repeatability, adequate process control, etc, etc. Those same devices being manufactured today, whether by the OEM or under license, are up to the parameters as stated within a very narrow statistical curve. 40 years ago 5-!0% die yields were the norm. Today 90% and above is the norm. When I retired from Intel in 2004, 96% die yield was the minimum acceptable for all devices and if it slipped, everyone was thrown into emergency mode to find the issue.

As far as the newer devices of the last 20 years, I still use the typical values, predominately, because the entire industry has had to improve to simply compete and stay viable. Imagine a manufacturer stuffing a board with robotic machines using cartridge, tape and roll component supplied parts from their suppliers. They cannot afford to test individual components, the suppliers cannot afford to test those components one by one, nor can the manufacturers. Process control is the key from start to finish.

The few exceptions are if I require matched components or those needing specific parameters/ratios met for my OWN design. In that case I will buy an adequate supply based on guess and test/cull.

In my experience, the designs I've worked on are usually too complicated to hand analyze, and so we use software tools to tell us if they will work within parameters or not. The software give us the options to analyze the designs using "pessimistic" or "typical" estimates of device performance. I've had success designing systems using the typical parameter, and have never needed the pessimistic analysis. But I've not completed that many designs ( I"m usually a contributor, not the design lead )

There was a term that surfaced in the late '40's/early '50's; "The Tyranny of Wires". I fear a new term will emerge in the future; the tyranny of complexity. If a designer or design team cannot get their head around their own creation, the only solution would be a machine intelligence that could think faster and peer at the minutiae with more depth. Some day we might just be cleaver enough to replace humankind with all sorts of AI's to do all work, manual and creative. GAD!

I imagine the decision depends factors other than miminim and typical performance. There are, for instance, the cost of rework, the availiblity of high performing devices, etc. I can tell you this much, when I was in charge of grading parts ( for the device manufacturer ), the devices that made the grade did so in the MINIMUM criteria, so take that for what's it's worth. It might be best to buy parts the meet the design goals with their minimum specs.

I think at the root are the design rule criteria (DRC's) one person, one team or one entity establishes at the outset. The chosen DRC's may or may not vary from time to time, project to project or even down to circuit to circuit. Tradeoffs are inevitable in just about every endeavor.

Well, I've rambled enough, methinks, and it's time for some sleep. I don't know if I've contributed anything of use here, but I think the main point is to remain flexible enough to produce something of value, which is not in the disposable category.

Cheers,
Merv