Which blue 3 mm LED to maximize battery life? (brightness is unimportant)

The blue 3 mm through-hole LED will be connected to a 3 V CR1025 coin cell, directly without resistor. It’s for decorative purposes. Brightness is not too important. What is important is maximizing battery life.

Any suggestions?

I already looked at datasheets, but I’m a bit lost. For example in the datasheet for the Kingbright L-1154QBW-D, forward current at 3 V is 10 mA. I just don’t know if that’s good or bad.

"Brightness is not too important. What is important is maximizing battery life.

  • Any suggestions?"*
    Add a current limit resistor. Reduce the current to 3mA, 5mA. Will still be plenty bright and battery life will be extended.
    Vf at 5mA is charted at 2.8V.
    (3V - 2.8V)/.005A = 40 ohm. 39 ohm is a standard value, try that.

CrossRoads:
Add a current limit resistor.

Not possible. Needs to be clamped directly onto the battery.

Then there is no suggestion to be made is there?

CrossRoads: Then there is no suggestion to be made is there?

Well, forward current at 3 V is not the same for all LEDs. That’s what I know. I just don’t know what are the best values to be expected, and in which LEDs to find them, hence my question.

feklee: Well, forward current at 3 V is not the same for all LEDs. That’s what I know. I just don’t know what are the best values to be expected, and in which LEDs to find them, hence my question.

If you say that, then LEDs don't work the way you think they do.

Either use a resistor as CrossRoads suggests, or there is no answer to your question.

Oracle: If you say that, then LEDs don't work the way you think they do.

Either use a resistor as CrossRoads suggests, or there is no answer to your question.

If there is no answer to my question then this implies that all blue LEDs drain the battery in the same way; really? That doesn’t match my experience. It’s not the first time that I hook up an LED directly to a battery, and using a resistor is not an option: Think LED throwie, although here it’s a different application.

If there is no answer to my question then this implies that all blue LEDs drain the battery in the same way; really?

Yes. The forward voltage of an LED and hence it's colour is dependent on the material it is made from.

It's not the first time that I hook up an LED directly to a battery,

You ought to be reported for component abuse.

although here it's a different application.

Which you are keeping a secrete in opposition to the open source sprite of this forum.

feklee: If there is no answer to my question then this implies that all blue LEDs drain the battery in the same way; really?

That's correct.

Well, forward current at 3 V is not the same for all LEDs.

Yes, you need to make sure you don't exceed the maximum forward current specified for the LED. You can certainly supply less if you want to save battery life and have a dimmer LED, but you can't exceed that amount. The LED does nothing to limit its own current.

That doesn’t match my experience. It’s not the first time that I hook up an LED directly to a battery

In your experience, you had no idea what you were doing and came to the wrong conclusion. If you connect any LED directly to a battery, you are effectively short circuiting the battery through the LED. It will draw the maximum current the battery can supply on a dead short and if this is greater than the maximum forward current of the LED, you will fry the LED.

You can only draw so much current from a battery because it does have an internal resistance which limits its current. These cheap little coin cells cannot supply enough current to instantly destroy the LED. But you're still abusing both components.

and using a resistor is not an option:

Then you are abusing the battery and cannot improve battery life. It takes about 15 seconds to replace an LED lead with an axial resistor if it's so important to clamp an LED to a battery.

Think LED throwie, although here it’s a different application.

It is damaging the LED and Battery, but the people who make throwies don't care because it only needs to last a short time. You said you care about battery life, so throwies are irrelevant to you.

Oracle: The LED does nothing to limit its own current.

Well, that turns out to be completely wrong. The LED actually has a very significant internal resistance.

Oracle: If you connect any LED directly to a battery, you are effectively short circuiting the battery through the LED. It will draw the maximum current the battery can supply on a dead short

And this also turns out to be completely wrong. There is an immense difference between the voltage drop of a LED and a short circuit. This combined with the internal resistance of the LED is used to effect in the popular $1 9-LED torches which have three "AAA" cells and no apparent current limiting at all. |500x500

Oracle: and if this is greater than the maximum forward current of the LED, you will fry the LED.

Which simply does not happen in these cases.

Paul__B: Well, that turns out to be completely wrong. The LED actually has a very significant internal resistance.

Really? Citation please?

Or are you being pedantic? The wire leads have a non-zero resistance. The real world LED semiconductor will of course not be "ideal". Is that what you are talking about?

At the Vf, the depletion region of the semiconductor, there is no internal resistance. Unless you're playing games with theoretical ideals vs real world fabrications like the component leads. The semiconductor becomes a conductor but not a superconductor.

And this also turns out to be completely wrong. There is an immense difference between the voltage drop of a LED and a short circuit.

Again, citation please. Unless you mean the voltage is dropped by the diode so the short circuit current through the non-super-conducting wires will be lower than it would be if the LED were replaced with wire.

So please, provide a source for LEDs having a "significant internal resistance" that makes anything I said "completely wrong".

And in any case, what are you trying to accomplish by your post? Even if my understanding of semiconductors is completely wrong, is your post helpful to anyone who wants to learn to connect and LED or is it just harmful and confusing. Is anything you said of help to the OP? Can he pick an LED from data sheets that is better to short across a battery?

These forums have become a very unpleasant place with people like you trying to show off how smart you are at the expense of people who come here genuinely trying to learn how to build things with their Arduino. Your tone is harsh and unpleasant, you are offering nothing of use to the OP. Your snippy little comments "completely wrong" are not useful at all. If I am wrong, rather than just trying to be an offensive snob, why don't you explain what you mean to make it a positive experience. And if I am right, why are you just trying to confuse everything. There is no such thing as a short circuit because all wire has resistance. And there is no such thing as an ideal semiconductor. How is that remotely relevant to this thread?

I really would like to hear what you mean in the case I am wrong. But even if you did have a point, you post in such a nasty way you are a negative influence on these forums, and I know i'm not this first to say this.

This combined with the internal resistance of the LED is used to effect in the popular $1 9-LED torches which have three "AAA" cells and no apparent current limiting at all.

I actually took the time to google that.

First, you have all 9 LEDs in parallel with no resistors. Are you suggesting that that is a good design? Or do you want to say there there really is resistance for each LED because the copper trace has a non-zero resistance. It is impossible to connect any components directly in parallel without resistance because all conductors have resistance.

Those lights are so shoddily made the battery spring alone has significant resistance. They are cheap Chinese junk that is designed to just last long enough to sell. It's actually usually the wiring or switch that fails long before the LEDs have a chance to be burnt out.

Are you seriously recommending someone who comes here to learn about electronics and wants to build a hobby project take design tips from that school of thought. Those flashlights are not so much designed as thrown together without any thought. They are horrible pieces of engineering that don't really work.

So how is that remotely useful to the OP or anyone else who comes along and wants to know how to wire an LED.

I get that you want to convince people how smart you are.

Your Ad hominem attack is misdirected. You can be wrong, but that does not necessarily make you a bad person.

Let's at least try to get beyond that.

I suggest - I advise - you demonstrate it for yourself and only then come back and discuss it.

Take two multimeters, a white LED and your regulated, variable bench supply. Connect the LED with a meter in series, set to read current (200 mA) then the second meter reading voltage directly across the LED. The impedance of the (digital) voltmeter will not affect matters while the "burden" resistance of the current meter will.

Starting from zero, gradually turn the supply voltage up, noting LED illumination and current readings. As soon as either shows, record both meter readings, then continue to advance the voltage stepwise continuing to record readings until you reach the specified maximum current of the LED.

Paul__B:
I suggest - I advise - you demonstrate it for yourself and only then come back and discuss it.

Take two multimeters, a white LED and your regulated, variable bench supply. Connect the LED with a meter in series, set to read current (200 mA) then the second meter reading voltage directly across the LED. The impedance of the (digital) voltmeter will not affect matters while the “burden” resistance of the current meter will.

Starting from zero, gradually turn the supply voltage up, noting LED illumination and current readings. As soon as either shows, record both meter readings, then continue to advance the voltage stepwise continuing to record readings until you reach the specified maximum current of the LED.

How about we do it a little more formally and trace the characteristic curve of the LED. This is what you’re asking me to do with the multimeter, but a curve tracer is so conveniently automatic and gives pretty graphs. Here is one image I found by googling. There are, of course, countless examples (sorry, I mean a large finite countable number of examples).

I’m not sure what you’re trying to say though. As the voltage increases the current rise through and well beyond the maximum specified current of the LED. There is nothing in place to limit the current and it will increase with voltage until you destroy the diode. And once you’re beyond the specified current, you damage the LED without instantly frying it.

As you can see, with voltage, the current through the LED rises asymptotically. An IDEAL semiconductor with no losses getting the power to the semiconductor will conduct unlimited current once the depletion region disappears (the voltage at which the curve touches the asymptote on this graph). Of course there is no such thing as an ideal semiconductor, and all the conductors involved have non-zero resistance, so this graph will never become truly vertical in the real world.

But what is your point? These curves will be slightly different for each LED even within the same batch. There is nothing there that will help you connect an LED directly to a battery. Especially when the battery’s own terminal voltage will vary as a function of current and will decline over time.

How does this tie in to the original poster’s question. In what way is anything you said useful to anyone reading this thread?

My post gave useful, accurate information. Your post is playing pedantic games. It is confusing and discouraging to new users who are trying to figure out how to properly connect an LED. And it is even worse in this specific thread where the OP actually wanted help picking an LED that would self-limit its current at a lower value to drain less battery life. The OP came in confused about how LEDs work, I tried to explain it to him, and all you contributed was further confusion.

It is also rude and discouraging to people like me. Why should I waste my time helping someone when along comes Paul__B to publicly tell me I’m “completely wrong” because he has some theoretical argument which has no relevance to the real world case being discussed. And of course examples of shoddy engineering that “prove” you right.

If i’m “completely wrong”, why not give some explanation why. I certainly don’t know everything and I’m happy to learn too. But your sniping shot doesn’t do anything except make the forum a negative place. Once again, how was I “completely wrong”?

Well, you have just illustrated the point nicely.

Given the typical curve for a white LED - and it is typical - then connecting a 3.3 V coin cell will limit its current to 10 mA - exactly what you want it to be.

Arguably, a 4.5 V battery as used in the cheap torches will drive the LEDs to 30 mA, but in practice with a significant current draw from all nine, the battery will not maintain 4.5 V, so the combined internal resistances of the battery and LEDs does provide the current limiting somewhat elegantly.

{And no, it is not the resistance of the bonding wires or the battery spring.}

Paul__B: Well, you have just illustrated the point nicely.

Given the typical curve for a white LED - and it is typical - then connecting a 3.3 V coin cell will limit its current to 10 mA - exactly what you want it to be.

Exactly, huh. Now I get to tell you you're completely wrong.

a) the graph is not precise enough. It is to convey the concept not select components. b) that is one LED they tested, every component is different due to manufacturing process. c) where do you buy a 3.3 V coin cell? d) terminal voltage of the battery varies based on the current drawn in this range

and once again, how does this help the original question which is how do you find a blue LED that will draw LESS power than a typical blue LED.

Arguably, a 4.5 V battery as used in the cheap torches will drive the LEDs to 30 mA, but in practice with a significant current draw from all nine, the battery will not maintain 4.5 V, so the combined internal resistances of the battery and LEDs does provide the current limiting somewhat elegantly.

So your advice is that is a good design worthy of emulation to connect 9 LEDs directly in parallel and then directly across a battery.

You will be burning out the LEDs with new batteries and running them too dim as the batteries age. Isn't it better to tell people good practices so their projects are more likely to work well and reliably rather than as a cheap kludge that's designed to fail?

And you call me completely wrong. Wow.