Looking for a high-efficiency (low-current) type LED?

Jeff101

Joined Jan 16, 2023
4
Hi all, Sorry if this is a stupid question. but I want to build a simple NiMh battery charger and the plan says I need a "high-efficiency (low-current) type LED" I have LED's but they don't say if they are this type? Down in the description in the bellow link...

Thanks!

Link here to schematic and info

dl324

Joined Mar 30, 2015
16,918
Welcome to AAC!
the plan says I need a "high-efficiency (low-current) type LED" I have LED's but they don't say if they are this type?
That's because they don't know what they're talking about. Since it's just an indicator, brightness shouldn't be a factor and any ultrabright LED should do.

He's not using the regulator in an unintended manner. Using it as a current source is described in the datasheet.

The efficiency of such a simple charging circuit for NiCd/NiMH is dubious.

BobTPH

Joined Jun 5, 2013
8,961
A 9V source and 50Ω resistor would do the same thing.

Ya’akov

Joined Jan 27, 2019
9,164
Welcome to AAC.

High efficiency LEDs are not “low current”, at least not in the misleading way that comes off. LEDs are often powered by constant current supplies, and the voltage is determined by the current provided. At the nominal current that appears on the data sheet they will produce the light output they are rated for. They can be overdriven and produce more light if they get more current, but more current means more heat and shorter life.

They have $$\mathsf{V_F}$$ (Forward Voltage) rating. The LED will draw its rated current—for a typical 5mm indicator type, usually 20mA—when forward biased at that voltage. A less than that voltage it will use less current and produce less light.

At some low voltage it will stop producing light, or conducting at all. In experiments, I have gotten perceptible light with very tiny currents, but not enough for an indicator. In your circuit, the LED will be forward biased at ~1.25V, so it will be considerably under the expected 1.7V or so for a normal, say red, indicator LED. This means it will draw much less current, any LED would.

But, a high efficiency LED will produce more light for that amount of current. It’s not using less current in this case, it’s producing more light for what it gets. High efficiency LEDs are often red because of the AlGaInP (Aluminum Gallium Indium Phosphide) chemistry.

It is easy enough to take your LEDs, put 1.25V across them, and see if they are bright enough for you. Or, you can just look for “High Efficiency Red LEDs” and forget the “low current” part.

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MrSalts

Joined Apr 2, 2020
2,767
LEDs draw more current depending on how much voltage you give them.
No, LEDs are not a resistive element so they DO NOT draw more current when you give them more voltage, they draw more current when you give them more current. You already defined the Vf if the LED and that changes some with different currents but, rightfully, few people look for the internal resistance of an LED to design a circuit.

Ya’akov

Joined Jan 27, 2019
9,164
The efficiency of such a simple charging circuit for NiCd/NiMH is dubious.
Without reference to the particular circuit, if you trickle charge NiCd and NiMH cells you can do it indefinitely. It is the slowest possible way to charge them, but it is also “safe” because no matter how long you do it, the cell will not be harmed.

Unlike other chemistries, when the electrolyte starts to decompose into gas it is converted back into electrolyte inside the inside the cell instead of outgassing. The safe current for NiMH is a bit lower than for NiCd and you can cook a NiMH battery with a NiCd tackle charger.

MrSalts

Joined Apr 2, 2020
2,767
Hi all, Sorry if this is a stupid question. but I want to build a simple NiMh battery charger and the plan says I need a "high-efficiency (low-current) type LED" I have LED's but they don't say if they are this type? Down in the description in the bellow link...

Thanks!

Link here to schematic and info
A reasonably modern, high-brightness LED can be clearly seen when run at 0.5mA (even though they are specified at 20 to 25mA).

The circuit design is rather cleaver and draws no current from the charger when the batteries are removed (the LED turns off as well). The designer allows 0.5 to 0.6mA to the LED and as specified, it does provide 180mA to the batteries in series.

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Ya’akov

Joined Jan 27, 2019
9,164
No, LEDs are not a resistive element so they DO NOT draw more current when you give them more voltage, they draw more current when you give them more current. You already defined the Vf if the LED and that changes some with different currents but, rightfully, few people look for the internal resistance of an LED to design a circuit.
You are right, I mistyped. When you raise the voltage, of course that increases the current. I skipped the step there. I will correct it, but the idea is the same.

Jeff101

Joined Jan 16, 2023
4
Thanks for all the replies, Love them all and I find this electronic stuff fascinating! I wanted a way to charge AA - AAA NiMH from a 12V source. Anyways from what I gather here it don't matter what LED or use a RED LED but when I search say Mouser for Ultra Bright LED or High Efficiency Red LEDs I come up with nothing.

Ya’akov

Joined Jan 27, 2019
9,164
[
You are right, I mistyped. When you raise the voltage, of course that increases the current. I skipped the step there. I will correct it, but the idea is the same.
I’ve rewritten it and tried to avoid the confusing conflation of voltage and current. The important idea is that “low current” is not what is sounds like as it is used in the posted story.

Ya’akov

Joined Jan 27, 2019
9,164
Thanks for all the replies, Love them all and I find this electronic stuff fascinating! I wanted a way to charge AA - AAA NiMH from a 12V source. Anyways from what I gather here it don't matter what LED or use a RED LED but when I search say Mouser for Ultra Bright LED or High Efficiency Red LEDs I come up with nothing.
Reviewing the current (no pun intended) offerings, “high efficiency” is now on most LEDs somewhere. Just look for a high brightness LED. In fact, a green one might be best because it will look brighter for a given luminous intensity. The circuit won’t care which LED you put in there so long as you can measure the right voltage at the terminals of the cell you will know it’s OK.

Take whatever you have and try it, see what you get.

sarahMCML

Joined May 11, 2019
370
Thanks for all the replies, Love them all and I find this electronic stuff fascinating! I wanted a way to charge AA - AAA NiMH from a 12V source. Anyways from what I gather here it don't matter what LED or use a RED LED but when I search say Mouser for Ultra Bright LED or High Efficiency Red LEDs I come up with nothing.
I'm using some Topbright High Efficiency Red LED's pulled up directly from the outputs of some 74ALS193 counters with 1.75V across the 22k resistors in series with each LED. That's just under 10uA through the LED's, and they're perfectly usable as indicators! They are model: TB5-V15-25-R18000

dl324

Joined Mar 30, 2015
16,918
Anyways from what I gather here it don't matter what LED or use a RED LED but when I search say Mouser for Ultra Bright LED or High Efficiency Red LEDs I come up with nothing.
The LED will have about 3mA flowing through it, so some standard brightness LEDs would work.

Here's typical data for some Kingbright LEDs:

These particular LEDs specify brightness at 10mA; many specify at 20mA. Both types show a luminous intensity of 50% of the level at 10mA. If you select an ultrabright type or a high brightness bin for a standard LED, the LED should turn on. Just don't plan on being able to see it in bright light.

From a different LED:

Jeff101

Joined Jan 16, 2023
4
OK Thanks! I also found some under sparkfun LED - Super Bright

dl324

Joined Mar 30, 2015
16,918
OK Thanks! I also found some under sparkfun LED - Super Bright
Personally, I'd find another source. Sparkfun prices are generally on the high side.

Jeff101

Joined Jan 16, 2023
4
Yeah I noticed, I have regular LED's on hand too so I'll just build it and see what happens.

DickCappels

Joined Aug 21, 2008
10,179
No, LEDs are not a resistive element so they DO NOT draw more current when you give them more voltage, they draw more current when you give them more current. You already defined the Vf if the LED and that changes some with different currents but, rightfully, few people look for the internal resistance of an LED to design a circuit.
Sometimes LEDs a powered directly by batteries with no intervening resistance or electronics, for example in some flashlights batteries alone directly drive a cluster of parallel white or UV LEDs, this being possible by selecting LEDs with appropriate voltage ranges.

LEDs can be considered to be voltage driven devices, though it is often preferable to think of them as current driven -it depends upon the application.

Pardon the digression.

MrSalts

Joined Apr 2, 2020
2,767
Sometimes LEDs a powered directly by batteries with no intervening resistance or electronics, for example in some flashlights batteries alone directly drive a cluster of parallel white or UV LEDs, this being possible by selecting LEDs with appropriate voltage ranges.

LEDs can be considered to be voltage driven devices, though it is often preferable to think of them as current driven -it depends upon the application.

Pardon the digression.
I disagree. It is done by using the internal resistance of the battery. Internal resistance of the battery is surprisingly constant for a particular size and brand. Simply use this method to determine internal resistance and you'll see that the particular AA battery they measure (at 375mA) is .275 ohms. The internal resistance of a battery is not perfectly constant, it goes up slightly with higher loads.

https://learn.sparkfun.com/tutorials/measuring-internal-resistance-of-batteries/internal-resistance

This means you can take an array of 13 white LEDs (with Vf of 2.7-2.8V), in parallel* and be limited to about 390mA.

* Some here would claim you shouldn't put LEDs in parallel without a resistor because...
- "all the current will flow through the LED with the lowest Vf". This is kind of true but keeping all LEDs within a small range, you'll be ok, especially for a device that isn't expected to last long
- "if an LED fails open, the other LEDs will get too much current and soon fail", true but, again, you get what you pay for.

In the end, you can consider an LED to be a voltage driven device but you'll be very unhappy if you measure the voltage of your battery and then try powering your flashlight from your bench power supply set to the same voltage as your battery. Poof.

Pardon the reply to the digression.

DickCappels

Joined Aug 21, 2008
10,179
You cannot be wrong. Just look at the current vs voltage curves of some white, blue, and ultraviolet LEDs. You probably already know that one can oder LEDs sorted by voltage so I won't bother you with it. Yes, eventually the battery's internal resistance limits the current to it load, including incandescent lamps when the batteries get old, but fresh alkaline cells have pretty low resistance.

Now back to our program in progress...