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LEDs from Lithium cells

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Barmybaz

Member
I wish to run 16 leds from lithium cells,
12 with a forward voltage of 1.8 - 2
4 with a forward voltage of 2 -2.4
all with a current rating of 20 ma
I have done this in the past using Alkaline cells but all the info suggests putting a resister to each Led,
Why is this ?
I wan't to make this project as small as possible, I seem to remember being told that with lithium cells you don't need a resister in line,
I have a number of CR2032 lithium cells rated at 3 volt
can I use these ?
to get a sensible battery life how many would I need to wire in parallel,
Help please,

Baz
 

Les Jones

Well-Known Member
Most Helpful Member
You need to define sensible battery life. 16 LEDs drawing 20 mA each will draw 320 mA A CR2032 has a capacity of 225 mAh I cannot find a value for their internal resistance but 30 mA seems to be the maximum recommended current drain with a recommendation not to exceed 15 mA (Without significant loss of capacity.) As these figures suggest that you would need about 20 in parallel I do not think it is a sensible choice of power source for 16 20mA LEDs.
 

alec_t

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all the info suggests putting a resister to each Led,
Why is this ?
Because it is essential to limit the current (in your case to 20mA in each LED), especially if you are using lithium batteries, otherwise you are likely to damage something or, at worst, have a fire :eek:. To determine the necessary resistor value, subtract the LED forward voltage Vf (you can find that in the LED's datasheet) from the cell/battery voltage and divide the difference by the desired LED current.
 

audioguru

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A little CR2032 lithium battery coin cell can power ONE or TWO LEDs for a little while. Its datasheet does not show its maximum allowed output current but shows 1ms pulses repeated every 15ms at 20mA and it does not last long. Pulses of light for a duration less than 30ms appear to be dimmed.
The datasheet for an AA alkaline cell shows it supplying 800mA for one hour which is about a few thousand times more than the coin cell.
 

Barmybaz

Member
Thanks for your suggestions, looks like I'm back to the AAs
about the resisters, why do they always seem to be put in series with each LED ?
Why can't I put several LEDs in parallel and use one resister,
I know it would not save much space but it may help,

Baz
 

Nigel Goodwin

Super Moderator
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Thanks for your suggestions, looks like I'm back to the AAs
about the resisters, why do they always seem to be put in series with each LED ?
Why can't I put several LEDs in parallel and use one resister,
Because they won't share the current evenly, making it very unreliable.

If you're got enough voltage, place the LED's in series, and use a single resistor (or constant current source) to feed them all.
 

Les Jones

Well-Known Member
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Because the voltage across LEDs for any given current through them varies slightly. The LEDs would not share the current equally between them so you might exceed the current rating of one or more of the group.
 

Barmybaz

Member

To determine the necessary resistor value, subtract the LED forward voltage Vf (you can find that in the LED's datasheet) from the cell/battery voltage and divide the difference by the desired LED current.


Would the above formular still apply,

Baz
 

Les Jones

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Most Helpful Member
Which above formula ?
Just for interest this is one method I use to drive a string of LEDs. It uses one of the cheap LM2577 step up regulators sold on ebay modified to be a constant current source.
This is the unmodified schematic.
Stepup_Regulator.jpg
This is the modified schematic.
Stepup_Regulator.jpg
The value of R3 is chosen to give a voltage drop of 1.25 volts at the required LED current. The zener diode D2 prevents the regulator from destroying itself by attempting to maintain the LED current in the event of the chain of LEDs failing open circuit.
 

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Barmybaz

Member
Thanks for all your help,
perhaps I should rely on the japanese To make things tiny,
have taken on all your advice and learnt a thing or two,

thanks
Baz
 

audioguru

Well-Known Member
Most Helpful Member
My very cheap Chinese flashlight has 24 white LEDs all in parallel and use the resistance of the three cheap AAA "super heavy duty" cells to limit the current. I tried Name Brand AAA alkaline cells and it became much brighter.
The LEDs are matched so they all have the SAME forward voltage.

You can also match your LEDs if you buy a few thousand LEDs and test every one then sort them into groups.
 
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