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A Basic Question of Efficiency - 4 LEDs in parallel or series!?!

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Agent M

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hi guys, this is my first post (love the forum, tons of people here willing to help out - BRILLIANT!!). now, i need to be treated as a novice, because i am one!

THE PRODUCT: 4 LED lamp powered by AAA batteries.

THE DESIGN: 100x100x15mm unit with one LED in the middle of each 100mm side (4 sides = 4 LEDs). power source will be housed in the center of the unit, 15mm height is just enough to accommodate diameter of AAA battery plus the battery holder.

THE ISSUE(S): the minimum voltage range of the LED is 3.0V, typical is 3.4V and max is 3.8V. 3x AAA battery holders are very rare, 2x and 4x AAA holders are more available and affordable (for mass producing the product!). 2x AAA would mean the LEDs would be at optimum-ish luminosity for only a few minutes before the voltage starts to drop and they start to dim. 4x AAA exceeds the max rating of the LEDs, and even if they worked, it would drastically reduce the lifespan of the LEDs.

i'm planning on placing the LEDs in a parallel circuit (i seem to have a misconception that parallel circuits are generally more efficient than series) and stick a resistor in to reduce the voltage but i have no idea what resistance these LEDs are and what the total resistance would be of 4 of them in parallel. also, i'm wondering about the power wastage of it all. the LEDs in series would be much easier, construction-wise but not sure if 6V from the 4x AAA power source is sufficient.

THE GOAL: to create the simplest and most energy efficient circuit to power 4 LEDs using either a 3V or 6V power source, giving the product it's maximum running time!

OTHER ISSUES: current - i know the LEDs are probably anywhere from 30mA to 80mA each but really don't know what current is generated from these banks of Duracell cells! mA hours - the Duracell pack doesn't say what their's is (unlike rechargeable ones), so i don't know how to calculate what the units max continuous usage time would be.

fellas, i know this is pretty beneath you but i thank you all in advance for all your help. thanks! :p

p.s. attached is the spec sheet of the LEDs. if not, then you can view them here: **broken link removed**
 

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Connecting the LEDin series is more efficient because the current is lower. Connecting LEDs in parallel would also require a separate resistor for each LED.

Unfortunately you can't connect the LEDs in series because the foward voltage will be too high, it can rage between 6V and 6.8V, if you're lucky it might work with brand new batteries but as soon as the voltage drops below 6V they won't work.

You need a series resistor for powering LEDs, else the battery will supply whatever current it feels like and the LEDs will overheat.

The folrmula for calculating the series resistor value is:
[latex]R = \frac{V-V_F}{I_F}[/latex]
Where:
V = Supply voltage.
Vf = Forward LED voltage.
If = Forward LED current in Amperes.
 
Your LEDs have an absolute max current rating of 30mA, so design the current-limiting resistor for 25mA with a brand new 6V battery and a 3.0V LED. The current-limiting resistor for each LED is 6V-3V/25mA= 120 ohms.

The LEDs might be 3.8V. Then their current with a new battery is 6V-3.8V/120= 18.3mA. When the battery voltage drops to 4V, the current with 3.8V LEDs is 4V-3.8V/120= 1.7mA which is very dim.

Measure the voltage of each LED with a current-limiting pot providing about 25mA, then calculate the value of a resistor.

Four LEDs on continuously use a lot of battery power. With four AAA alkaline cells used to power the four LEDs, they will be at half brightness in only about 1.5 hours and the battery will be dead in 3 hours. You will see the dimming. Four AA cells will last more than twice as long and cost less.

Consider adding a simple circuit to blink the LEDs to save power. Then the battery can be small and will last much longer.
 
The easy and fast way to solve your problem is to use a 9V battery instead which takes the same space as two AA batteries. You can run 2 in series with one resistor, and several parallel strings if you want.

The more sophisticated approach with two AA's is to use a boost converter. And the easy but expensive way to do that is to get a MicroPuck. Do a search on boost converters and you might find some other good alternatives.

-- Dan
 
Hi Dan,
A little 9V battery doesn't have enough voltage to power two sets of two 3.8V (their max) LEDs in series because its voltage will quickly drop lower than 7.6V then the LEDs will produce no light.
Little 9V batteries are designed for low current applications so 50mA will kill it soon.
 
Yea, my rechargeable Energizer 9V battery rated at 150mAh is around 9V when fully charged, but it drops quickly less than an hour, 7.x volts even not being used.
 
bananasiong said:
Yea, my rechargeable Energizer 9V battery rated at 150mAh is around 9V when fully charged, but it drops quickly less than an hour, 7.x volts even not being used.

That's because it's NOT 9V, it's probably only 7.2V (6 cells in series) - most rechargeable PP3 batteries are like this!.
 
Why not look on its datasheet:
 

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Spoka!!

hey peeps! thanks so much for all the ideas, really appreciate it. but to tell you the truth, i got a bit despondent thinking 'this ain't gonna be as easy as i thought it would be!', until...

a few days ago i went xmas shopping with my girlfriend to IKEA! boring, boring, boring... and then i come across a re-chargeable kiddies glow lamp (called 'SPOKA', and comes with a power supply, must be a new product as it is still not on the ikea website). and it looked like it was powered by LEDs!!! i decided to part with some money so i could take it home and use a sledgehammer on it for the sake of scientific progress (yes! that's what i'm calling it.). booyah! it was perfect:

1. 4 LEDs
2. 3x AAA re-chargeable cells
3. continuously on

all the criteria i want for my project. needless to say i was chuffed. as you can see from the pics, it's got the 3x AAA power source wired to the main board with the push-button on/off switch (this board, i assume, also houses the circuitry to control the charging of the battery - correct me if i'm wrong please), and 2 other boards housing 2 LEDs and 2 corresponding resistors each.

now, when fully charged, the individual LEDs read between 3.21-3.25V each. but the terminals from the battery reads about 4.14V! how's this working out fellas? it shouldn't be more than 3x 1.2V equaling 3.6V, should it???

the corresponding resistors for each LED is reducing the voltage and current to the LEDs required rating, right? so, i can take the existing circuitry and fiddle about with that to cater for my max 3.6V LEDs.

so, if my prototype works, i'm intending on getting in touch with the manufacturers (it's made in china, what a surprise these days, eh?). you think if i approached ikea, they would give the manufacturer's details? seeing me as competition and all?

as much detailed feedback as poss will be so appreciated. hope you're all having a fab xmas.

hero999: that was a much needed formula, gonna be needing that for sure. thanks.

audioguru: "With four AAA alkaline cells used to power the four LEDs, they will be at half brightness in only about 1.5 hours and the battery will be dead in 3 hours. You will see the dimming. Four AA cells will last more than twice as long and cost less." - absolutely right. very important factors for producing a consumer product, hence AAA wouldn't be an option. thanks for pointing that out. i was compromising overall efficiency for 'good looks' of the product.

nigel: thanks for your input, 9V battery is not an option either. also, if you don't mind, would you tell us (or just the newbie, maybe everybody else already knows) a bit more about your background, current job, etc. i'm very interested in people's backgrounds. also, whereabouts are you based? i'm in london.

my background: computer science bsc. (never really used it in industry), had my own web dev business for 4 years, then just been doing various businesses (import/export, property - quite entrepreneurial, i guess). but my passion is in design! product design to be specific. always loved electronics as a kid, but never really bothered to find out the basics of it - never had a toy i didn't take apart! i want to create products that merge technology with furniture. been thinking about it for the past 6 years or so, recently someone told me if i didn't start now, i would 'think' about it for the rest of my life. i believed her and now want to start with a low cost project. again, thanks for all your help, guys.

marshall

p.s. a cell is something that gives out 1.5V and a 'battery' is a series of 2 or more 1.5V cells. so calling one AA duracell a 'battery' is technically incorrect. am i right? it's just what i recall from GCSE physics (20 blessed years ago!).
 

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Agent M said:
now, when fully charged, the individual LEDs read between 3.21-3.25V each. but the terminals from the battery reads about 4.14V! how's this working out fellas? it shouldn't be more than 3x 1.2V equaling 3.6V, should it???
Ni-Cad and Ni-MH rechargable cells average 1.2V while discharging, but are 1.4V when fully charged fresh out of the charger. Then 3 of them would measure 4.2V.

the corresponding resistors for each LED is reducing the voltage and current to the LEDs required rating, right? so, i can take the existing circuitry and fiddle about with that to cater for my max 3.6V LEDs.
The battery would drop slowly to 3V. If the LEDs are 3.6V then they would be dim for a while then will be off for most of the battery's charge.
When you go into production then you can order millions of LEDs with a lower voltage. The LED manufacturer will select them for you and will sell the higher voltage LEDs to hobbiests.
 
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