parallel led array

[dorke]

Here is another very simple idea for you to try:
Use 1 Battery to drive directly(no resistor!) 3 X IR LEDs in series.
This way you get no losses at all.

The maximum voltage you can get from a fully charged Li-On battery is 4.2Volts
Each LED will get a maximum of 4.2/3= 1.4Volts.
So it is safe for the LEDs.

With the battery voltage dropping with use,
the LEDs will get less and less voltage and illuminate less.

At 3.7volts each LED will get 3.7/3=1.23Volts
At 3.3volts each LED will get 3.0/3=1.1Volts

Worth a try,
If you do try it, what do you get?

 

[lightman]

ok I am still waiting on the new led to get here they are also 3w led so I will wire three in series like you say and see what happens ok I can put it up against the other one that's wired in parallel see which one is brighter and last the longest

 

[lightman]

could something like this be used ? http://www.ebay.co.uk/itm/DC-DC-Boo...423475?hash=item4637798cb3:g:ex4AAOSwEeFVCh~A

 

[BobK]

I just want a simple way to wire them to get the best light I can from them I was told if I wire them in series they would not be as bright I really thank you for all your time but I am not the brightest in the bunch so not sure of the parts I would need to run it your way or were to solder each bit I can not read diagrams so not sure of what the simbles mean

Well, you were told wrong.

As long as the voltage across the LED matches the LEDs specification it will put out the same light whether in parallel or in series. But 2 in series uses exactly half the power of 2 in parallel.

I have given you the best way to get the most light out of the LEDs with the longest battery life.

I disagree with putting 3 in series. The battery will quickly be down to 3.7V, giving you 1.2V for each LED. Since the current depends exponentially on the voltage, this will take the current down to something like 1/2 and hence 1/2 the light output.


Bob

 

[lightman]

cheers will it be ok to put two in series from a 3.7 battery ?

 

[Gryd3]

cheers will it be ok to put two in series from a 3.7 battery ?

How about you try to figure that out

Look at the specs for the LEDs.
They will have a 'forward voltage' specified.
Simply add the forward voltage together for each LED you want to connect in series. As long as it's under the battery voltage you're fine.
There is a catch though, the Battery voltage is very rarely an exact multiple of the LED forward voltage, so when you add up the forward voltage of the LEDs, you will find that there is some 'left overs' of anywhere from 0.5V to 3V depending on the LEDs you are using. Simply use a single resistor to take care of this excess voltage.
This step is important. The value of the resistor chosen should be calculated using the left-over voltage AND the forward current for the LEDs.
This resistor will serve as protection for the LEDs to prevent excessive current from flowing through.

V=IR
( Battery Voltage - LED voltages in Series ) / LED current = Resistor Rating in Ω (Ohms)
If the voltage of the LEDs is close enough to the battery voltage, you can sometimes ignore the Resistor... because batteries have an 'internal resistance' that can sufficiently limit current flow.
Internal resistances are usually very low, and are much lower with more powerful batteries.

 

[lightman]

ok I will see if I can work that out thanks bud

 

[Gryd3]

ok I will see if I can work that out thanks bud

No prob. give it a try and post your work.
You'll find that there won't be an 'exact' answer because the numbers provided in the specs are usually ranged . Minimum, Maximum, Typical.

When I do these calculations I think of a worst case scenario.
I take the highest battery voltage I can expect.
I use the minimum forward voltage for the LEDs.
I use the maximum or normal current for the LEDs.

This way, the resistor and math I use will ensure that I never go over the LED's maximum current rating.
The forward voltage on an LED isn't what the LED 'runs at'... it's a measurement of the voltage that just so happens to be across the LED when you give it the desired current. So a properly operating bright LED running at 20mA may be operating anywhere between min and max.
If I did the calculations with the LED max voltage rating.. then if the LEDs happen to run at the minimum rating, then I would have more excess voltage across the resistor than I planned. This way means that the LEDs will always operate a little under-powered though.
Of course, LEDs are cheap, you can drive them HARD and replace them when they die.

 

[lightman]

I would need a 1ohm resistor two led = 2.8 taken from the 3.7 leaves 9 so next best it 10 = 1ohm that will give me 3.8v ?

 

[Gryd3]

I would need a 1ohm resistor two led = 2.8 taken from the 3.7 leaves 9 so next best it 10 = 1ohm that will give me 3.8v ?

2.8V taken from 3.7V = 0.9V
What kind of current do the LEDs draw?
Voltage = Current * Resistance
0.9V = ??? * 1Ω
Current = 0.9V / 1Ω = 0.9Amps (900mA)

 

[lightman]

led 1.4 forward

DC Forward Voltage (VF): 1.4V
DC Forward Current (IF): 1050mA
Maximum Pulse Voltage: 1.7V
Maximum Pulse Current: 1500mA

 

[Gryd3]

led 1.4 forward

DC Forward Voltage (VF): 1.4V
DC Forward Current (IF): 1050mA
Maximum Pulse Voltage: 1.7V
Maximum Pulse Current: 1500mA

Excellent!
I didn't realize these were such big LEDs though
Well.. Here's another formula for you:
Power = voltage * current
That resistor will drop 0.9V and we calculated 900mA a couple posts ago.
The power rating here is important. The resistor will waste over 810mW of power, so you should get a 1Ω resistor that is rated higher than 810mW and you'll be fine

 

[lightman]

have you a link to the resistor I would need thanks

 

[Gryd3]

Something like this :
http://www.digikey.ca/product-detail/en/RSF100JB-73-2R/2.0W-1-ND/18018
Note that I linked a 2Ω resistor!
You can use 2 of these in Parallel to make a 1Ω, 2W resistor.

Of course, you could just as easily buy a 1Ω with a higher rating. You can do it any way you want.

This would be ideal though > http://www.digikey.ca/product-detail/en/2-1625892-6/A105941CT-ND/3477508
It's a 1Ω rated at 3Watts and is less than a dollar.
finding something similar from a local supplier would be ideal

 

[lightman]

how about this http://www.ebay.co.uk/itm/180709149407?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

 

[Gryd3]

how about this http://www.ebay.co.uk/itm/180709149407?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

That will do, but it's pretty close to the rating.
It will work, and will run hot. If it burns out (after a while)
Simply bundle some of them together to make a higher rated equivalent bundle of resistors and use that instead.

 

[lightman]

I have a 2.2 ohm that should do it I seen this light how the hell can he run 12w from a 3.7v battery and get such a long battery life and he gets 130 meter distance I am getting 90 feet with 9w just don't add up

http://www.ebay.co.uk/itm/12-WATT-I...244389?hash=item3f56fb3925:g:1nIAAOSw5VFWGo6L

 

[Gryd3]

I have a 2.2 ohm that should do it I seen this light how the hell can he run 12w from a 3.7v battery and get such a long battery life and he gets 130 meter distance I am getting 90 feet with 9w just don't add up

http://www.ebay.co.uk/itm/12-WATT-I...244389?hash=item3f56fb3925:g:1nIAAOSw5VFWGo6L

2.2Ω will run the lights dimmer but will work.
If you have two of them. Make a 'new' resistor with both of them in parallel, and you will have a 1.1Ω resistor

The ebay guy you listed is lying.
No way is it possible to power a 12W load with a 3.7V 4Ah battery for any more than maybe an hour.
Some people are tricky liers though... Some lights are advertised as 'equivalent' Watts.
So a 60W LED light bulb actually runs much much less, but outputs the same amount of light as a 60W incandescent. ... but even then... 23 hours sounds like horse droppings.

 

[lightman]

I will not give up I want to make the best light I can for my use not bothered with selling they will be for the team to use this fella makes really good ones it a some sort of dimmer fitted to I wish I was clever lol

http://ghostlightco.com/lights.html

 

[Gryd3]

I will not give up I want to make the best light I can for my use not bothered with selling they will be for the team to use this fella makes really good ones it a some sort of dimmer fitted to I wish I was clever lol

http://ghostlightco.com/lights.html

Hang around for a while and you'll figure it out

You can easily buy very very cheap 'constant current' LED drivers online. These are way better than using resistors, and some are adjustable. This takes care of your 'dimming' question