Need Help with LED reisistance

Hi I'm not too sure if this is the right place to ask this but...

I'm working on my first every Mod making a 5x 3mm LED drop in for the Maglite Solitaire. I thought of it after seeing the mod section on Flashlight review and the use of a single 3mm LED in place of the bulb but i wanted to take it an extra step.

But to the point. I have made the actual drop in and it works really well (except it's rather long and the head doesn't screw all the way down) but i'm using a 12V MN21 battery and a spacer and common sense tells me i need a resistor in there but i'm not sure what i need.

The LED's spec is:
Forward Voltage: < 4.5V
Reverse Current: < 50 uA
Max Power Dissipation Pm = 80mW
Max continuous Forward Current: I(fm) = 30mA
Max Peak Forward Current: I(fp) = 75mA

The 5 LED's are RUNNING IN PARALLEL so i'm not too sure how that effects everything

I'm not that well experienced in the workings of LED's really (I'm only 18!) so i would really appreaciate the help. I will be posting pictures of the end result when i know i can safely run it!

Best Regards!

Danny

Ogg Vorbis said:

Hi I'm not too sure if this is the right place to ask this but...

I'm working on my first every Mod making a 5x 3mm LED drop in for the Maglite Solitaire. I thought of it after seeing the mod section on Flashlight review and the use of a single 3mm LED in place of the bulb but i wanted to take it an extra step.

But to the point. I have made the actual drop in and it works really well (except it's rather long and the head doesn't screw all the way down) but i'm using a 12V MN21 battery and a spacer and common sense tells me i need a resistor in there but i'm not sure what i need.

The LED's spec is:
Forward Voltage: < 4.5V
Reverse Current: < 50 uA
Max Power Dissipation Pm = 80mW
Max continuous Forward Current: I(fm) = 30mA
Max Peak Forward Current: I(fp) = 75mA

The 5 LED's are RUNNING IN PARALLEL so i'm not too sure how that effects everything

I'm not that well experienced in the workings of LED's really (I'm only 18!) so i would really appreaciate the help. I will be posting pictures of the end result when i know i can safely run it!

Best Regards!

Danny

Click to expand...

The method about to explain errs on the side of caution so that you will hopefully get some confidence doing this....i don't want to pop your leds for you either hehe.

have you got the data sheet for the LED? You need a little more info than you quoted ideally.

If you do, somewhere there will be a Vf against If. Given that continous should be 30mA this is prbably what you want, unless you want to try pushing the LEDs little harder. This will give the Vf you are aiming to drop the voltage down to.

next you need the battery voltage...to be safe use the voltage of the battery when fully charged. this is higher than the stated voltage. If you have a multimeter just meaasure the voltage fresh off charge (when not being charged).

Subtract the Vf you got off the graph from this battery voltage. That gives you the required voltage across your resistor.

if you want to use 1 resistor for all you need to add all the LED currents up. EG 30mA x 5 in this case so 150mA.

For the resisitor size. Good old ohms law. R = V/I

V across res / 150mA = your resistor in ohms

it is worth while calculating the power the resisitr needs to dissipate too now....which is V across Res X 150mA (in this case). Make sure the resistor power rating is higher than this number...or it will get hot. You should be ok at these levels though.

If you want to use indivivual resistors its the same calc, but use the 30mA figure and use 5 resistors this size 1 for each LED.

The single res methjod is smaller physically, and less soldering. But if one of the LEDs pop, the currentin the circuit and hance the resistor will drop...this means the voltage across the resistor will drop which therefore means the voltage across the LED must go up by the same ammount...this will push more current though the remaining leds...probably popping on or mor of them and the cycle goes into a run away anmd they all pop hehe.

with individual resistors this won't happen.

I would atfirst start driving the LEDs gently...then play with driving them harder once you have more confidence in this procedure and the results.

Hope that makes sense and good luck .

Happy modding

Stu

Wow thats a great help... i would like to have been able to put resistors on each LED but the solitaire is really limited for space so i plan to the resistor in a dummy cell. I should be ok to work it all out now. I'll post back when i've finished, will probably be middle of the week when i get the resistor i need.

Thanks again!

no worries.

yeah show us how it works out. look forward to seeing it.

Stu

Ogg;

The best use of your 12 Volt battery would be with three of the LED's in series.

In parallel with that, add the 2 remaining LEDs with a resistor value of 120 ohms.
The resistor can be a 1/4 or 1/2 watt size.

Larry Cobb