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Topic: Calculating led resistance? (Read 601 times) previous topic - next topic

Arduino?

Hi
Ok so i just ordered some leds for my little brothers pine wood durby car! I got them from digikey but im a little confused how i would calculate the resistor values for them. Basically is the forward voltage the same as the supply voltage? Like do i use the forward voltage for calculting the resistor value? Here is a example of one of the leds http://search.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&site=us&keywords=1080-1068-ND&x=0&y=0 Im assuming i would just do Forward volts/ Current. Is this correct?

kd7eir

First of all, how many do you plan to use?  Then you need to decide what voltage you want to power them from.  Each one needs 2.2V, but the total voltage you will need to provide to them depends on whether you wire them in parallel, series, or a combination of the two.

This page http://led.linear1.org/led.wiz is what I use to design my led layouts.

Let us know more about how you want to hook them up, and we can provide more specific information.

Arduino?

ok well since all the leds im using are 2.1-2.2v and 20ma i was hoping i could use only one or 2 resistors for all of them, as far as series or parallel im not shore yet which ever works for using only a single or couple resistors for all the leds. Im going to have a total of 9 leds, 2 on the front, to on the back, and 5 on top!

AWOL

How are you going to power them?
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

Arduino?

I will be using dual cr2032 cell holders

AWOL

"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

jackrae

CR2032 has a capacity of 225mAH but only if the load current is limited to 0.2mA.    Each of your LEDs has a rated current of 20mA which is 100 times what the cell can deliver.  So, your first problem is to determine a battery size that can power your 9 LEDs.   Assuming you use say 4 x AA batteries in series to give 6 volts you can drive your system as 4 banks of 2 LEDs in series and 1 bank of 1 LED.

Each series bank will require a resistor calculated as follows R = (6-4.4)*1000/20  =  80 ohms (say 82)  so you need  4 off 82 ohm resistors

The single LED requires R = (6-2.2)*1000/20  =  190 ohms (say 220) so you need a 220ohm resistor

Note that 80 and 190 are not standard E24 values so go for the next higher which are 82 and 220 respectively

Your total load current is 100mA so the ideal battery is size C which is rated at around 6AH at 100mA current.   However this may be way too large so using AA cells will work but these are only rated at around 2.5AH at 50mA discharge, so you might expect say 1AH capacity which will give you 10 hours of operation.

Alternatively you could use LEDs which demand less current or flow less current through the type you have chosen (by increasing the resistor values) to give reduced brightness.


AWOL

Quote
so you might expect say 1AH capacity which will give you 10 hours of operation.

Presumably the OP is only interested in operating the lights for the duration of a race.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

floresta

Quote
Forward volts/ Current. Is this correct?

Absolutely not.

'Forward volts' refers to the typical voltage that appears across a typical LED when the nominal 'Current' is flowing through the LED.  

You must start out with a power supply that provides more than this 'Forward volts' and then limit the LED current with a series resistor.  So you must have a resistor in series with the LED and the combination is connected to the power supply.

The difference between the supply voltage and the LED forward voltage appears across the resistor.  Divide this voltage by the desired current to get the required resistance.


Don

DVDdoug

Quote
Forward volts/ Current. Is this correct? 
FYI - That will give you the effective resistance of the LED (under those specific voltage/current conditions), which is not that helpful here.   LEDs are non-linear...  their resistance changes when the voltage & current change, so Ohm's Law needs to be applied to the resistor (which stays constant) rather than to the LED.

Arduino?

thanks for all the info. yeah i can really put 4x aaa in the car because of space and weight ...im limited to 7oz total. What would happen if i used 3 of the dual cr2032 battery holders? would that give me the 100ma of current draw that i will be using?

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