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# Resistors for LED setup

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#### lptoole

##### New Member
Hello.

I am running 2 4-pin high intensity LEDs from a 9v battery. LED FW current is 70mA and FW supply is 3.3V. First, what resistor do I need, second, do I need one for both the positive and negative wires of the battery?

Thanks so much for your help.

-Logan

In any case you will need one resistor. You will need to decide if you conenct them serial or parallel. You can use this **broken link removed** to calculate the resistor and then you will put this value to this **broken link removed** to get a resistor value that does exist in the market. You can use either serial or parallel connection.

The absolute max allowed current for a Luxeon SuperFlux red LED is 70mA only if it is well cooled by 25 degrees C max ambient air and plenty of pcb copper. The other colours have a max allowed current of 50mA.
Chinese copies might be less.

I have some Luxeon SuperFlux red LEDs crowded together on stripboard at 53mA and they get extremely hot.

You will need to decide if you conenct them serial or parallel.

How does a parallel resistor limit LED forward current?

he meant you can connect the leds in series with yr resistor or in parrallel but it would be best to use seperate resistors for each led because sometimes leds can be a bit current hungry....

I would use two transistors in constant current mode. This would give you a constant brightness from a 9v battery as it dropped to about 5v.
This circuit is just an example. The Darlington transistor could be replaced with an ordinary transistor.

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How does a parallel resistor limit LED forward current?

the leds, not the resistor

the leds, not the resistor

Maybe Murphy was involved in constructing your sentence.

No LED is mentioned there, but (singular) a resistor becoming plural in the same sentence.

Maybe Murphy was involved in constructing your sentence.

No LED is mentioned there, but (singular) a resistor becoming plural in the same sentence.

no, murphy was not involved at any time.

you ask: "How does a parallel resistor limit LED forward current?"
I answer "the leds, not the resistor"

What is it that you do not understand and accuse little murphy? Not the resistors to go in parallel but the leds.... He can connect the leds either in parallel or in series...

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