Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
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?
In any case you will need one resistor. You will need to decide if you conenct them serial or parallel. You can use this LED resistor calculator to calculate the resistor and then you will put this value to this standard resistor value calculator 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.
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.