Not sure which resistor I need

[JoeRodge]

Hello, I was wanting to make a little LED solar light for fun. I'm new to this and having trouble figuring out the resistor value I would want. I see the ohm's law calculation. Just not sure what I am doing with it.

I was hoping to use this COB LED or something like it: https://www.aliexpress.us/item/3256805620746224.html

I would power it using a 3.6V Lithium Ion cell. With a capacity of around 900-1200mAh. I want the power draw to be around 200mA. So that it can run for 3-5 hours. Can anyone help me figure this out?

 

[Dave_H]

I play around with solar lights of all types.

The Li-ion cell will start out at 4.1v-4.2v fully charged and run down to around 3.3-3.4v with little capacity left. Try to not discharge below 2.5v, and for better life say 3.0v. Using resistor, LED current will decrease, and brightness also, but this is common on commercial products for cost and simplicity.

Your LED should have forward voltage in the 3v range. Depending on how well it's spec'ed, you might have to approximate and/or do some simple tests.

Say the LED has vf=3.2v at 200mA which you take as maximum. Resistor will be:

(4.2v - 3.2v)/0.200 = 5 ohms.

You could use 4.7 ohms or 5.1 ohms which are common values,close enough.

If Vf is different, some resistance adjustment may be required.

Power rating of resistor should be 0.2W, I would use at least 0.5W or 1W.

On average, with cell at 3.7v, current will be 0.1A (100mA). Near end of charge, LED current may be very low. If this is too low, resistance could be lowered, if maximum LED current is not exceeded.

For example if LED can tolerate (say) 300mA at full charge, average current at 3.7v will be 150mA, using 3.3 ohms (0.5W or higher) but runtime will be shorter.

Dave