# Potentiometer + 100 led nightmare. How many ohms I need?

#### menteausente

Dec 30, 2014
1
Hi I don't have high level electronics knowledge and I need to know a few things, maybe someone could help me. I want to make a replica of a lightsaber and for that...
I need to connect arround 100 leds in paralel to a 3,7v li-ion battery and I want to regulate the amount of light they give with a potentiometer. Now the cuestions are:

1. What resistance I need to lower down the voltage from the battery to 3v so I dont burn the leds? (1 resistance between the battery's possitive and the led chain is enough right?)

2. What is de amount of ohms that the potentiometer's maximum value must have to barely turn off the leds? (because the more resistance, the less light they will give... RIGHT?)
Specs:

Battery: Li-ion 3,7V 2400 mah (DO NOT ASK MORE ABOUT IT, that's everything I know and all batteries I bougth are the same)

LED: Pink LED
-Max. Forward Voltage: 3.0-3.2V
-Max. Forward Current: 20-30mA (x100 LEDs)

-Potentiometer:
Here it becomes confusing, If I'm not wrong, I need a potentiometer that cand handle 6W of power (?) because 3V x (100 x 0.02Ah) = 6W. What I dont know is the amount of Ohms, I've seen 5k, 10k... I don't know, Is that enough?

#### Gryd3

Jun 25, 2014
4,098
I suggest you start here : https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/

tldr;
Parallel requires one resistor for EACH parallel LED. (All LEDs will be side-by-side like rungs on a ladder.)
Series requires one resistor for EACH series of LEDs. (All LEDs will be strung together in a chain.)

So.. you have a 3.7V battery, you can't hook up any more than one LED in series without a 'boost converter'. Because each LED you add to a string increases the total voltage you need for the string.
Using 100x LEDs each rated at 20-30mA will draw 2000mA - 3000mA when connected in parallel. This is 2-3 Amps! You would be incredibly lucky if your lights lasted anywhere near an hour of being on.

Also, your potentiometer will not work.. as the power handling capabilities are not there... you would need something like a 555-based PWM driver to dim the LEDs. This you can control with a potentiometer.
(* Note, there is a reason the cheap power drills you buy don't use potentiometers to adjust motor speed... the faint whining you hear when you pull the trigger is the PWM frequency going to the motor. This is an ideal setup.)

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