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### Topic: Efficient way to power LEDs with a 1S LiPo (Read 1 time)previous topic - next topic

#### Zealot

##### May 17, 2015, 12:47 amLast Edit: May 17, 2015, 01:02 am by Zealot
I'd like to power a bunch (30 pieces) of 3.2V / 20mA LEDs with a 1S LiPo battery with 2600mAh.

The LEDs use a total of 600mA - so the discharge rate of the battery is only ~0.2C.

First I thought about using a step-down converter, but they don't work with such a low voltage drop of only 0.3V (3.6V - 3.3V).

So I'd use a step-up/step-down converter to bring the 3.6 to 4.2V of the LiPo to 3.3V. Then I'd use a 10 Ω resistor (series resistor) for each LED to bring down the voltage to 3.1V (and protect the LED).

Does that make sense?

How would you use a 1S LiPo to power your LEDs?

#### knut_ny

#1
##### May 17, 2015, 02:09 pm
make sense to me, but U'll be running the converter at its max.
Ny

#### PaulRB

#2
##### May 17, 2015, 03:00 pm
Hi, what about a constant current driver? A simple one can be made from one or two transistors and a couple of resistors. This would be able to take a range of input voltages and provide your constant 600 mA without need for series resistors.

My only concerns are whether it would still operate down to 3.6V input which is, as you say, only 0.3V more than the led forward voltage. Also choice of exact circuit and components to achieve this is beyond my analog circuits expertise. But someone on this forum will know.

Paul

#### Zealot

#3
##### May 18, 2015, 12:04 pm
Hi, what about a constant current driver? A simple one can be made from one or two transistors and a couple of resistors.
Can you please give more details?

#### PaulRB

#4
##### May 18, 2015, 01:33 pm
Well, I was thinking of something like this.

But like I say, I'm no expert in analog electronics. What you need to establish is whether a circuit like this will work with a supply voltage only around 0.3V more than the led forward voltage.

#### Archibald

#5
##### May 18, 2015, 09:31 pm
I'd like to power a bunch (30 pieces) of 3.2V / 20mA LEDs  . . .
I believe the voltage of LEDs can vary significantly. 3.2V may be only typical. Does the data sheet for the LEDs you are using give a tolerance range at 20mA?

Will an Arduino be switching your "bunch" of LEDs?  If so, how?

You may like to consider purchasing a step-up/step-down converter with adjustable output voltage so you can use about 4 volts for the LEDs:
https://www.pololu.com/product/2118.

If your Arduino is operating at 3.3V and you use a higher voltage such as 4V for the LEDs, do not connect Arduino outputs directly to LEDs (or to their series resistors).

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