LED array power queries

[Gitza]

New user here so hi everyone!

I am making several lights with a number of LEDs (around 25-60) that I am powering with a single cell li-po.

My first question is, will this circuit work and be efficient ....

A 3.7V single cell li-po connected to a small single cell mini USB li-po charger. Those connected to a DC-DC converter (MP2307) to smooth the voltage out. Then from that going to rails for the LEDs. Apparently you can use caps and voltage regulators but those are less efficient and obviously more hassle.

My second question is that I can change the output voltage of the DC-DC to exactly the forward voltage required by the white LEDs so when I put those figures into a resistor calculator it says to use a 1 Ohm resistor. Does that sound right? If I was to use red etc LEDs it would be like 20 Ohm so if I had the LEDs alternating red/white/red/white I would just use the corresponding resistors off the V+ rail for each LED?

I hope that makes sense, please let me know if there is any other information you need.

Many thanks in advance,
Ollie.

 

[Gitza]

Well, I have just noticed that DC-DC converter is minimum 4.7v input.... whoops. Will have to find an alternative.

 

[Gitza]

Ah! I also forgot to ask...

Any ideas on how to auto shut it off when it reaches ~3.6v? As not to damage the li-po.

Cheers

 

[Harald Kapp]

Welcome to EP.

Those connected to a DC-DC converter (MP2307) to smooth the voltage out. Then from that going to rails for the LEDs. Apparently you can use caps and voltage regulators but those are less efficient and obviously more hassle.

Caps are no hassle, not typically. Even the DC-DC conveter uses caps to smooth the output voltage.
A DC-DC conveter is a kind of voltage regulator, too. Your reference to less efficiency is for linear regulators. You need to make a distinction between the two kinds ofvoltage regulators as they operate quite differently.

My first question is, will this circuit work and be efficient ....

It will probably work. Efficiency depends on the actual setup and components used.

My second question is that I can change the output voltage of the DC-DC to exactly the forward voltage required by the white LEDs so when I put those figures into a resistor calculator it says to use a 1 Ohm resistor. Does that sound right?

Yes and no.
Yes: you may be able to adjust the output of the DC-DC converter and theoretically get away with a resistor as small as 1Ω to limit the current.
No: the smaller the voltage across the current limiting resistor, the more the circuit becomes sensitive to changes in parameters like DC-DC converter output voltage, LED pass voltage etc. A small change of e.g. 100mV in LED passs voltage will change the current by 100mA! This is not a reliable setup.

A much better solution will be to use a DC-DC converter with controlled (regulated) current output set to the requirements of the LEDs such that the DC-DC converter automatically adjusts the output voltage to match the LEDs' characteristic (which btw. changes with temperature, age etc.). A voltage output DC-DC comverter may be used provided it has a built-in current limiter which ideally is adjustable to match your LEDs' needs.

 

[skenn_ie]

It depends on whether you want to vary the number of LEDs that are illuminated, or the brightness. Usually, LEDs are driven by a current regulator rather than voltage. Most LED's need at least 10mA, and putting a very low value resistor with just a slight bit more voltage is not a good idea ... as the forward voltage (1.2V) will vary quite a bit from one diode to another, and with temperature. I would suggest at least 10ohms per LED, preferably more, with power supply allowance for the resistive voltage drop.

 

[Gitza]

Okay, thanks for the replies

I didn't think that sounded right! With all that in mind, do you think a dc-dc convertor with current control would be the most efficient but more importantly cost effective?

Any suggestions on a better setup would be much appreciated.... I have an ring of 3mm LEDs that could be of different colours, powered by a single cell li-po with a little usb charger. The enclosures are very small, always only 130mm high but in various shapes. I will be milling a PCB to power and hold the LEDs.

A means to automatically power off when the voltage is low, to not damage the li-po, is also needed I think.

I want to keep cost down as much as possible as I will be making a lot of them.