Step-up/Step-down booster for a flashlight

Thread Starter

Alex Jurtan

Joined Jul 14, 2015
14
Hello everyone!

I have been researching and experimenting with making a step-up booster circuit. So far I made the SparkFun 3.3V Step-Up Breakout - NCP1402 (as seen in the picture). It works with one AA or two AA batteries, but just outputs around 3V instead of 3.3V. Not sure what the problem is.
I have mounted a 1W power led on it, but it gets hot without a heatsink (not ideal for my use). Before I did test it with a 5050 SMD LED, but it was not bright enough.

Ideally I would like to have a circuit that outputs a fixed voltage (3.3V) with a variable input (1.5V - 6V) and this all with a really bright LED.
____________________________________________________
I would really appreciate some advice on how to make a step-up/step-down booster and what kind of LED would be smart to use for a flashlight.

- Step up/step down booster
- fixed output at 3.3V
- input range 1.5V - 6V

Thank you!
 

Attachments

RichardO

Joined May 4, 2013
2,268
You have 2 problems.
The first is that LED's must be powered by a constant current source, not a voltage. The second is that the LED will get hot dissipating 1 watt if you do not use a heat sink. The life of an LED is greatly shortened if it is too hot to touch.

You need to use a switcher IC that has a feedback input that lets you set the maximum output current. I have never seen a buck/boost switcher for driving an LED but one may well exist. I have always used step-up switchers such as the LT1937.
 

#12

Joined Nov 30, 2010
18,167
You don't tell the LED how much voltage it is supposed to require. You give it the amount of current it requires and the LED shows you the voltage that corresponds with that current for that particular LED at the temperature it is at that moment.

As for buck/boost: Why make this so difficult? I have a LED flashlight in my pocket that runs on a single AA battery and a boost circuit. $6 and my incentive to work out a design and build something much more crude seems to disappear. However, you are free to experiment if you find it educational.
 

Thread Starter

Alex Jurtan

Joined Jul 14, 2015
14
Hello,

Thank you for the replies. I see what I have been doing wrong. I am still a beginner trying to hack something working together.
Are there any examples for using the LT1937? I can just find the Datasheet and it would help me alot if I could see a working project.

I thought a buck/booster is the same as a booster circuit. I am not sure what I have to look for. Does the voltage matter or just the current?

I bought this LED driver circuit board but I dont know how to find suited components (Like LED or power source).
It has a 800mA ouput, which is too high for my 5050 LED. It also does not say what output voltage it produces from a single AA battery.

Ideally I would like to make one circuit that can power a LED with 1.5V input or 4.5V. So either 1xAA or 2xAA or 3xAA, same circuit.

This is all really confusing still to me, but I hope you can give me some starting points. At the moment I am just reading all over the place and not sure what I am looking for.
 

dl324

Joined Mar 30, 2015
9,321
You're going about this wrong. Your LED has a typical forward voltage (Vf) of 3.2V, but it could be as high as 4.0V and no lower limit is given. Using a 3.3V supply isn't going to give you much brightness if your LED happens to be closer to 4.0V than 3.2V.

As mentioned earlier, LEDs are typically driven from current sources; but in applications where absolute brightness or brightness matching isn't important, a series resistor is sufficient.

A 20mA current source would be more appropriate for your "flashlight"; it needs to be able to provide at least 4V so any LED of the type you're using would give the same brightness.

LED "flashlights" without reflectors are of limited usefulness. I have made several of these on tiny strips of copper clad that operate from 6V SLA batteries to give some "ambient" light during power outages. As flashlights, they'd leave a lot to be desired...
 

#12

Joined Nov 30, 2010
18,167
Ideally I would like to make one circuit that can power a LED with 1.5V input or 4.5V. So either 1xAA or 2xAA or 3xAA, same circuit.
At the moment I am just reading all over the place and not sure what I am looking for.
A typical beginner, trying to be helpful, tries to work it out, asks for what he WANTS, and we spend 3 days figuring out how to do it the hard way. :( If you just ask for what you NEED this will hurt less.

I question the requirement to operate on 3 different voltage sources. Are you designing this for customers that can't count to two? They keep trying to shove three batteries into the flashlight tube? :confused:

Break it down to the basics. A dozen excellent people are trying to figure out what you NEED. :)
 

Thread Starter

Alex Jurtan

Joined Jul 14, 2015
14
Hi everyone,
First of all, thanks for being so proactive about my question.
I am sorry if I am making this sound very complicated and I also see that it might not be common to use 3 different voltage sources. I am building a flashlight that is flexible in size, which the user can adjust -> The reason why I am looking for such a circuit.

Basically what I NEED:
A circuit that can power a LED with one or up to three AA batteries and always have the same brightness.



This source shortly describes different power regulations in flashlights. What I understand is, that I need a Buck-Boost Circuit to either lower or boost the input voltage.
http://flashlightwiki.com/Driver#Buck-Boost_Circuit
 

#12

Joined Nov 30, 2010
18,167
What you need is a 20 mA constant current driver which will work from one volt to 4.5 volts input and is capable of producing 3.2v to 4v output, as needed. I can reference a page with similar, but not quite right circuits, but this is where I get off the bus because this is outside the range of my skills.

http://www.romanblack.com/smps/a05.htm
 

Attachments

Top