Hello,

I'm trying to make an efficient LED grid backlight using x16 (5mm) white leds. The leds are rated with Vf of 3.2-3.6v and a max forward current of 25 mA. I have to use 3 AA batteries (4.5v) to power the grid.

To test the brightness I wired them up in parallel without resistors. It worked fine and the light output was very good. An online calculator said I should use a resistor (39 ohms) in front of every LED. I would also like the grid to have a high low setting, possibly mid level setting. My current plan is to use a two or three way switch to control light output...My questions are...

1) can I get away (reliable use) without using resistors to get the full brightness?
2) Can I use one resistor for the whole grid? instead of one in front of each LED. how do you calculate that resistance?
3) And if #2 is possible...Is using different resistors the simple/best way to control the light level via a 2 or 3 way switch?

4) I'm I missing another simple way hooking these LED's up? I would like to use the least amounts of parts, keep it simple but reliable, and keep costs very low.

5) What kind of life can I expect, or how is the calculated. I want the low setting to last a while but I know that will depend on the resistance and mAh of the batteries. What can I expect ballpark High/Low?

thanks

 

The only thing that stopped your LEDs from burning out was the internal resistance of the battery!!! Use the resistors!!!!!!

For the life do (Ah of battery) / (16x 25mA) and you will be left with hours

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Mike
My website: www.ElectroBird.net

 

Can I get away with one resistor for the group?

Life question if my 3 AA batteries are 2600mah wired in series do you use (2600) / (16x 25mA) or (3 x 2600) / (16x 25mA)?

 

No, you can't use one resistor for the full group you don't have enough voltage head room. To use a single resistor you'd need to run them in series on a 60 volt or higher supply, then one resistor would work for all of them.

Answers as I see them.

1) No, you'll have to limit the current somehow, a single resistor per LED at the voltage you're limited to is the dead simplest way of doing this.
2) No, see above.
3) Again no, to control brightness of an LED you have to reduce the voltage using a bigger series resistor or pulse the LED with a PWM signal. A bigger series resistor will use the same amount of power, it'll just be burned off in the resistor. You could use a simple 555 to allow variable duty cycle to the LED which will allow full range power control with very low losses for the least complexity.
4) You're at the limit, mind you the cost of micro controllers is very low now days and something to take into account, it may be simpler in the end to make the whole thing smart rather than trying to find some passive way to do it.

5) Given what you've said so far the LED's will use 3.6v (max listed) @ 25ma of power is going to be a constant 400ma draw during backlight usage. Using a stab in the dark estimate of 1200maH on AA batteries that's 3 hours of continuous backlight use.

More batteries in series would allow you to run more LED's with only a single resistor, this is good. More batteries in parallel would allow longer run times because of the increased total maH capacity available, however then you're dealing with 4 batteries, 2 sets of 2 in series in parallel. To make things more sane I'd say double your battery count to 4 series AA's, double up on the LED's so you have two in each series with only 8 resistors and use a 555 attached to either a switch for the duty cycle, or a POT so you can set the lighting level to taste.

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Simply look at the datasheet for AA alkaline battery cells:
1) Three in series make 4.5V when new which will burn out your 3.2V to 3.6V LEDs.
Your battery cells were not brand new. So your LEDs survived.
2) With a current of 400mA then three AA alkaline cells will drop to 3.0V in only 3 hours and you will see the LEDs dimming for the entire time.
3) Each LED has a different forward voltage which will cause some to burn out and others to be dim unless you test many and pick ones that are the same.

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Uncle $crooge

 

Thank you for the replies, my expertise is in graphic design not electronics. I see my option as...

Simple
1) 4.5v (x3 AA), 16 parallel LEDs, (16) 39 ohm resistors, switch -> no light control with an estimated 3 hour run time with decent light.

or Sceadwian's idea that might be beyond my knowledge base, but here is how I understand it..

2) 6v (x4 AA), 2 LEDs in series with one resistor ( ? ohm rating) -> Do that 'set' 8 times wired in parallel. (see image)


I really like the idea of using a POT. How do you spec a POT? or calculate?

I don't know anything about micro controllers or 555 timers, I've read about others on this site using them but didn't fully understand how to accomplish it myself.

Would my attached image work? What resistor should be used for the brightest/safest light output or how can I calculate that?

 

Your circuit will not work from 6V because you have two LEDs in series and they need at least 6.4V to 7.2V plus additional voltage for the current-limiting resistors.

Four brand new AA alkaline cells will start at 6V then will quickly drop to 4.8V. then slowly drop to 4.0V where they will be dead. Don't you want the lights to work for most of the life of the battery?

A pot will smoke and burn unless it is huge and expensive. A pot can feed a small current to a transistor that applies a variable voltage to the LEDs. The transistor needs to have additional voltage.

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Uncle $crooge

 

Ok, thank you audioguru. I was hoping that would work with a POT, I liked the idea of adjusting the light level, but big and expensive will not work for this project.

To keep it simple I drew up another basic plan and went back to the 3 AA batteries. Resistor A would be the 39ohm and resistor B would be a higher resistance to provide the low light setting.


I wouldn't be opposed to learning more about micro controllers to have more control but I have never used them and couldn't find anything similar to my needs and setup in other posts.

 

Now you have LEDs that slowly dim until the battery is almost dead.

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Uncle $crooge

 

I'm fine with that. On switch A I should get 3hrs of consistent light then they will start to dim correct? Switch B should last longer with consistent light, then dim to dead?

If I go back to 16 LEDs in parallel, 4.5v, 39ohm 1/4w resistor on each LED, what other components and where do I need to add them to use a POT. assuming 4.5v for 3.6v LEDs is enough voltage excess for a POT?

 

One a side comment, I like your schematics, they are, how to say, very graphical .

I should get to work on a standard schematic to good looking schmematic conversion software.

I personally would go for NiMH AAs to power your thing, because you can recharge them (obviously) and because they have a much flatter Voltage-stored power curve. This means intensity will not change too much depending of battery life left, which fits your static current limitation better.

Pretty much 1.4 full to the brim, falling a 1.2ish very quickly under load, and staying close to that until practically empty. (from memory)

You can find fujicell for really cheap on ebay and they have been good for me if you do not recharge them at more than third capacity/h (that is 4 hours charge give or take, taking in account losses), or go for long storage life ones, which have less capacity, but can be used as replacement for normal batteries, as they do not loose their charge over time by themselves.

Hope this helps

 

superfrog, I have read that about NiMhs and lithiums pros and cons and overall savings in the long run but when this leaves my hands the people are going to slap AA's into it out of convenience so I would like to design for them.

 

S, just to get things clear.

You need to light 16 white LED's, which generally have a Vf for 3.5-3.7V. For decent brightness you'll need at thevery least 10mA, 15mA average...20mA would be pushing it for standard 5mm.

Available power voltage is 4.5V, probably 4.0 - 4.7 for 3 AA's.

Now, simply using resistors, it seems because of the limit of your power voltage, you would need a resistor for each LED, very wasteful. The value of which would be:

R =V/I, = (4.5-3.5)/ 0.015 (for 15mA) = 1.0/0.015 = 66ohm. = 68ohm standard value.

Altohugh I'm sure you are after 'simplicity' rather than having to venture into the world of switch-mode power supplies, I would strongly suggest either purchasing a small boost convert module, or building one yourself.

The power you require per LED is: 3.5 * 0.02 (for 20mA) = 70mW max.
For 16 LED's thats: 16 * 70mW = 1120mW = 1.12Watts.

Using a boost power supply would not only be more efficient, but also may negate the need for many resistors. A 'true DIY' boost power supply would be tricky to build, so I would look around for modules on ebay or something, or even buying a cheap 'LED flashlight', which generally have built in boost power supplies, but that would probably need to be modded in some fashion.

If you decide to use a boost power supply, then let me know and I'll see if I can find a cheap and convenient way of doing it. Either by using standard parts, or, more likely, buying a relatively cheap 'chip' which will do almsot all thework for you.

Blueteeth

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Inconsistency is the key to flexibility!

 

Blue,

I'd like to keep this simple and inexpensive, primarily my cost in making it. I know the resistor route isn't the most efficient use of power not to mention there is 32 of them now to get high/low.

Would Sceadwian's micro controller suggestion be more simple. As I mentioned before I have no experience and would need some direction. Or using 4 AA batteries 2 in series and the other 2 in parallel, but again for the full brightness I would need a booster for the power to get 3.6v, I suppose...

here is my led spec from the manufacture: pdf link

 

The cel phone industry has this exact problem and they use simple boost devices like an LT1932 or similar part which is wired as a current source. That way the brightness of the LEDs does not change with battery voltage or temperature (which has a big effect on brightness). This increases battery life by not wasting so much power.

http://cds.linear.com/docs/Datasheet/1932f.pdf

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