LED drivers ?

Hi,

I'm new to this topic and beginning to look at all of the info available in this forum.

I'm sure it is a frequent subject, but I could sure use some advice on available white LED driver options. There is just some much info to sort through.

I'm doing a project that will use 6 or 8 nicha LEDs (3.6V, 20 mA)in parallel using alkaline batteries for juice. I'm hoping this will not turn into a huge electronics project.

I'm going to need high efficiency, low cost and a low parts count. What is a good, simple scheme?

Are there any off the shelf boards around? With the popularity of white LEDs seems like there should be some ready made stuff.

Any help is appreciated!!

Mark E

At that low power, you should just use some series resistors in some vector-board

3x2AA - is going to be 4.5v, you will need to "get rid of" .9v, using good old Ohms-Law, that would be

R=E/I
R=.9/.020
R=45 Ohms

More reliable to use 1 45ohm resistor (51 is closer) per LED

It's better to drive LED arrays in series so you don't have to worry about the variations in Vf between LEDs. Try this circuit (it works with multiple LEDs in series as well as a single LED):



Efficiency will probably be in the mid 70%s. This circuit can be made to give better efficiency by substituting Q2 (and Q1 if desired) with a ZTX449. Naturally, you'll have to adjust R2 to get the desired LED current. If you decide to use a simple resistor limiting scheme instead then use an individual resistor for each LED so that each LED has (more or less) the same current flowing through it.

Thanks for the help, Mike. I guess my concern to just splitting the voltage is how fast the output from batteries drops. Once the LEDs are seeing 3V they are a bit dim. I'll set it up that way and test.

I tried the LT1086 ldo regulator, but it seemed to heat up and eat the batteries.

Like your site. I spend about half my time in White Salmon, across the river from Hood River. Hope you dodged the weather.

thanks again!!

mark e

Thanks for the help.

I'm actually a bit far into this thing and am hoping to use 3 alkaline C cells for power. Will this circuit run with 4.5v input?

What sort of circuit is this? A switching regulator?

mark e

Its just best to keep it simple, run 3xC batteries and one 45 ohm dropping resistor per LED

[ QUOTE ]
Mark_E said:
Thanks for the help.

I'm actually a bit far into this thing and am hoping to use 3 alkaline C cells for power. Will this circuit run with 4.5v input?

What sort of circuit is this? A switching regulator?

mark e

[/ QUOTE ]
Correct, the circuit is a step-up switching regulator. It runs just fine at 4.5V. It has no regulation, so as input voltage increases so does LED current, and when input voltage exceeds the forward voltage of the LED string, the LEDs end up driven directly off the battery (less the small series resistance of the inductor). Naturally, this isn't a problem with a string of multiple LEDs, but with only 1 LED the circuit would go to direct drive with 2 fresh alkalines. I used this circuit (with a few modifications) to drive my bike headlight consisting of 2 series strings of 10 LEDs (total Vf ~30V) off of 4 AA NiMH cells. The limit to the total number of LEDs is based on the Vce of Q2 and the average Vf of whichever LEDs you are using. In my case, the average Vf was about 3V and Vce was 30V, so I had to use to two strings of ten LEDs instead of one string of twenty. Efficiency is around 87% with my mods. Mid 90s should be possible with a few tweaks which I'll incorporate eventually. There is no regulation, as I mentioned, but battery voltage with NiMH cells is pretty steady anyway so there isn't much variation in light output until the cells are almost fully discharged anyway. Battery life is around 5 hours running the array at about 1.6 watts.

For your purposes I think a few series resistors are better unless you're very comfortable with electronics. The Vf of Nichias is somewhat more than my LEDs anyway, so you're not wasting as much energy in the "ballast" resistors as I would be.