Using Linear 1930 for current regulator to run LEDs in series

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Jonathan

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Just some thoughts. The Linear 1930 uses a 'constant current' switching control method, not a constant output current. The output is regulated with a standard resistive divider and a 1.25V reference. In the usual connection the output _voltage_ is regulated.

You could use the device to produce a regulated output current, by placing a current sense resistor between the load and ground, and connecting the load-resistor node to the feedback pin. The device will adjust the output voltage until the voltage drop across this current sense resistor is 1.25V. This is similar to the technique of using an adjustable three terminal regulator as a current source.

The only problem with the above scheme is that you will have full output current at 1.25V across your sense resistor...if you are running something like single Luxeon, then you will see about 1/3 of the power used by the Luxeon simply dissipated in the sense resistor.

I'd suggest looking at the Linear LT1618, which has a 50mV current feedback as well as a 1.25V voltage feedback. It also has a _shutdown_ pin, which means that you could do the switching with a very low current switch, eg. a reed switch.

In terms of which LEDs to use, I think that the Luxeons will have the benefit of better efficiency and a tighter beam. If efficiency _of the LED_ is your only concern, then the Luxeon will do better both for high power lighting and for low power lighting. Ideally, for lower powers, you would pulse the Luxeon rapidly so that the current level remains high enough to prevent color shifts.

But using a Luxeon will present a very serious problem: the Luxeon is a single LED with a voltage drop of about 3.2V. You can't use a boost converter with a 4.5V supply to drive a 3.2V load. You would need some sort of 'buck-boost' circuit to deal with the battery voltage falling through the load voltage.

The losses in the boost diode (about 0.5V) and in the current sense resistor will also be a very large fraction of the load voltage, meaning that efficiency would be poor. So from the point of view of actually driving the things with a boost converter, I thing that you would do better with the series strings of 5mm LEDs rather than the Luxeon.

You might be able to get away with 4AA cells, operated 2 series 2 parallel to provide a nominal 3V supply. The 1618 will operate down to 1.6V, and the combination of the Luxeon and the boost diode will be sufficiently above the battery voltage that you won't have too much leakage through the boost converter.

Regards,
Jonathan Edelson
 
Thanks for your thoughts, Jonathan.

I think I was mistaken, and the chip I was actually referring to was the Linear 1932. It is a purpose-built white-LED driver, and can drive up to 8 LEDs in series, directly controlling the current thru them to match brightness. I think it would also increase the lifespan of the batteries, too, since it is using much less output current than a similar number of parallel LEDs would.

I am concerned about battery life, since this is one of the reasons that I am shying away from the lithium 123 batteries - cost. I use a flashlight a lot in my job, and so it needs to be cheap to run.

As for the LS, I would simply build the Zetex circuit as developed on this forum elsewhere. I don't think it allows for dimming, as they went for minimal components w/maximum efficiency.

Wayne and Mr. Al etc., could your circuit be added to to provide adjustable brightness? (I have a larger board area to play with, say 1" diameter, since this will be a one-off, scratch-built item).

Any and all help and comments appreciated!
 
I am trying to design a custom flashlight for my own needs, and really like the looks of the Surefire lights. I don't like the idea of the 123A batteries, though, so I am thinking of turning my own flashlight out of Aluminum. I am planning to use 3 or 4 AAA batteries instead of the 123's (should be much cheaper to run).

I am thinking to drive 2 series strings of 8 or 9, 5mm Nichias, at about 35ma each string. This will give me the option of having the longer runtime of one string, or switching in the extra brightness of 2. Naturally I would like to use a regulated setup, and looking at the Linear 1930 current source converter chip, it looks like a good chip to use.

What kind of battery lifespan do you experts out there think I'd get out of this kind of converter? I'm kind of set on using 3 AAA's side-by-side in a triangle for the form factor they give (a la the Ledda Clipper), but could also do the 2 pairs side by side like an Everready Double Barrel configuration, if that would be a much better solution.

I figure I could make the LED board in the head about an inch across, and have picked up some 1.25" Al round bar to make the housing. I'm using 1.125" round for the body/battery holder (it could be easily turned smaller).

Another question begs an answer, though: would it be a better light if I just bought a LS and used that for the light? I would want to provide a low setting for the light, either running the LS at low power, or adding a few 5mm Nichias around the LS. Which would be a better solution, in terms of battery life?

All of these questions, and I still haven't figured out a good waterproof switch. It will be up by the thumb on the battery holder body, (slide or rocker switch), and the light may have a tailcap momentary switch, although I've pretty much decided against that, (due to technical difficulties).

This light I intend to use in my capacity as a phone technician, while on the job. It'll be used and abused, as I am out in the weather a lot, and so it will have to be weatherproof (it'll never be used underwater, for example).

So, lots of questions, I hope you guys can help me out, as you've helped others.
smile.gif


Doug Speck.
 
dat2zip,

I've looked at the thread (been following it for some time, actually), and you seem to be driving a LS. This is OK, as it would be cheaper to just buy a LS than to buy all those Nichias, but does having a dimming function reduce battery consumption proportionately? That is the reason I was proposing having 2 strings of 8 LEDs, rather than all of them in parallel, or 2 groups in parallel. I am interested in battery life in this application, as I am usually working in close quarters, so max brightness is not my main concern. I see from your thread that I could use the Maxim 1675 for a max battery life solution.

Can your circuit be modified to use a higher input voltage, as from 3 cells? Since I want to use AAA batteries, are they able to provide enough current to run the circuit without damage or shortened lifespan? I know you are using AAs in your tests - do you have any AAAs you could test this with?

Thanks.

Thanks
 
The 1675 is probably not a good choice but is still possible. One, the Maxim part can only put out Maximum 6V and you cannot put more than one white LEDs in series with this voltage. (2 * 3.5V = 7). So, hooking up 5 or 6 in series is out of the question with this part. I was considering driving the Nichia as well as a single LS but, as noted above you cannot run the LEDs in a series string. The ZLT+ with a different current limit should be able to handle the current and voltage requirement. I need to check the Zetex specification.

The LT1932 might be a good choice. I'm looking at building a proto of the 1932 for use with the 5mm Whites and blues. You can use a simple pot to control the LED current. That's a nice possible option.

The Potentiometer on the fb pin of the MAX1675 allows you to change the output voltage which would control the brightness on the MAX1675. Efficiency stays up around 90% or higher until the output current drops below 10mA or so. Don't quote me on that, I'd have to check the data sheet for sure.

So, running the LS with the MAX1675 is a possible option and turning the voltage down for less brightness will give you a good battery life and a single LED solution.

The only reservation to putting together the circuit yourself might be rather tricky, is soldering such a tiny part on a board.
 
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by dat2zip:

The LT1932 might be a good choice. I'm looking at building a proto of the 1932 for use with the 5mm Whites and blues. You can use a simple pot to control the LED current. That's a nice possible option.
<HR></BLOCKQUOTE>

I worked already with the LT1932 but could not continue on it the last two weeks because of renovating my home. Here you will find my first results with this chip: LED driver with the LT1932 from Linear Technology

I already made a rev2 board with a bigger inductor (10uH instead 4.7uH) but it has lost 1 . 3% of its efficiency without any improvement of a higher output current at a low input voltage.

So i think this chip is for sure not usable for a single cell application. Using it with three or more AA/AAA/C/D cells would be a good solution for driving 3 to max. 7 white Nichias in series. In this case the LT1932 reaches a very good efficiency (look at the results in the mentioned LT1932 thread)
 
remuen, your results with the 1932 look excellent. This is what I was planning when I began this thread. I put the wrong chip number in the thread title, but don't know how to change it...
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Do you have enough components to do a test run for me? I would really like to to know how the circuit runs with 1, and then 2 branches of 8 series LEDs, in parallel with each other. I am planning to run my flashlight with 3 AAA Alkaline batteries, in series for 4.5v as Vin. If you could try this in your test, that would be great.

I'd do it myself, but I haven't got the components, nor a simulator. If you haven't got the parts (I realize 16 LEDs is quite a few!), don't worry about it.

BTW, which circuit configuration and component values are you using? You don't mention either in the other thread.

Doug Speck.
 
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by DSpeck:
Do you have enough components to do a test run for me? I would really like to to know how the circuit runs with 1, and then 2 branches of 8 series LEDs, in parallel with each other. I am planning to run my flashlight with 3 AAA Alkaline batteries, in series for 4.5v as Vin. If you could try this in your test, that would be great.

I'd do it myself, but I haven't got the components, nor a simulator. If you haven't got the parts (I realize 16 LEDs is quite a few!), don't worry about it.
<HR></BLOCKQUOTE>

DSpeck, I am just going to publish some more results in the correct
grin.gif
LT1932 thread.
 
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