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luxeon emitters

The three should work fine. However, using a simple pot means that all the current will have to go through it. At 350mA, if your battery is 13.8V, and the Vf is 3.42 (see

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for datasheet) then the pot will have to dissipate about 1.2W. Thus, one of those tiny radio shack pots will just burn up. If you do this, make sure the one you buy can take at least 2W.

One simple way around buying a pot like this would be to use a transistor to set the current. One approach is to use a single NPN power transistor (like one in a TO220 case) and use the pot to set the base voltage. This has the disadvantage that the current (and thus the brightness) will vary widely depending on the temperature. However, it may be good enough. Note that the NPN will dissipate that 1.2W as above, so it may need a heatsink, depending on location.

If more precision is required, a 1 ohm resistor between the NPN's emitter and ground can be used to measure the current. This can be used as negative feedback to an opamp, which then is used to control the base of the NPN with its output. By setting the voltage on the positive input, you can control the current with good precision, and the current won't change with temperature. A pot can be used to set the non-inverting input of the opamp. However, even more precision can be obtained by using a temperature compensated voltage reference. This way, your current will no longer be proportional to the input voltage.

However, it just occurred to me that you could use a low voltage TLV431, which uses a 1.2V ref, and a 3.5 ohm sense resistor, like this:

-----------------o-------------- 12V | | | | [led string] [330] | | c\\| | |-b---------o e/| | K | --- o---------R-/ \\ TLV431 | --- [3R5 1W] | A | |

----o------------o--------------- GND

When the current gets above 350mA, the TLV431 will turn on, pulling the base of the NPN lower, thus decreasing the current. This gives you an accurate, temp compensated current source which doesn't depend on the input voltage. It only requires a drop of about 1.5V at 350mA, so it can handle a voltage down to a bit lower than 12V with 3 luxeon emitters in series.

Reply to
Bob Monsen
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can you get away with running 3 luxeon emitters in series from a 12V supply?

I know that 12V means 13-14V in practice, so in theory it should be possible to set them up like:

12V --->|--->|--->|---[===]--- gnd

with the (variable) resistor set to whatever resistance was needed to get the right current from whatever voltage drop was left over. But is this likely to work OK in practice, or would it be better to use 2 in series and a more reliable 4-5V voltage drop through the resistor?

Also, is using a variable resistor a good-enough way of dimming the lights, or should I build a more complex circuit?

This is for lighting on a friend's canal boat.

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Reply to
andy

thanks - I'll try the transistor current source first, I think, and then try your circuit if that doesn't work.

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Reply to
andy

away with running 3 luxeon emitters in series from a 12V

that 12V means 13-14V in practice, so in theory it should be

them up like:

you didn't mention which luxeons you were planning to use

the nax forward volltagge is 4v so unless you intend using a switching regulator two in series is the highest I'd go - otherwise use a boos regulator and wire them all in series....

for lighting on a friend's canal boat.

from reading the specs it seems some of them may be effected by humidity (they mention humidity next to the longevity chart)

Bye. Jasen

Reply to
Jasen Betts

you didn't mention which luxeons you were planning to use

the nax forward volltagge is 4v so unless you intend using a switching regulator two in series is the highest I'd go - otherwise use a boos regulator and wire them all in series....

from reading the specs it seems some of them may be effected by humidity (they mention humidity next to the longevity chart)

Bye. Jasen

Reply to
Jasen Betts

luxeon star III, white, lambertian radiation pattern.

I've checked this since, and 4v is the max forward voltage, not the typical forward voltage, which is more like 3.6v, so I think I can get away with using 3 as long as the battery isn't run too low.

What I'm thinking of doing now is to make a dimmable current source with a voltage reference, op amp, and transistor, and see if that works.

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Reply to
andy

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