How do I regulate a Luxeon from 3.7v?

[BatteryCharger]

I'm making a light that will use one of the batteries from my cell phone, leaving the battery intact so I can still use it in the phone. The battery is 3.7v, about perfect for a Luxeon. But how do I regulate the current? I mean, if I use a resistor it would also lower the voltage right?

 

[Burnt_Retinas]

You use a boost converter.

Sounds odd I know, but all boost circuits will have circuit losses due to a diode drop and inductor resistance. This creates a situation where the boost circuit actually needs to perform the boost operation to boost the output to the right current. Even a fully charged battery will work ok (assuming LiIon/4.2V max charge), but it won't be boosting as such but rather like driving the LED via a series diode and inductor resistance. You shouldn't get more than a tad over 1A, perhaps say 1.2 max in this config (fully charged) with an LIII unless it's of particularly low Vf.

Works a treat. You'll have to live with the efficiency losses that come with the converter of your choice however.

>>>Edit<<< This assumes you want to drive an LIII and assumes current to be LIII @ max. A tad too little info in your initial post to provide perhaps the real solution. May help anyway...


Chris

 

[BatteryCharger]

I want to use a regular 1 watt, actually. So is there no way to do this with a simple resistor or something? Maybe at a slightly reduced light output?

 

[BatteryCharger]

^

 

[Ding]

Depends on what bin of luxeon 1W you are using. If you use the J bin, 3.27-3.51V Vf, then you can just use a dead cheap rectifying diode to drop a ~0.5V and lit the Luxeon without problem. If you can find a H bin, then you have more luck. The luxeon will be at its rated brightness and fully drain the battery.

 

[Burnt_Retinas]

A resistor doesn't regulate current, it merely reduces current, a value being chosen to prevent damage to the LED when the batteries are fully charged.

Your options are resistor (doesn't regulate), SEPIC converter or perhaps a low dropout linear regulator.

Chris

 

[Doug Owen]

This sounds like an ideal application for a Low Drop Out linear regulator as suggested above. I've been using one made from the LM334 for some time now with much success. The version driving the LS at 350 mA uses that IC, a transistor, a small capacitor two resistors and a piece of wire (as a third low value resistor). Typically .5 X .75 inches when built. Should be cheap, give you constant output, and be more efficient than any other solution I would think.

Doug Owen

 

[Chop]

If you want regulated, you'll need to use a circuit. I've modded many lights using surefire boost converters and badboys to drive 1W and 3W luxeons, feeding them with Pilas. I typically use J and K Vf luxeons and the luxes are driven to bias.

This comment only holds true for the current badboys, as I don't know about the new ones that are supposed to be coming out.

 

[Crux]

I'm looking at using the Fairchild ICL6363 Step-Up Step-Down regulator.
It is specifically designed to work with one Li+ cell. Output is limited to 500mA.
I haven't tried it yet but, i'm gathering parts to test it...

Crux

 

[BatteryCharger]

I would love to make some sort of regulator for this, but putting together anything on a circuit board is above my abilities. I was just thinking about my everled though. How do they get a regulator like that in such a small package? It must be pretty simple and not need a board. Are there any schematics anywhere for such a device?

 

[Doug Owen]

[ QUOTE ]
BatteryCharger said:
I would love to make some sort of regulator for this, but putting together anything on a circuit board is above my abilities. I was just thinking about my everled though. How do they get a regulator like that in such a small package? It must be pretty simple and not need a board. Are there any schematics anywhere for such a device?

[/ QUOTE ]

Yes, they are about. But don't confuse small with simple. The circuit involved is complex, using surface mount parts, it is far from simple and for sure uses a (quite demanding to build) board.

The sort of regulator I suggested is probably the best trade off between easy to build and high performance. If 'anything on a circuit board' is above your abilities, for sure the sort of circuit in the Everled will be there...with bells on.

Send me a PM, 'let's talk'.

Doug Owen

 

[tylerdurden]

Why not just direct drive it? Luxeon-I emitters routinely handle up to 4.5v with no problem. 3.7v should be just fine.

 

[JonSidneyB]

remember that Litium Ion will actually be closer to 4.2 than to 3.7 at first.

 

[3rd_shift]

The amount of current most cellphone li-ion batteries can put out is also somewhat limited.
A small cellphone battery may simply be able to direct drive that luxeon.
Ever notice how some li-ion cellphone batts drop considerably in "charge remaining", after a short conversation?
And then comes back up a bar, or two?
Many of these batteries can only supply a limited amount of current.
Many cellphone batteries are not the high output/high quality ones, like the Pilas that jsburlys has.

Assuming that your luxeon has a nice 486 sized heatsink block to mount onto,
this may be simpler for a circuitboard greenhorn.
#1 Try a high wattage reostat variable resistor in series with that setup, with a series amp meter, until current stays just under 350ma no matter what.(less is ok, if you don't mind it being dimmer and cooler running)
#2 Then measure the reostat's ohm reading.
If you have the reostat at near zero added resistance, then you may direct drive that badboy, and can skip the following steps and enjoy it in direct drive. /ubbthreads/images/graemlins/thumbsup.gif
If there is some added resistance, read on.

#3 Go get a couple 1/4 to 1/2 watt resistors with the same added resistance for the luxeons, and carefully solder them into the circuit with electronics solder, and a small 25 watt soldering iron.(not your grandaddys 100 watt one)
#4 enjoy. /ubbthreads/images/graemlins/smile.gif

 

[BatteryCharger]

I don't know how much current this battery will put out, but it's a 1000mah battery and my phone will run it dry in 2 hours. Even less if I'm on the internet, so it must be capable of 500ma+. I have a bunch of 1 ohm resisters, I guess I'll just try it out with some of those and see what happens. First I have to make some way of connecting to the phone battery without damaging it.

Doug - do you have a schematic for the circuit you're talking about? Might it be possible to make it without a circuit board, and then encase the whole thing in epoxy? My soldering skills stop at soldering wires together. /ubbthreads/images/graemlins/smile.gif

 

[Doug Owen]

[ QUOTE ]
BatteryCharger said:
Doug - do you have a schematic for the circuit you're talking about? Might it be possible to make it without a circuit board, and then encase the whole thing in epoxy? My soldering skills stop at soldering wires together. /ubbthreads/images/graemlins/smile.gif

[/ QUOTE ]

Er, ah, of course I do, I just can't post it (it's the Luddite part of me....). Try National's LM334 specs , it's the circuit called "Higher Output Current", right side of page 8. For a sense resistor I use a couple feet of wire wrap wire for full blast LS driving. You also need a hotter transistor, I use one of the Zetex PNPs.

Yes, you can build it on perf board, or even 'dead bug' if you're good, and epoxy or otherwise insulate it (minor heat issue in the transistor), I've done both.

Tell ya what, send me a PM, let's see what we can work out?

Doug Owen