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Old 05-19-2008, 07:52 AM #17
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Default Re: simplest boost LED driver, please modify...

That is what i wanted to do at first as well. You can boost the voltage to 10V (no load voltage) and then, when you pull some current this will drop, as the battery drops, it will drop even more, so you need to boost to more than the minimum needed.

In the end, the efficiency will not be very good, but it could work from a single AA cell. The 317 has around 30-40% efficiency,and the boost circuit can be 80-90% if it's good, so the combined will be low. But since you're only driving a Blu Ray at under 100mA most likely, it will work for a while. The current draw on the AA cell will be close to 1A tho.

The numbers are just a quick guess, and they depend on what you actually use and how good you adjust it. I would adjust it so, that the boost circuit would still provide enough voltage for the 317 to regulate, even when the battery is close to empty.


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Old 05-19-2008, 08:14 AM #18
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Default Re: simplest boost LED driver, please modify...

Quote:
Originally Posted by adgmeijin
in my test the output of this on mine was 41vDC without load, and when you put load it only gives out what the LED voltage needed

the current of this in the out was 300mA without load from input of fresh 1.5v AA, when you put LED load on it, the current goes down from 250mA to 150mA, depends on LED type.
BTW, there is no such thing as no-load current. There is no-load voltage, but not current.

You obviously put your DMM into a current measuring range, and put it on the circuit output. A DMM in current measuring mode is a short. It has no resistance (theoretically). By doing that, you shorted the output, and the circuit was putting out the max current it can through the DMM. Your DMM was the load.

If you were to measure the voltage with a second DMM during this, you would get approximatelly 0V. Doing this doesn't really tell you anything about the circuit's behavior and the fact, that the output was 41V without a load means it is pretty much useless for an LD. Even if you put a regulator after it, the current capabilities could be too low. You need a better, more efficient boost circuit with a regulator after it, or a real current regulated boost driver, but that will mean SMD components and PCB etching.
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Old 05-19-2008, 08:36 AM #19
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Default Re: simplest boost LED driver, please modify...

Igort thanks for the kind suggestion on how to use my DMM right, i hope i could find solution on this matter... :-/
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Old 05-19-2008, 10:26 AM #20
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Default Re: simplest boost LED driver, please modify...

Of course you can. Try the DX circuit i suggested with an LM317. That circuit packs more power, than the one you posted here, and can be adjusted with a pot. A 317 after it would keep a constant current, or at least prevent it from going over the top.

You could also get a driver from rckstr for $5 built or $3 kit, and use it with the same boost circuit, to get longer battery life and better efficiency than with a 317..
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Old 05-19-2008, 11:03 AM #21
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Default Re: simplest boost LED driver, please modify...

Oh, and about the dummy loads. Use several 1N4001 diodes in series, power them with 300mA from a 317 and measure their combined Vf (voltage across them). Then you will know exactly what diode they compare to or how many to put in series (maybe 5-6 in series). Try to get 4.5-5V voltage drop across them.


When testing, if it works, you need a lab style power supply. Then you can set the voltage to 1.5V and check if the 317 gets enough from the boost circuit to regulate. Then you slowly lower the voltage, while measuring the current the 317 is putting through the dummy load with a correct Vf (similar to the LD you will be using later).

When you see the current is dropping, adjust the voltage up and down just enough untill the current is still what you set it to. Then you will know what the minimum voltage is, where the boost circuit can still give the 317 enough for regulation. If it can go low enough (1.1V would be enough, less would be better), you can then use it with the LD. Don't forget the filtering i mentioned in the PM.


That's pretty much it i think. If you need a more detailed explanation, just tell which part you don't understand.
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Old 05-20-2008, 11:16 PM #22
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Default Re: simplest boost LED driver, please modify...

To be honest, this circuit is absolutely useless to power any kind of laser diode. It has no regulation at all, output power depends on the coil, transistor (and its temperature) and rectification diode, and probably shoots up and down by just looking at it.

This circuit is cute for driving white leds from penlite batteries (nearly dead ones work too), but thats all.

If you want to go switched mode, you should really look for a boost converter with good current regulation. Plenty of chips that can do that exist, but getting your hands on loose ones can prove very difficult.
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Old 05-21-2008, 06:03 AM #23
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Default Re: simplest boost LED driver, please modify...

I think he gave up on this circuit now. I was talking about a particular DX circuit, with a 317 or rkcstr's driver after it for regulation, since the circuit in this thread wouldn't supply enough to the driver.

He is going to try that now. But eventually, it might really be better to learn how to etch and handle SMD components and go for an "all in one" solution.
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