Help with voltage regulation...

lm2575 constant current regulator

Hello all, I'm VERY green to electronics, but after reading some information on small circuits, I'm trying to design one that will power 20-30 3.6v 20ma LED's for around 9-12 hours. I'm orgininally was thinking about using 7.2v batteries with a LM137 circuit, something like:

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However, since this requires about 3v of headroom, I'm a little worried about the battery voltage dropping during use. My main problem is that I ABSOLUTELY need the current (and voltage?) to stay constant in the LED's because it is a certain wavelength that it really needs to be at or extremely close to. So I guess I need something to regulate the voltage as it drops from the battery and keep it at a constant voltage and then use resistors to keep the LED's at a constant current? Again, this is my first electronics project and first time I've ever dealt with anything electrical, so please excuse my ignorance, but any help would be GREATLY appreciated.

Thanks![/img]

lm3578 substitute

317 voltage regulator is not a very good choice for battery based applications.
You will be loosing alsmost 50% of battery power into heat on this linear regulator.
Much better option will be a switchmode step-down dc/dc converter such as LM2572ADJ (adj stands for adjustable):
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(free samples option)
It will easily convert 7.2V to say, 4V and up to 1A of current.
The rest of your circuit remains the same: resistors and diodes, but in order to further extend battery life, I would rather lower the voltage to say, 3.8V (or even lower) and lower the values of resistors to <10Ω (or less) ..
Regards,
IanP

Ok thank you very much. I would need to keep the output voltage higher than 3.6 though to be able to use resistors to get the correct current for the LED's correct? And when you say the rest of the circuit stays the same, you mean keep the capacitators also and just basically substitute the LM2572ADJ for the LM317? Also for the R1 and R2 resistors, if it still uses them, I heard it was better to use a combination of higher ohm resistors to achieve the voltage rather than lower ohm resistors. Do you have a schematic of a circuit using one of these? EDIT: Sorry I just saw the picture of the schematic you put up. Thanks

Thanks again for the help!

Download full data sheet from link provided in my previous post. In this document you will find application hins on Scotky diode selection (in your case you can use very popular 1N5817), output capacitor size (again, in your case output ripple voltage is not very important, so you can select smaller capacitor than 2000µF) etc. etc.
And finally, consider the output voltage at ≈3.8V, then for ≈25mA diode current you will be using ≈8.2Ω resistors ..
Regards,
IanP

Ok well I ordered 2 samples.. may I PM you if I need any help when they arrive? I appreciate the help you have given thus far. One question though.. the T package says it can only support 8 - 40v? Is that right, do I need another package to use say 6 or 7.2v?

Thanks.

The iput voltage of 7.2V can be a problem, specially when the battery voltage drops, and the LM2575 may not cope well in this situation (there is only one type of 2575, no other input voltage ranges are produced).
Safer will be to use another switchmode IC, the LM3578, which input voltage can be as low as 2Vdc .. (see circuit below) ..
Here is its data sheet:
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So better oreder samples of LM3578 too. It wouldn't heart to test both of them ..
Regards,
IanP
PS: PMs are OK ..

Ok, Thanks. Again, that circuit doesn't look too bad but I may need some help figuring out what ohm resistors/capacitators/whatever I would need to get it down to 3.8v's. I've tried to use Nationals workbench thingy but it does it for AC and sometimes it doesn't work. The LM2575T-ADJ says it can support a minimum of 4V's in... but then some places it says 8V in.. so I dunno what's up with that. I was also looking at a LM3475 possibly?

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Here's probably a stupid question, but how come this works:
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Yet this does not:
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Yes to complicate things more.. I need a photoresistor circuit that turns the LED'S off in light and on in dark. I figure this could just be put after the lm2575's or whatever's Vout and the LED's. Several ways I could think to do this, but who knows if any of them are right. Especially since the LED's will be in parallel...

Thanks again for all the help.

I also made the lm317 circuit using 4.5v in and dropping it to 1.85v... I felt no heat at all on the LM317 after a couple of minutes. Is the voltage so low that it won't matter.. and if so would it be about the same if I dropped 7.2v to 3.8v? Still being a little worried about the 3v headroom..though I've also seen places that say the lm317 only needs about 1.6v headroom.. it's a simple circuit that's for sure. Of course I was only running one 1.7v 20ma LED off of it.. maybe with more led's and more and drawing like 600ma it would heat up alot more...I guess it has alot more to do with battery effiency also?

Added after 43 minutes:

Someone on another forum suggested this:

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looks like it may be a preassemble buck or some sort? It says up to 16v output 30v input and 1000ma for the CC5WA.... maybe this would be my best best instead of trying the circuitry myself? Does it look decent?

Firstly to your 2 circuits:
1. At the transistor's base you need just ≈0.7V to conduct enough current to turn on the LED. In "saturation Uce can go below 0.5V, so you will have ≈2.5V for LED..
2. In this case the Ub voltage, to turn this transistor on, will be Uled+0.7V and that will exeed your 3V battery supply. It is possible that in a dark room you will see the LED just "on"..

I need a photoresistor circuit that turns the LED'S off in light and on in dark.

Click to expand...

Some voltage regulatrs have a dedicated input called "shut-down". You can use additional transistor (OC + puul-up resistor) controlled by a photodiode or phototransistor to switch the regulator OFF.

I also made the lm317 circuit using 4.5v in and dropping it to 1.85v... I felt no heat at all

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This is because the current was vety low and thus the heat generated by this regulator was low too. Its package can despatch some limited heat ..
If the you drew more current the situation would be totally different..

looks like it may be a preassemble buck or some sort? It says up to 16v output 30v input and 1000ma for the CC5WA.... maybe this would be my best best instead of trying the circuitry myself?

Click to expand...

This stuff is a constant current regulator and in your case you rather need sm voltage regulator ..

If I were you I would build something by myself.
I know there are hndreds of options but at some stage you will need o make up your mind and just build something ..
Try the below circuit .. disconnect pin 7 from the circuit and with an external transistor (described above) you will be able to turn the output on/off ..
Regards,
IanP