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Old 10-07-2007, 11:58 AM #1
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Default 1 question about lm317

im going to use 2 cr123's so i will have 6v.
1 laser diode
1 lm317
and 1 capacitor

so would it be alright if i just use these and will they give me the right voltage?
If the lm317 is used as a current regulator, then will i always have that amount of current until the batteries dies out completely

eg i will use a resistor and a pot
1.25/.0033=378ma
so will i always get that much current input until the batteries are almost dead and not like normal batts where they start to dim after a while.
thanks


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Old 10-07-2007, 12:39 PM #2
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Default Re: 1 question about lm317

Quote:
Originally Posted by izzy007
im going to use 2 cr123's so i will have 6v.
1 laser diode
1 lm317
and 1 capacitor

so would it be alright if i just use these and will they give me the right voltage?
If the lm317 is used as a current regulator, then will i always have that amount of current until the batteries dies out completely

eg i will use a resistor and a pot
1.25/.0033=378ma
so will i always get that much current input until the batteries are almost dead and not like normal batts where they start to dim after a while.
thanks
You are cuting it close. The drop across the LM317 will be between 1.5-2.5V and another 1.25V across the sense resistor. That's 2.75-3.75 V drop total before the load.

If you want the circuit to stay in regulation uuntil the batteried are almost dead (~2 V each) that would give you an operational battery range of 4-6V. The LD will require around 2.8-3V to operate at 378mA. So:

For fresh batteries: 6-(2.75->3.75V)= 2.25->3.25V ---> you may be OK
For dead batteries: 4-(2.75->3.75V)=0.25->1.25V ---> not enough to remain in regulation

With your 0.0033 figure above, I assume you are using milli-ohms, otherwise the calculation would yield 378 Amps!
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Old 10-07-2007, 03:23 PM #3
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Default Re: 1 question about lm317

okay i bought this
i connected the resistor to pin 2 and then a pot. joined the pin 1 to it and pin 3 is input
is that correct.
|------------
| | |
|___ | __ |
| | |input
adj out
| 3ohm resistor
| |
| Pot
\ /
\/

http://www.maplin.co.uk/Media/PDFs/Module%208067-2.pdf
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Old 10-07-2007, 06:09 PM #4
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Default Re: 1 question about lm317

Diagram looks ok as long as you connect your positive wire to the LD to the "adj" end of the pot, as you may be indicating with \/ on your drawing. I have not tried at 3ohms, it may be a tad low........
Good luck with it anyway.

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Old 10-07-2007, 06:28 PM #5
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Default Re: 1 question about lm317

Quote:
Originally Posted by rog8811
Diagram looks ok as long as you connect your positive wire to the LD to the "adj" end of the pot, as you may be indicating with \/ on your drawing. I have not tried at 3ohms, it may be a tad low........
Good luck with it anyway.

Regards rog8811
ok thanks
for a 20x diode, i think 3.3 ohms should be fine
which should give around 378ma input and about 200mw output
yeah the adj ang out wire of the pot are connected together like this
|----------
| |
|__ |_|_ |
| \ /
in adj/out

thanks
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Old 10-07-2007, 07:02 PM #6
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Default Re: 1 question about lm317

You're ok, but the problem is stated above in what Chimo wrote. As soon as the batteries begin to drop in voltage, so will the regulator. You would be much better off using 2 RCR123's.
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Old 10-07-2007, 07:14 PM #7
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Default Re: 1 question about lm317

Quote:
Originally Posted by Gazoo
You're ok, but the problem is stated above in what Chimo wrote. As soon as the batteries begin to drop in voltage, so will the regulator. You would be much better off using 2 RCR123's.
the only thing is that they are quite hard to find and rechargeables ones are only 3v i think
i just wanted to know that if it will output the same amount of current if the voltage is around 5-6v rather than it dimming when the batteries start to die out when using just a pot.

i'll probably only use it for a total of 1-2 hours before giving the batts a recharge, just waiting for my parts to arrive.
so here goes my 6th diode.
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Old 10-07-2007, 07:32 PM #8
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Default Re: 1 question about lm317

Rechargeable lithiums come off the charger fully charged at 4.2 volts. From there they discharge rapidly to around 3.6 volts and then slowly discharge. You would need to be careful not to over discharge them....this can result in batteries venting or even exploding. But treated properly, they can be safe. If I recall correctly they should be charged when the voltage reached is 3 volts.

Honestly I do not know how well CR123's hold up at various loads. Try them out, and if you see the voltage drop or sag when the laser is powered on you might want to think about moving to rechargeable's. You can monitor the voltage across the voltage out and adjust on the LM317. When the voltage there is anything less than 1.25 volts, the regulator is beginning to drop out.
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Old 10-07-2007, 07:44 PM #9
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Default Re: 1 question about lm317

alright thanks
i just figured out that i ordered the 3*aaa battery torch rather than the cr123, i'll see if they fit in there otherwise would it not just be a good idea to get a single cr123 battery torch and use a resistor and just a pot with that?
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Old 10-07-2007, 08:02 PM #10
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Default Re: 1 question about lm317

Well it depends on if the torch has a regulator built into it. Remember most 1 watt LED torches should work. Probably the cheapest way around all of this would be to go ahead and use the 3 aaa torch with this regulator:

http://www.dealextreme.com/details.dx/sku.3160

It will work very nicely with 3 nimh aaa batteries and there are no risks..I plan on using one with an open can diode.

Stone has tested the AMC7135 in this thread...in case you missed it.

http://www.laserpointerforums.com/fo...num=1191417983
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