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Old 04-02-2011, 07:34 PM #1
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Default Is a variable resistor required for Drivers?

Hello, I'm currently trying to build my own laser driver. More specifically the one called LM317. I've read through the instructions (here Laser driver - It can be done) a few times but I cannot seem to find more detailed information on the specific use of the Variable Resistor/Multiturn Pot. I am trying accomplish a ~100mA output throught the driver, so I will be using a 15 and 68 ohm resistors in parallel. That should be enough to allow no more than about 102mA through correct? If so, what purpose does the variable resistor serve? Could I just replace it with a 100ohm resistor instead? I'm asking this partly because Radioshack doesn't seem to carry multivariable resistors with such small resistances, the smallest I could find was 5k Ohm =O

Thanks in advance for any help =D

Sorry I forgot to add that I found my calculations through the instructions found in that website, using the voltage as 1.5v.



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Old 04-02-2011, 07:43 PM #2
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Default Re: Is a variable resistor required for Drivers?

The pot is to adjust the current. A lot of people here, including myself, don't use them if you can use resistors alone to get the desired current. There is really now reason for them. You're fine with no pot.
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Old 04-02-2011, 07:45 PM #3
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Default Re: Is a variable resistor required for Drivers?

An adjustable resistor (potenciometer) is only needed if you want to adjust the current.
If you know you want a fixed 100mA, your 16+68 in parallel is just fine.
Just leave out the pot, don't replace it with a 100Ohm resistor, otherwise you'd have ~112Ohm -> ~10mA.
What diode, a PHR?
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Old 04-02-2011, 07:51 PM #4
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Default Re: Is a variable resistor required for Drivers?

Yes! Awesome! =D And yes, it's a PHR-803t. Actually, I might as well ask now. It has also been bothering me that in his instructions his picture says that a 9v source is needed in order to properly drive the laser. However, I plan on only using a single 18650 battery. How will that change my calculations?
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Old 04-02-2011, 07:55 PM #5
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Default Re: Is a variable resistor required for Drivers?

It won't work.....
The PHR LD will require 4-4.5 vollts...
the regulator dropout is 2.5 volts...
so you will need at least 7 volts to regulate
the current...

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Old 04-02-2011, 07:55 PM #6
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Default Re: Is a variable resistor required for Drivers?

you cant use a single cell...
you need to use atleast 2 cells
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Old 04-02-2011, 07:59 PM #7
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Default Re: Is a variable resistor required for Drivers?

Ohh, shoot =P My guidesman host only fits one 18650 (although I DO have two available...) haha. Well, since there's no other way around it?, i'll just wait around until I find a fitting host. Thanks so much guys =D
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Old 04-02-2011, 08:01 PM #8
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Default Re: Is a variable resistor required for Drivers?

and if you wanted to use your single 18650 batter you would need a boost type driver.
otherwise, like jerry said you need at least 7. but if you use 7 it is good to have a battery combo with a high mah rating so you don't fall out of voltage regulation too quickly. thats why it is recommended to use a higher voltage like 9. but not the 9vdc rectangular batteries. they don't supply enough current and drain too quick.

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Old 04-02-2011, 08:06 PM #9
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Default Re: Is a variable resistor required for Drivers?

If it fits one 18650, it might fit 2x16340 with a little modification of the spring perhaps.
Or one of those 3xAAA battery holders, with 2X10450 LiIons inside...
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Old 04-02-2011, 08:07 PM #10
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Default Re: Is a variable resistor required for Drivers?

Quote:
Originally Posted by chipdouglas View Post
and if you wanted to use your single 18650 batter you would need a boost type driver.
otherwise, like jerry said you need at least 7. but if you use 7 it is good to have a battery combo with a high mah rating so you don't fall out of voltage regulation too quickly. thats why it is recommended to use a higher voltage like 9. but not the 9vdc rectangular batteries. they don't supply enough current and drain too quick.

michael.

Well, I bought some Sanyo 18650 from LarryDFW, they're rated at 2,600mah so I assume that's at least enough. However, I'm surprised that a 405 LD needs so much voltage to function haha. When I bought the 18650 host from Jayrob I immediatly assumed it would be able to fit the whole bad boy haha.
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Old 04-02-2011, 08:10 PM #11
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Default Re: Is a variable resistor required for Drivers?

Quote:
Originally Posted by anselm View Post
If it fits one 18650, it might fit 2x16340 with a little modification of the spring perhaps.
Or one of those 3xAAA battery holders, with 2X10450 LiIons inside...
in my 6x build i use the 3aaa cartridge with 3 10440's and a rkcstr. it works pretty good. i only have it set at 150ma though.

@ rolando, look at what lasersbee posted about the voltage requirements..

driver needs 2.5, laser diode needs 4.5. you need those 2 voltages combined minimum.
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Old 04-02-2011, 08:15 PM #12
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Default Re: Is a variable resistor required for Drivers?

So yeah 7v minimum...Hmm, interesting... Well, the host did include one of those 3xAAA battery holders! I guess I'll order some of the 10440-10450 batteries that you guys are talking about. In the meantime I'll make a host out of cardboard to hold my 18650's =P lol.
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Old 04-02-2011, 08:16 PM #13
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Default Re: Is a variable resistor required for Drivers?

use the heatsink though
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Old 04-02-2011, 08:19 PM #14
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Default Re: Is a variable resistor required for Drivers?

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use the heatsink though
Oh yeah yeah, haha. I wouldn't wish to bust yet another diode XD thanks.
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Old 04-02-2011, 10:06 PM #15
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Default Re: Is a variable resistor required for Drivers?

in a lot of the 3 aaa cartridge hosts you can mod it to fit the 18650. i have. you will usually need to cut the tail spring. and since there will be wobble room, you will need to secure the indside as well.

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