Old 12-17-2010, 02:04 AM #1
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Default pot before or after resistor?

About to get all my goodies in to make a DDL driver. Thought i had all my thoughts together when I just hit a kink. I've become unsure if I should put my pot right after the LM317 and before the resistor(s) like in this guide that so many people mention to beginners :
Laser driver - It can be done

Or if i should use a resistor across the LM317, before the pot like in this:

YouTube - DIY: How to build a laser diode driver

The second one seems more logical to me because the current will have been set at max and the pot will only adjust from max and downward.


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Old 12-17-2010, 02:16 AM #2
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Default Re: pot before or after resistor?

Shouldn't matter, resistors in series just add up, addition is commutative. Put whichever is rated for higher Watt first.
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Old 12-17-2010, 02:25 AM #3
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Default Re: pot before or after resistor?

Just use 1 resistor to set it. The pot is really not need as your not going to be changing the current once you have it put together.
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Old 12-17-2010, 02:30 AM #4
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Red face Re: pot before or after resistor?

Ohhhhhh That definately makes since. Gonna go with the pot first then since it's rated at 3W
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Old 12-17-2010, 02:34 AM #5
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Default Re: pot before or after resistor?

Only reason I thought I may need the pot is bc it's a lpc-815 and I was gonna use a 3.3 ohm resistor, setting it 380mA max and use the pot to turn it down if the hestsinks didn't do a good enough job. Still going to be my first build so I didn't know. But if you guys think I don't need it then I will not put it in bc it would definately keep my driver smaller.
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Old 12-17-2010, 02:36 AM #6
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Default Re: pot before or after resistor?

Post a Picture of your Heatsink. Then i can tell you if the current is going to be to high for that heatsink or if its enough if you dont mind.
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Old 12-17-2010, 02:53 AM #7
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Default Re: pot before or after resistor?

Ok don't hate me for this but I can't upload a pic bc I'm on my iPhone so I uploaded one to Facebook ha.
The pic shows the heatsink in relation to the LM317. The LD heatsink is quite large and it isnt really a concern I don't believe.

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Old 12-17-2010, 05:22 AM #8
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Default Re: pot before or after resistor?

How do you plain on mounting it to that heatsink.? If you are going to drill a hole in the center of the heatsink where the LM is sitting and stick the Aixiz module there you will be more then fine t 380mA.
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Old 12-17-2010, 09:01 AM #9
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Default Re: pot before or after resistor?

The aixiz module is going to be inside a much larger heatsink that I'm sure disperses heat better. That HS was for the LM317 itself bc I heard that it is the other component that gets hot. Is this more uselessness?
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Old 12-17-2010, 09:03 AM #10
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Default Re: pot before or after resistor?

I thought you were talking about the Diode getting to hot. Anyways that is more then enough heatsinking for the LM.

Good luck. Post some pictures when you finish.
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Old 12-17-2010, 09:25 AM #11
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Default Re: pot before or after resistor?

ok so back to the other topic, no reason to use a pot to lower it below 380mA then?
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Old 12-17-2010, 09:54 AM #12
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Default Re: pot before or after resistor?

Nope you are good to go at 380mA.
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Old 12-17-2010, 11:11 AM #13
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Default Re: pot before or after resistor?

Awesome. I'll post results when I finish up. Thanks for the help
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Old 03-01-2012, 05:34 PM #14
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Default Re: pot before or after resistor?

Quick question about resistor choice. Guys talk alot about 10ohm resistor. but I notice they come in different watts. Is there one that's better then others?
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Old 03-01-2012, 05:59 PM #15
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Default Re: pot before or after resistor?

1/4W or up is fine.
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