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Old 09-15-2011, 10:37 PM #1
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Default Adjustable current laser

Hello all,

I've been told by an experienced forum member that having an adjustable (via pot) laser (445nm 1W) is a bad practice. Is this true? if so, is there a better way to do it? thanks.


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Old 09-15-2011, 10:42 PM #2
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Default Re: Adjustable current laser

the problem i have read with pots is as you turn them they can do some nasty things or jump straight off value and give you a burst of power.
But those are only for very cheap pots.
If you get a good one besides from some electrical noise crated as you turn the pot it should be fine.
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Old 09-15-2011, 11:22 PM #3
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Default Re: Adjustable current laser

i would say having and external adjustable pot is ok..... IF... you have current limiting resistor set to the max safe limit you would want your laser to be. so if the pot goes, you still have good ole mister back up.

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Old 09-15-2011, 11:24 PM #4
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Default Re: Adjustable current laser

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Originally Posted by chipdouglas View Post
i would say having and external adjustable pot is ok..... IF... you have current limiting resistor set to the max safe limit you would want your laser to be. so if the pot goes, you still have good ole mister back up.

michael.
Of course I do
I'm more concerned about what kiyoukan said - is there anything I can do? inductor and/or small filter cap maybe? :/

I have a uh. I forget the wattage. I think its uh 5W or something. I know its definitely >2W. cost me 5 bucks. It's a wirewound pot.

Edit: and it goes from ~0 - 30 Ohms or something like that. in any case it changes the current between ~0.2 and ~1.3 A on my LM317 current regulator

Last edited by Limecat; 09-15-2011 at 11:26 PM. Reason: more info
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Old 09-15-2011, 11:29 PM #5
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Default Re: Adjustable current laser

well kiyu's reads similar to mine... the current jump wouldn't matter because of your resistor.

the main problem of pots is the wiper on them goes... but this is after tons of use. just get a good pot and the correct resistor and you won't have any problems.

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Old 09-16-2011, 12:41 AM #6
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Default Re: Adjustable current laser

There is nothing inherently wrong with using a potmeter to vary the current - it makes a fine input signal for any analog driver.

Things like wearing wipers and bounce can be a problem though... you should design the circuit in a way that it doesnt do anything bad when the wiper gets disconnected. Adding a simple pull down resistor and/or capacitor over the wiper to ground will likely solve this matter permanently.
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Old 09-16-2011, 12:42 AM #7
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Default Re: Adjustable current laser

That's what I thought too. But I was just told otherwise by someone, so I was here asking for clarification :P
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Old 09-16-2011, 01:57 AM #8
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Default Re: Adjustable current laser

On a proper driver that will not do you any good, as it connects the wiper to ground, and it will never output any current. Perhaps it will work with lm317 style drivers where the main current runs through the potmeter... but please just stop designing those!
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Old 09-16-2011, 04:29 AM #9
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Default Re: Adjustable current laser

When doing this, Im just gonna suggest do it slowly and with a stable circuit... like drlava stuff.

I also strongly suggest that if you potmod, use a very sturdy potentiometer, like the one on the boostdrive or the microdrive, or soemthing of similar sort, because you clearly do not want to short or damage anything in the worse possible event. Do note that the pot on the flexdrive 5 is rated for only 15 adjustments max, along with other pots that look like it... so be careful when doing this.

Lastly, eliminate static.
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Old 09-16-2011, 08:28 PM #10
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Default Re: Adjustable current laser

Um he has a 5w wire wound pot and you say he should use a "sturdy" pot like on a flexdrive? You know those little smd micropots are like 1/8th watt right? And far from sturdy.

You also say he should use a drlava driver... So you want him to take a flexdrive, remove the pot, and then replace it with a pot from a flexdrive? Please tell me how the hell that solves anything.

What he wants to know is if he can safely adjust the pot on an lm317 driver while it is running. I say that as long as you connect the wiper to one side of the pot and use a resistor in series with it to set max current then you should be fine. If the wiper ever comes off you will have the max value of the pot plus the resistor for Rsense so there isn't any chance of damage to the diode.
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