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Old 03-17-2012, 07:57 PM #1
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Default Variable laser power?

Hi all,

Is there any driver out there that can adjust laser brightness like a variable power supply? If not, how can that be accomplished? I want to change my laser output to make it brighter or dimmer. Thanks!


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Old 03-17-2012, 08:10 PM #2
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Default Re: Variable laser power?

I'm not sure of the specs, but I've read you can put a pot between the diode, and driver. I do know regulating the power to the driver will not work.
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Old 03-18-2012, 12:57 AM #3
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Default Re: Variable laser power?

^Not on a constant-current driver. That would just serve to increase the voltage output of the driver to overcome the resistance you introduce - possibly to the point of damaging the driver.

Pretty much any driver that has a pot can do this. However, the pots on most drivers are not meant to be twisted as often as you might intend and they require small screw drivers. In other words, they're not beefy enough to stand up to constant adjustment. Either build your own driver with a panel-mount pot, or replace the pot on an existing driver with a panel-mount pot.
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Old 03-18-2012, 02:10 AM #4
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Default Re: Variable laser power?

You know I asked this same question a while ago, and was told to put a pot between the diode and driver.

What drivers are not considered constant-current? And if you add an panel mount pot, how do you limit the current so that you dont fry the diode?
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Old 03-18-2012, 02:57 AM #5
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Default Re: Variable laser power?

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Originally Posted by Tech_Junkie View Post
And if you add an panel mount pot, how do you limit the current so that you dont fry the diode?
By putting a resistor in series with the pot. The resistor's value is set to allow the maximum safe current through. At this point the pot serves to only reduce the current further, as a dimmer.

The first problem is that the pot needs to withstand the electrical wattage of the diode. For example: a 1W blue is 4.5V @ 1A, which is 4.5W of electrical power. Pots over 3W rating tend to be expensive and bulky. You'll also need fairly low value resistance for this task.

R = V / I

R = 4.5 / 1

R= 4.5 Ohms. So your series resistor should be 4.5 Ohms and also be able to withstand the 4.5W load. A 5Watt resistor should do fine. This is just for a 1W laser, once you start trying to dim 1.8A monsters you're looking at expensive parts.

You want your pot to be of the same order of magnitude as your resistor generally unless the load has a very wide range of operation. In this case, 5x additional resistance would yield a drop to 16% power, which is probably good enough. So, you would want a 5W rated pot with a value of between 20 and 30 ohms.

So:
LD+ lead from driver to 4.5 Ohm 5Watt resistor (or resistor network) to wiper (center pin) of 5Watt ~25 Ohm pot. Right pin of Pot (with the shaft facing you) to LD + pin.

Problem two: anything except a linear driver will try to raise the voltage being applied to the load (diode) to compensate for the reduced current getting to the load. This means that effectively only LDO and DDL style drivers can be used for this.
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Old 03-18-2012, 08:51 AM #6
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Default Re: Variable laser power?

Thanks for the reply everyone!

From what I've read, I need to use a linear driver and attach a potentiometer behind it and a resistor to control the current? Does anyone know a durable potentiometer that will withstand being adjusted constantly? Also, I've heard that linear drivers are not very efficient. Would that be a problem running say, a 445nm diode of 500mW-1W of power? Also, green DPSS modules can also be powered this way?

Thanks again!
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Old 03-18-2012, 09:19 AM #7
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Default Re: Variable laser power?

5W 25Ohm Pot
http://www.mouser.com/ProductDetail/...g0ppBEzrwiM%3d

5W 4.7 Ohm Resistor
http://www.mouser.com/ProductDetail/...GIwh1ZiapqA%3d

Then just build a simple "DDL" LM317 constant current driver set for 1A to use with a 445nm diode.
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Old 03-18-2012, 01:38 PM #8
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Default Re: Variable laser power?

Thank you sir.
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Old 03-18-2012, 05:17 PM #9
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Default Re: Variable laser power?

Wouldn't it be more effective and efficient to use a PWM circuit? They can be pretty compact. This way you are not burning so much energy as heat through a resistor.
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Old 03-18-2012, 05:22 PM #10
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Default Re: Variable laser power?

Quote:
Originally Posted by Jubathoph View Post
Wouldn't it be more effective and efficient to use a PWM circuit? They can be pretty compact. This way you are not burning so much energy as heat through a resistor.
Speaking of PWM, since you introduced the subject, couldn't we run these diodes @ currents over ~1.8A if they were PWM... Any chance we could get higher outputs that we currently are with CW, if we were using pulse width modulation ?

Has it ever been experimented with ?
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Old 03-18-2012, 05:53 PM #11
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Default Re: Variable laser power?

Your approach may be possible but I havent tried that. What my approach is to set the max current and vary the duty cycle from 0 to 100%. At 100% duty cycle the diode is essentially operating at constant wave. IIRC All you need is:

(1) lm555
(2) 10 microF Caps
(1) cap any value >100microF to set the frequency
(1) Potentiometer
(1) diode


I could be missing something but this is pretty much it. The duty cycle is controlled by the Potentiometer and the frequency is controlled by the cap. You can get some neat effects by using a large cap because it slooowwwsss ddddooooowwwwwnnn the duty cycle. There are several threads that go into much better detail than this if you are interested.
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Old 03-18-2012, 05:56 PM #12
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Default Re: Variable laser power?

Quote:
Originally Posted by Jubathoph View Post
Wouldn't it be more effective and efficient to use a PWM circuit? They can be pretty compact. This way you are not burning so much energy as heat through a resistor.
The problem is unlike with a motor, a short burst of power, at say 1.8A, doesn't put out less light than a long burst of 1.8A does, only the time on changes. In other words the eye hazard doesn't diminish when using PWM. PWMing a motor you control the speed because the motor doesn't have enough "on time" to reach full rotational speed. With a laser the very uSec the diode see's 1.8A it's putting out >1.7W, there is no equivalent to a spin up time. All PWMing a laser does is reduce the heat generated by the diode because it is extending the duty cycle. It's just as dangerous to yours eyes and afaik just as bright.

Also, diode's don't only die from thermal reasons at high currents, the sheer amount of light generated destroys the optical cavity. It's called COD.. catastrophic optical destruction. That's the primary reason diodes fail when pushed too hard. PWMing at a high current won't prevent COD, you could PWM at the highest "safe" current and squeeze out some extra lifetime but it's not worth it to most people.
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Old 03-18-2012, 09:00 PM #13
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Default Re: Variable laser power?

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Originally Posted by Sigurthr View Post
5W 25Ohm Pot
026TB32R250B1A1 CTS Electronic Components Potentiometers

5W 4.7 Ohm Resistor
MOSX5C4R7J KOA Speer Metal Oxide Resistors

Then just build a simple "DDL" LM317 constant current driver set for 1A to use with a 445nm diode.
I have a 200mW green operating at 500mA and 2.0V after the driver. Is this normal? Also, to dim it I would need a ~1W resistor and pot?
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Old 03-18-2012, 09:11 PM #14
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Default Re: Variable laser power?

Yeah that sounds pretty darn efficient actually, if it actually is doing 200mW anyway... Remember greens are pumped by high power IR diodes. It's not uncommon for 5mW green to use 200mW IR.

Right, your resistor and pot need to withstand at least 1W, and it's always better to overshoot by a bit, so you want 1.5W or higher. Remember you can use multiple resistors and add up the power ratings - four 250mW resistors = 1W rating.

You'll need a 4 ohm resistor (or four 1Ohm 1/2Watt resistors in series - very cheap) and you can use the same pot I linked above.
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Old 03-18-2012, 10:05 PM #15
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Default Re: Variable laser power?

Just wondering, how do you know figure out the pot to use? I understand the resistance of the resistor can be calculated using Ohm's law but I'm confused about the potentiometer. Thanks!
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Old 03-18-2012, 10:42 PM #16
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Default Re: Variable laser power?

I just look for what resistance will yield an 80% or greater reduction in current.

R = V / I
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