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Old 01-12-2014, 01:16 AM #1
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Default PWM drive: increased operating times?

Hi everyone,

Does anyone have any good resources for the benefits of driving a laser with a PWM signal?

I'd like to know if pulsing the laser will increase the operating time and reduce the rate at which the diode heats up, but I don't have any resources to see just how much of an improvement I'll get.

I'm comfortable setting up a temperature circuit and logging temperature with various frequencies, but I suspect that there must be some resources out there that says whether or not pulsing the laser will increase its operating time.

Thanks!


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Old 01-12-2014, 01:22 AM #2
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Default Re: PWM drive: increased operating times?

If you operate it at PWM speed/width where it appears identically bright to how it was when running CW you won't get any thermal improvement. PWM is useful as a means of dimming the perceived brightness, but that is all. Unlike analog dimming, PWM dimming doesn't actually reduce the danger of a direct hit.

In other words if you don't mind it appearing half a bright you can run it at 50% duty cycle PWM, and generate half the amount of heat when averaged across a large enough time scale (usually seconds), but this is no better than running at half brightness by reducing driver current.
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Old 01-12-2014, 01:28 AM #3
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Default Re: PWM drive: increased operating times?

So driving a laser with a pulse train signal will only reduce apparent brightness?

What uses does reducing apparent brightness have if a microsecond or two is enough to cause permanent damage to someone's vision and doesn't actually add any layer of safety?
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Old 01-12-2014, 01:44 AM #4
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Default Re: PWM drive: increased operating times?

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Originally Posted by poofjunior View Post
So driving a laser with a pulse train signal will only reduce apparent brightness?

What uses does reducing apparent brightness have if a microsecond or two is enough to cause permanent damage to someone's vision and doesn't actually add any layer of safety?
Exactly.

The only actual use is in data transmission and for laser shows (for blanking and mixing colors to get a certain white/color balance).
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Old 01-13-2014, 10:33 PM #5
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Default Re: PWM drive: increased operating times?

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I'd like to know if pulsing the laser will increase the operating time and reduce the rate at which the diode heats up...
Yes. However, reducing the drive current is far more simple and has the same effect.
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Last edited by Cyparagon; 01-14-2014 at 04:34 AM. Reason: typo
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Old 01-14-2014, 04:14 AM #6
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Default Re: PWM drive: increased operating times?

The lasers peak power will be higher with PWM, but average power (which is basically all you care about in a pointer) will be the same/lower as others have said.
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Old 01-14-2014, 05:19 PM #7
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Default Re: PWM drive: increased operating times?

^And this effect may diminish with higher frequency longer duty cycle PWM, I would guess. Idea is that instead of being on the whole time, it is on for a bit, and off for a bit, allowing time to cool. And a cool chip is generally more efficient than a warm one, so same power in, the one with the more efficient chip will put more power out.

As others have said, this can be more dangerous because it LOOKS lower power, but still has a very high peak power.

PWM drivers are used in flashlights a lot, because that IS what you care about: the perceived brightness.
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Old 01-18-2014, 07:48 AM #8
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Default Re: PWM drive: increased operating times?

Not only that, PWM is also used in switchmode driver which steps up voltage, or step it down for to safely feed the load, in this case, the laser diode. I have green laser pointer, as an example, which I feed it single 18650 A123 Li : FePO4 cell - which has voltage way too low to even light up the diode at all (3.2 Volts), but the module in my laser has XBoost (the usual driver DTR installs before sellin' the Gallium-Nitride diode based laser modules off) to step 3.2 Volts up to 7 Volts.

And no, PWM will not make it run cooler, but for diode life expectancy, maybe. If you want it to run cooler, try using Aluminum flashlight host which is actually a very good heatsink.
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Old 01-18-2014, 03:55 PM #9
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Default Re: PWM drive: increased operating times?

I don't have any practical knowledge to bring to this discussion, but do have a couple thoughts:
  • I understand the peak brightness safety concern. It does, though, seem that many of the high power diodes are still quite dangerous (and still require same safety glasses) at low non-PWM levels. Plus, there is the concern you may accidentally switch to a higher mode anyway.
  • Are we sure PWM doesn't bring down diode temp? PWM in LED's certainly drops operation temps.
  • In LED's, not only perceived light output drops, measured output also drops. Maybe lux meters average reading over time. I don't know.
This is an interesting topic to me as there are tons of PWM based drivers out there in the market. I currently have one in my m140. Medium pulls about half the amps, but still burns pretty good. So it's nice to extend run time (battery and temp) be using medium when I can.

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Old 01-18-2014, 08:14 PM #10
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Default Re: PWM drive: increased operating times?

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no, PWM will not make it run cooler
Lower average power to the diode means less heat. What are you talking about?
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Old 01-18-2014, 10:29 PM #11
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Default Re: PWM drive: increased operating times?

Well, depends on amperage in pulsed mode, it may lower temperature of the laser diode but the reason I said no is because it wouldn't drop temperature very much. You can experiment with power LED, though (easier to use as a thermal test load). Almost all LED flashlight I have come across are always heatsunk even though they're PWM'd.
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Old 01-20-2014, 10:03 PM #12
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Default Re: PWM drive: increased operating times?

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Originally Posted by DrMario View Post
Well, depends on amperage in pulsed mode
It's simple: PWM assumes typical drive current, but at lower duty cycle than 100%.

Quote:
Originally Posted by DrMario View Post
it may lower temperature of the laser diode but the reason I said no is because it wouldn't drop temperature very much.
Then say it won't very much. Don't say "no."

If someone asked if having properly inflated tires increases fuel efficiency, would you say "no"?

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Almost all LED flashlight I have come across are always heatsunk even though they're PWM'd.
That point proves nothing. A 50W LED needs to be heat sinked. A 1W LED also needs to be heat sinked.
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Last edited by Cyparagon; 01-20-2014 at 10:04 PM. Reason: typo
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