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Thread: PWM - What is it, How does it work and how to detect it.

  1. #61
    Flashaholic* archer6817j's Avatar
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    So I know linear drivers like the battery voltage to be close to the led Vf...but what about pwm? Could I get a pwm driver to handle the range from 3-12V? What would be the advantage/disadvantage?

  2. #62
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    Default Re: PWM - What is it, How does it work and how to detect it.

    I missed this thread the first time it was posted. Good explanation of PWM.

    I find I am quite sensitive to PWM. Even on lights that have higher a PWM, I can sometimes notice the flickering from the corner of my eyes when I move my head.
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  3. #63

    Default Re: PWM - What is it, How does it work and how to detect it.

    @archer6817j
    You could use a buck driver to convert the V_in(3-12V) to V_out(3,6V) and a µC+MOSFET for PWM.

    Buck/buck-boost drivers usually are more complex and thus bigger and more expensive.
    But you can use a wider voltage range at a decent efficiency.

  4. #64

    Default Re: PWM - What is it, How does it work and how to detect it.

    Quote Originally Posted by odd View Post
    @archer6817j
    You could use a buck driver to convert the V_in(3-12V) to V_out(3,6V) and a µC+MOSFET for PWM.

    Buck/buck-boost drivers usually are more complex and thus bigger and more expensive.
    But you can use a wider voltage range at a decent efficiency.
    I certainly appreciate your clear explanation. Flashlight electronics are still a mystery to me. My knowledge of buck and boost is limited to control transformers and isolation transformers, where they are used to slightly raise or lower a critical voltage.
    Last edited by precisionworks; 08-10-2011 at 05:59 AM.
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  5. #65

    Default Re: PWM - What is it, How does it work and how to detect it.

    Incredible thread...

    I was searching about for info on PWM as I had been noticng it on two recent lights that I purchased. I suspected that I might be a tad more sensitive to the effects as I posted about the PWM I was seeing and one member confirmed he had not noticed it at all.

    I did the "good" vs "bad" PWM tests, and by all acounts, the PWM is good, but I still really notice it, especially when I catch reflections as the result of movement off of reflective surfaces like chrome, etc.

    Anyway, the two lights are my new McGizmo XM-L Mule and the Oveready Triple XPG H-M-L head. I'd be curious if anyone else is seeing this on either of these lights?

  6. #66
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Very good thread about PWM, Cataract.

    I would like to add a nuance to the initial post

    Quote Originally Posted by Cataract View Post
    As you can see, we have now changed our constant voltage input for a square wave. In this graphic, our voltage is on half the time and off half the time. The technical term here would be that we have a 50% duty cycle (50% of the time on.) With such an input on our LED, the amount of light given off appears diminished by 50%. The reason I say it appears, is because it gives off its maximum amount of light, but only 50% of the time, so our eyes perceive only half the amount of light (more on this in the next section).
    It would be more exact to say

    "With such an input on our LED, the amount of light given off appears diminished as much as 50%" or "With such an input on our LED, the amount of light given off appears diminished by 50% if there is no any flickering"

    The reason lies on the Broca-Sulzer effect that states that when light pulses are short enough (below 0.1 or 100ms, but usually 50ms and less), perceived brightness is enhanced. How much it is enhanced depends of the frequency, pulse duration and overall true brightness level, but its often of 30-40% and can reach more than 200% (perceived brightness more than double than actual).

    This effect is due to human visual system not being a perfect integrator of light and it's related to the own flickering effect. When there is no flickering, there is no Broca-Sulzer effect. So its just a nuance because if the system is designed to avoid flickering fully, the initial statement is right. Actually, with this explanation, I think there is no need to edit initial post.

    But I think it is worth to point out it because when a small flickering is tolerated, it is possible to get an enhancement on perceived brightness by using PWM, being able on some situations to overcome the inherently more efficient way of Constant Current dimming (at least, considering LEDs alone, and not efficiency of the driver/compatibility with batteries).

  7. #67
    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    I revive this thread to thank Bullzeyebill for making this a sticky.

    I also encourage everyone to share their own experience on PWM detection and good or bad experiences with specific PWM frequencies.

    I also see my picture links in post 1 have somehow been corrupted and will update them when I get a chance, probably next weekend.
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  8. #68

    Default Re: PWM - What is it, How does it work and how to detect it.

    Cataract. Thanks in advance for reposting pics, etc. I just found this thread and was going to ask if you could do that.

    As for PWM, I just started to notice this in some of my lights. The first time was when I was walking back to my house and caught the reflection off of the parked cars reflectors out of the corner of my eye. I thought it was just from the reflection, but started to notice it more frequently. When I saw that I did the very unscientific experiment of pointing the lights at a moving fan and you can see the different levels of PWM based on whether the light is on Low, Med or High (then I found out about the shower trick).

    It seems that I am more sensitive to this now than I was before too.

    Cadillac tail lights for sure, really annoying.
    Last edited by greatscoot; 09-29-2013 at 08:28 AM. Reason: Typo
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    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Pics have been updated.

    greatscoot: you're welcome. The pics should definitely make things a lot clearer...

    PWM is often easy to notice when your light shines back at you from reflective surfaces an you move our eyes or head around. Some lights, like Zebralight before they finally went for constant current regulation, are very nice to use in general but you still see the PWM on surfaces like a stainless steel faucet while moving about.

    I believe we tend to notice PWM more once we have seen the effects. I also notice a lot of newer cars (like Caddys and Volvos) seem to have gotten away from PWM tail lights (thank god!) but other manufacturers like KIA have started using it in their newer luxury models. It hopefully is only a matter of time before they get enough complaints and start doing things the sensitive way.
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  10. #70
    Flashaholic* RI Chevy's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    It is odd for me to say, but I notice the PMW in my drop ins more from the high pitched whine rather than the flickering of the light.
    My ears supersede my eyes.
    The "whine" sound drives me battey. On the other hand, I have a drop in with a linear driver that does not show any effect of PMW at all. This I prefer.

  11. #71
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    Default Re: PWM - What is it, How does it work and how to detect it.

    M​y eyes are very susceptible to pwm I notice it almost all the time even in the higher end lights. It's current controlled for me now a days
    The power used to go out a lot more before I got these expensive lights...

  12. #72
    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Quote Originally Posted by RI Chevy View Post
    It is odd for me to say, but I notice the PMW in my drop ins more from the high pitched whine rather than the flickering of the light.
    My ears supersede my eyes.
    The "whine" sound drives me battey. On the other hand, I have a drop in with a linear driver that does not show any effect of PMW at all. This I prefer.
    Interesting, what brand of dropins? It is possible you have an ear that is sensitive to higher frequencies than most people - I do, but I have never heard PWM so far and some of my lights have a PWM frequency that is definitely in the average hearing range.

    Quote Originally Posted by Jakeyb View Post
    M​y eyes are very susceptible to pwm I notice it almost all the time even in the higher end lights. It's current controlled for me now a days
    Some people are extremely sensitive, but not all higher end lights use a high enough frequency for PWM to be unnoticeable.

    47's are the only ones so far that are just fine with me and I can endure the PWM regulated Zebras that I own, but they're borderline to me as I immediately notice it in reflections on the faucets. Those will be replaced with the newer current-controlled models as I can afford it...
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  13. #73
    Flashaholic* RI Chevy's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    The drop in is my Malkoff M361N. The PMW is extremely loud in my particular drop in.

  14. #74
    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Thanks. It could be an unusual inductor whine. I also have a few lights with high pitch sound, but I personally don't find it annoying and it has nothing to do with PWM. It would be interesting to know what frequency PWM is used and if others hear it as well.
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    Flashaholic* RI Chevy's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Everyone in my house can hear it. I am unsure on the frequency of the PMW, as I am only an end user and know nothing about the mechanics of it.
    But it is louder on medium than on low. Go figure.

  16. #76
    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Quote Originally Posted by RI Chevy View Post
    Everyone in my house can hear it. I am unsure on the frequency of the PMW, as I am only an end user and know nothing about the mechanics of it.
    But it is louder on medium than on low. Go figure.
    OUCH! I hope you only mean they can hear it from a short distance, not wake up in the middle of the night while you're having a midnight snack! LOL

    I only own one Malkoff and it only has one mode, but your case sounds unusual. Does the whine change pitch between modes? Do you still hear it on high?

    I ask because it could be a regular (but not necessarily normal) inductor whine that has nothing to do with the PWM
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    Flashaholic* RI Chevy's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    No. They can only hear it when it is close to their heads. But at a distance of about 2 to 3 feet.

    I have no PMW on high, PMW on medium, and PMW on low. But the thing is it is louder on medium than on low, and their is a different, higher pitch tone on medium than on low. Low is quieter and has a lower pitch tone.

  18. #78
    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Extremely interesting stuff... do you hear it from farther away than the others? Does it annoy everyone just as bad? Do you (or anyone else) hear a whine on high?

    Note: High mode is not commonly controlled through PWM; high mode is usually current-controlled or direct-drive.
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    Flashaholic* RI Chevy's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Everyone can hear it and they don't like it.

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    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Even on high? If so, then I'd say there is a faulty inductor, or bad solder/trace inside...
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    Default Re: PWM - What is it, How does it work and how to detect it.

    The loudest light i have owned is a lumapower lm21. The noice from the pwm filled the room... The funny thing is i could hear it used way different frequencys on med and lo mode. Lo mode was growling in a lo pitch noice (crappy low frequenzy), medium mode was screaming in a higher frequency. A simple shaketest comfirmed it visually.

  22. #82
    Flashaholic* RI Chevy's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Quote Originally Posted by RI Chevy View Post
    No. They can only hear it when it is close to their heads. But at a distance of about 2 to 3 feet.

    I have no PMW on high, PMW on medium, and PMW on low.
    But the thing is it is louder on medium than on low, and their is a different, higher pitch tone on medium than on low. Low is quieter and has a lower pitch tone.
    Quote Originally Posted by Cataract View Post
    Even on high? If so, then I'd say there is a faulty inductor, or bad solder/trace inside...
    No PMW on high. Only on Medium and Low.

  23. #83
    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Quote Originally Posted by jorn View Post
    The loudest light i have owned is a lumapower lm21. The noice from the pwm filled the room... The funny thing is i could hear it used way different frequencys on med and lo mode. Lo mode was growling in a lo pitch noice (crappy low frequenzy), medium mode was screaming in a higher frequency. A simple shaketest comfirmed it visually.
    Ouch! room-filling PWM noise is as bad and even worse to the ears as low frequency PWM to the eyes!
    Because of the square wave's shape, it should sound and look like higher frequency -in most cases- when using a higher output, but the frequency is normally the same. This is due to the time off being shorter at higher output, making it seem like the flicker happens more often (thing of a faster strobe, but in this case, it is only our perception that is tricked into believing that). Some light could use a different frequency for each mode, but that's not a very economic way to go as it means having more complex or just more IC's (chips) on the board.

    Quote Originally Posted by RI Chevy View Post
    No PMW on high. Only on Medium and Low.
    I know it might have seemed like I wasn't paying attention, but a high pitch whine on high means an inductor is whining, not having anything directly to do with PWM as high is most often direct-current or current driven (think of the straight line in my graphs.) It still could come from a bad solder of faulty inductor (small copper wire coil) in your light but, seeing that it might not be an isolated case, it couldt be a side-effect of low frequency PWM as well... I never heard that in person but I take both your words for it. I definitely don't like PWM in all its forms, period, but PWM noise must be a total wretch!

    So, there ya go: Another bad side effect of PWM could include annoying noise (perfect or faulty board and/or components and/or soldering or whatnot)
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Frequency is not the same on lo and med on the lm21. Lo mode uses a really slow and easy to see pwm. Med is faster and harder to detect.

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    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    I'm trying not to get too technical to keep the thread as simple as possible in the spirit of the original post:

    They probably use 2 different chips with fixed frequencies and pulse width.
    Changing the frequency instead of pulse width is also a means to change the perceived intensity: less "beats per second" will be perceived as dimmer than a higher (faster) frequency, but that also means it is guaranteed you will perceive the strobing effect very easily on lower modes. That is not really PWM, though, but it is the same basic principle. It makes for a much simpler circuit and possibly cheaper, but not necessarily smaller.
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    Flashaholic* RI Chevy's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Quote Originally Posted by Cataract View Post
    I know it might have seemed like I wasn't paying attention, but a high pitch whine on high means an inductor is whining, not having anything directly to do with PWM as high is most often direct-current or current driven (think of the straight line in my graphs.) It still could come from a bad solder of faulty inductor (small copper wire coil) in your light but, seeing that it might not be an isolated case, it couldt be a side-effect of low frequency PWM as well... I never heard that in person but I take both your words for it. I definitely don't like PWM in all its forms, period, but PWM noise must be a total wretch!

    So, there ya go: Another bad side effect of PWM could include annoying noise (perfect or faulty board and/or components and/or soldering or whatnot)
    I got you. I am not sure I could check the soldering on the M361N now though. Funny thing is I can't see the flickering, but I can hear the whine. I have 2 distinct whines on medium and low only. Just my luck.

  27. #87
    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Quote Originally Posted by RI Chevy View Post
    I got you. I am not sure I could check the soldering on the M361N now though. Funny thing is I can't see the flickering, but I can hear the whine. I have 2 distinct whines on medium and low only. Just my luck.
    Ah, things are clearing up now...

    Malkoff drop-ins are potted, so you can't open them up. If you can't see any flickering, even using my technique or any other, then you might just have a noisy inductor. Doesn't mean your module is not using PWM for med and low (I don't know Malkoff devices all that well); the sound frequency from an inductor will change with different pulse widths, with a higher pitch for a wider pulse - or higher output as in medium mode. I could bet this is it, as I do have a hard time believing Malkoff was cheap enough to use a slow frequency modulation. There is however another possibility: a fit that is not even in the host could also cause high pitch noise on certain modes. Basically, the "vibration" can be transmitted through smaller contact points. The physics of this can be complicated to explain, so I prefer to leave it out at this point, but it has happened to me once and was cleared up with a good maintenance and rotating the dropin a bit (sounds like shameless self-publicity and cross-ref between my own stickies) Try these steps to see if it changes anything: Basic Flashlight Troubleshooting Guide (I should have started with that before elaborating, but I'm leaving all the good info on the thread.)

    If that doesn't change anything, it could be interesting to start a thread on your specific drop-in and ask if anyone else has noise issues. Something tells me your case is exceptional and you might be able to get a replacement - provided the problem has not been solved with maintenance by now.
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  28. #88
    Flashaholic* RI Chevy's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    Thanks for the informative response. I have it in a Surefire bored 6P. The fit is flawless inside the host, but I'll try a few of the tips and see if I get a different outcome. I have many, many Malkoff modules, and I really do not want to make a big deal out of it, as I have to much respect for the Malkoff's. All of my others are absolutely perfect. Thanks again.

  29. #89
    Flashaholic* Cataract's Avatar
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    Default Re: PWM - What is it, How does it work and how to detect it.

    My pleasure, I think people deserve to know.

    I know about the Malkoff quality, so I find it a little odd. It is most likely a very exceptional case, but do let us know if maintenance makes any difference. If it does I would be grateful if you would mention it in the troubleshooting guide to share with others.
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  30. #90
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    Default Re: PWM - What is it, How does it work and how to detect it.

    I experimented with the M 361 N drop in and hosts that I have. Unfortunately I hear the PMW in any host I place it in while on low and medium power. So I think it is the drop in, not the host. The PMW is pretty consistent regardless of what host I place it in. I am running an 18650 battery.

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