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

mhphoto

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:huh: PWM can be easily seen by pointing your light at your shower head, with water running of course! The water may look like separate drops depending on the PWM frequency. My Pelican 9410 does use PWM for the lower mode, but it is of sufficient frequency that it's only noticeable in the way described.

Try this with your lights, let me know what you see.

A ceiling fan works too ;)

The explanation that was given for the blinking on the Quarks involved graphs and such to show that, while it looks like PWM, it has a different signature on an oscilloscope (or whatever that neat machine is called).
 

indychris

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Incredible description and illustrations, Cataract. For a newbie like myself who wasn't even aware that PWM existed, this was a huge eye opener.

So is that any type of near comprehensive list that notes which popular lights utilize PWM and those that do not?
 

Cataract

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Thanks, I took my time coming up with this one and I'm glad people appreciate it.

There are no lists that I know of, but this would be a good time to start one. Of course, such a list should be split in two to separate the lights that show "good" and "bad" PWM, unless "someone" could measure the PWM frequency and include it in the list :poke: :devil:

If no one offers any help with this, then this thread would be a good place to start listing those lights. Later, I could compile the list and append it to the original post or put it in a separate thread.
 

mhphoto

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On the subject of "good" and "bad" PWM (in my opinion):

Good: 4Sevens' Mini line, Newer Maratac AAA lights (just got my Copper AAA today :D)

Bad: Princeton "Fuel" headlamp. Its PWM is ridiculous. If it were any slower it would just be a strobe. Makes me nauseous just thinking about it… :sick2:
 
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calipsoii

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Great thread, very informative. The important thing to stress to people reading this is that PWM isn't inherently bad. Low frequency PWM is bad, yes, but if done properly (over 1000hz preferably) it's unnoticeable.

If you gave me a choice when ordering a multi-mode light: high-frequency PWM or current-controlled, I would choose PWM. There's nothing I hate more than a light that's a crisp, pure color on High and then suddenly goes to an ugly green when dropped down to Low.

(I'm looking at you, Preon Revo on my keychain)
 

Burgess

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Great thread here !


Thank you, Cataract !


:goodjob::kewlpics::thanks:lovecpf
_
 

Burgess

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BTW --

David (4Sevens) wrote (November 2009) this very interesting & informative post,
comparing and contrasting PWM vs. current-regulation.


Here is the Link


Nice to have all this great info in one place !
 

LedTed

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Nice article Cataract! :twothumbs

Here is a trick you probably already know, to "look at" PWM. Shine the beam at the water from an indoor faucet.
 

mrlysle

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GREAT thread Cataract! BTW, most, if not all of selfbuilts' reviews show the PWM frequency used and the respective output level for lights that use PWM. Some interesting reading there for those interested!
 

LukeA

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Excellent post.

One relatively inexpensive way to measure PWM frequency is with an RC airplane propeller tachometer, which uses a photocell to measure pulses of light and dark caused by passing blades. Set it for a 2-blade prop and multiply the rpm number by 2 pulses per revolution/60 seconds per minute to get the frequency in Hertz.

Or, using a photograph, you can calculate frequency by counting the number of pulses and dividing that number by the picture's exposure time in seconds. In the first image in the OP, there are 8 pulses shown in a time of 1/8s for ~64Hz.
 

uplite

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how do you think Zebralight has such massive efficiency when using PWM? The SC51 which uses PWM is more efficient than a quark AA which is CC
Good question.

Are we comparing apples to apples? What output level? Emitter? Battery?

I haven't looked in a while, but I seem to remember that Zebralight quoted their runtimes from a NiMH cell, and Quark used an Alkaline.

If the output, emitter, and battery are the same, then it comes down to the driver. There can be big differences in efficiency based on the quality of the components and circuit design. Perhaps Zebralight uses a better quality driver.

-Jeff
 

HooNz

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Use a portable shortwave receiver if it has a LW (longwave) on it , turn on the light , hold it close to the Rx'er , tune about to find a strong noiseless signal , turn off the light and if the signal disappears that would be the PWM frequency , then just to make sure with the light on tune about in that area to see if there is a stronger signal ..

Mine is 380khz or so , i posted this up yonks ago somewhere , i have seen mentioned over the net over a few months 12khz , 30khz , 60khz and up to around 1 mhz for PWM.

And obviously if there is no signal the torch might not be PWM .
 

Cataract

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GREAT thread Cataract! BTW, most, if not all of selfbuilts' reviews show the PWM frequency used and the respective output level for lights that use PWM. Some interesting reading there for those interested!

I had a feeling he might state something in there, but got too lazy to go check. Thanks, I'll append that to the original post, among a lot of good info being shared here.


Use a portable shortwave receiver if it has a LW (longwave) on it , turn on the light , hold it close to the Rx'er , tune about to find a strong noiseless signal , turn off the light and if the signal disappears that would be the PWM frequency , then just to make sure with the light on tune about in that area to see if there is a stronger signal ..

Mine is 380khz or so , i posted this up yonks ago somewhere , i have seen mentioned over the net over a few months 12khz , 30khz , 60khz and up to around 1 mhz for PWM.

And obviously if there is no signal the torch might not be PWM .

The RC plane tachometer was a good one, but this is amazing! I don't have a shortwave radio, so I couldn't try it, but I knew there was something to do with this when I turned some of my lights on and off and my old tube TV lost the signal for half a second... I'm just finishing upgrading everything to digital and now you'll have me buy a shortwave/longwave radio...
 

Joe_Beam

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This is a great thread. I have been wondering how pwm would affect shortwave/ longwave am radios. Now I would like to have somebody explain the different types of current regulation.
 

HooNz

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I had a feeling he might state something in there, but got too lazy to go check. Thanks, I'll append that to the original post, among a lot of good info being shared here.




The RC plane tachometer was a good one, but this is amazing! I don't have a shortwave radio, so I couldn't try it, but I knew there was something to do with this when I turned some of my lights on and off and my old tube TV lost the signal for half a second... I'm just finishing upgrading everything to digital and now you'll have me buy a shortwave/longwave radio...

LOL , if i was wealthy i would! , there are some good SW rx'ers out there , some get down to 15khz , some really good old ones that can be got cheep at secondhand shops or even garage sales (yard sales there?) , EE/bay is a spot too . :) . I go to/Visit the "Special shop" .

ps-Technically if one wanted to really tune about , a VLF combination rx'er would be the go 1-400khz .
 
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