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

[BYD-bateria]

Nice job
I suggest you reverse engineer your solution. There are dozens of plug-them-in-an-outlet emergency lights on the market. Find one that matches closely your ideal, buy it, and hack it. You will likely be seeing the UL aspects battery needed in the device you dissect.

 

[Hiker]

I have noticed that when I put my single AAA Fenix against a window I hear a high pitched whining sound. I assume that is the PWM so this is probably another way to test it.

 

[jon_slider]

… high pitched whining sound. I assume that is the PWM…

thanks for that insight, it is new info to me. I usually notice PWM when taking photos, or I can detect it by waving.

Feel free to add the Fenix to this thread if you like: (please include specific model info)
List Lights that use PWM

I read recently that the Malkoff MDC is a PWM light known to have a circuit noise like you describe

Yes, the multi-level MDCs do use PWM. Some lights exhibit louder whine than others, with mine only being audible when the light is pressed against my ear.

 

[Heavy]

Fantastic explanation! Thanks so much.

If I am not mistaken this is the same type of system that is used in florescent bulbs yes?

They run, or are supposed to run, at a rate that the eye can't see.

This is why I ask. I have a huge problem with fluorescent lights. Just a few minutes under them and my eyes are super tired. I notice that if the ballasts that control that rate of speed gets old I swear I can see them pulsing.


So my question is this. Is this the same way LED bulbs for your house are controlled? Because if they are then I worry that they will bother me also.


As you can see from my post I know just enough about lighting to know that I don't know much.

 

[FRITZHID]

Fantastic explanation! Thanks so much.

If I am not mistaken this is the same type of system that is used in florescent bulbs yes?

They run, or are supposed to run, at a rate that the eye can't see.

This is why I ask. I have a huge problem with fluorescent lights. Just a few minutes under them and my eyes are super tired. I notice that if the ballasts that control that rate of speed gets old I swear I can see them pulsing.


So my question is this. Is this the same way LED bulbs for your house are controlled? Because if they are then I worry that they will bother me also.


As you can see from my post I know just enough about lighting to know that I don't know much.

Older florescent lighting used the A.C. line freq of 60hz so the bulbs flashed on and off at a rate of 120/sec. Newer versions use electronic ballasts that on higher end use very high freq so flicker "sickness" is minimised. LED lighting that uses PWM control can cause this as well however many use either high freq PWM or current control to fix that issue, espc in home LED lighting.
There are many reviews on cpf that will show you which are the worst/best.

 

[kj75]

A example I want to share with you.
Found during my first dinner at my holiday resort, I noticed this terrible bulb.
Look at the picture taken with my cell phone.

 

[jon_slider]

terrible bulb.

great photo!
before taking the photo, did you think there was PWM happening?
what did you notice about the lighting that made you investigate?

 

[kj75]

great photo!
before taking the photo, did you think there was PWM happening?
what did you notice about the lighting that made you investigate?

I was cleaning the table and noticed this while whiping with my hand.
Took the picture to show and explain my sons.

 

[jon_slider]

noticed this while whiping with my hand.

I had a similar experience with my LED house lighting. I was wiping my hand back and forth on a piece of leather on the table, to polish a knife. It looked like my hand kept stopping. I went online and researched home led lighting and PWM. Turns out many home systems use PWM for dimming, including unfortunately, my Phillips Hue System.

But thanks to your photo, I just did a series of tests, to see if Phillips Hue only Pulses during dimmed output. Unfortunately no, it pulses even on high. So, Im the guy that sold all his PWM based flashlights, paid to swap drivers to no PWM multiple times in others, and here I am sitting in my kitchen under a PWM light source.. hmmmm.. I guess people should feel welcome to laugh at me , including myself..

warm white shows PWM in the photo, both on high and also on a dimmed mode not shown

cool white on high the PWM is not visible in the photo

cool white dimmed, reveals PWM in the photo

the red led does not show any PWM in photos

not all conditions will capture PWM in a photo, I found red and blue do not reveal PWM, but yellow, white, and even green do show the interference bands in the photos. At first I though that it would not happen on high, only on dimmed levels, but no, PWM is present even at full brightness, and can be photographed quite easily on the Warm White setting. All photos from my iPhone 5, no special white balance nor other adjustments.

 

[kj75]

Thank you @jon for your effort!
Now we have to check all our house led lighting too

 

[SemiMan]

There are a several things to consider about whether a light is going to bother you, one is modulation depth (how deep the flicker is), the other is frequency of the flicker, and the shape of the flicker can come into play as well.

- The fluorescent lights that bothered you were likely old magnetic ballasts, who flickered at 100 or 120Hz. They did not turn on/of, but they would have peak-peak flicker up to 50% of the average. I.e. they went from 75% - 125% light output
- Electronic fluorescents may have flicker at 100/120Hz, but it may only be 5-10%, and then riding on top of that may be flicker at 20+KHz which is invisible
- CFLs are usually pretty similar
- LEDs for the home are all over the map
- The one in the hotel may have been a so-called AC drive LED which really does turn on/off at 100/120Hz. They are headache machines. Other LED lights for the home typically have anywhere from near 0 to 40-50% flicker, with the bulk it seems in the 20-30% range though as LEDs get more efficient, companies are willing to sacrifice a bit of efficiency for light quality.

 

[SemiMan]

Accidental double post

 

[jon_slider]

LEDs for the home are all over the map… headache machines

Are there ANY home LED lighting systems you know of that dont use Pulses? I removed my Phillips Hue due to the PWM. It did not give me headaches, but the light seemed to make for a "fuzzy" ambiance. (no idea how to quantify that impression)

 

[FRITZHID]

Are there ANY home LED lighting systems you know of that dont use Pulses? I removed my Phillips Hue due to the PWM. It did not give me headaches, but the light seemed to make for a "fuzzy" ambiance. (no idea how to quantify that impression)

GE bright stick. I have a dozen around the house. No noticeable PWM at all.

 

[Modder]

nice post !

 

[jon_slider]

There are a several things to consider about whether a light is going to bother you, one is modulation depth (how deep the flicker is), the other is frequency of the flicker, and the shape of the flicker can come into play as well.

Thanks for the rundown.
You mention fluorescents, CFLs and LEDs all have flicker.
What about incandescent, do they flicker too?

GE bright stick. I have a dozen around the house. No noticeable PWM at all.

thanks, I looked it up,
they are rated for 80CRI..
no "noticeable" PWM? Have you tried taking a cell phone photo?

For home lighting, Warm White incandescent with 100CRI and no? PWM are sounding better and better to me.

 

[SemiMan]

Home LED lighting does not have PWM almost as a rule. The flicker is from the 50/60Hz line frequency and circuit\cost tradeoffs.

Incandescent does flicker, just not a lot.

 

[timsdl72]

I apologize ahead of time. I didn't read all 270+ pages before replying so if it's been mentioned before, I'm sorry!

I work as an electrical specialist with the engineering department at a large university. One thing we had noticed is an inconsistency between products that were supposed to be the same item but the flicker was different. What we found was that manufactures were changing suppliers of the LED drivers. I'm sure cost was their reasoning but the result was that the PWM frequency (in the case of motor control VFD's, we call it switching frequency) was different.

Interesting stuff. Thanks for all of the info!

 

[jon_slider]

Home LED lighting does not have PWM almost as a rule.

thanks for your comments

 

[Bullzeyebill]

PM to jon slider.

Bill