That PWM Reference Light… the Ultrafire WF-602C.

In every review I make for the last …. two years or so, I have included data on PWM.  Not once have I featured or even shown the reference light I use for bad PWM.  Not even a photo of it, best I can recall.

The Light!

Well here it is!  An UltraFire WF-602C!  Not technically unbranded or generic, but it’s about as generic as they come.

The emitter is stated as a Cree Q2-WC.  Your guess is as good as mine.  Of course this light is more than 10 years old, so….

Not hard to see why I was into this light – a GITD tailcap!!

It’s funny about this light.  I bought a bunch of lights “back then” and this one is the only one left.  It’s not a good light, but it’s still “right” for so much.


More about PWM

What is PWM?

PWM is Pulse Width Modulation.  Here’s a wikipedia page on PWM, which will describe it much better than I can.  PWM is used for all sorts of electronic applications, but in the case of flashlights, PWM is used to achieve modes other than the highest level of output.  A light utilizing PWM has two states – off and on.  For higher modes, the light is on more than off, but it’s still binary – off or on.  Lower modes, the light will be more off than on.

Ability to “see” PWM

High frequency PWM is (or almost is) imperceptible, by even the most sensitive users.  As the on/off frequency slows, it becomes much easier to see PWM.  It’s seen as a flicker in the light.  At sufficiently slow frequency, the will will appear stroboscopic.  Regular strobe modes in fact, could be considered very slow PWM.

Noticing PWM in a light is typically regarded as unpleasant, and slow PWM is not desired.  But PWM is not entirely bad.  Fast PWM is even good, in some cases, and fast PWM is unlikely to be annoying (or even noticeable.)

PWM and Color Accuracy

There are claims that utilizing PWM maintains the color accuracy of the LED.  There are plenty of articles to cover that.  Here is more commentary, too.  This is well past my level of expertise.  Between visible PWM, and a mode related color shift, give me mode related color shift any day….

Why included the info?

Some people are extremely sensitive to PWM.  I’m one of those people….  Unfortunately I notice PWM that’s so fast most people don’t notice or aren’t bothered by it.  Blessing and a curse, I suppose.

I include it because I find it a good measure of how interested in pleasing a consumer the maker is.  I see it as lazy to use slow PWM.  If the PWM is so slow I can notice it, then chances are there’s something else about the light that has been designed or made lazily.  But no PWM, or fast PWM, is the next right thing.  Andúril uses fast PWM.  The Dr. Jones h17f driver (one of my favorites) uses fast PWM.  So it’s not all bad!

Reference PWM of UltraFire WF-602C

Here’s PWM for all 3 modes of my PWM reference light, the UltraFire WF-602C.  If there was interest, a full review of this $10.78 flashlight could be in order….  Also this isn’t a “How Do I…” post on my measurement of PWM as seen below – I can do that too if there is interest.

The mode order of the WF-602C is High (no PWM), Medium, Low, and the images below are in that order.  Note also that the PWM on this light is so bad (read: such a low frequency), that my usual time scale has to be changed.  Normally the full scale is 50us, whereas here the scale is a much bigger window of time: 2ms.

What does that mean though?  For this light I need a bigger window of time to show a peak-to-peak of the on/off cycle.  Better lights (lights with faster or no PWM) don’t need such a big time window.  So having to use the 2ms window is very bad.  Any user would absolutely be able to see the PWM on this light.

In the two graphs you see at right, the yellow line traces low then high.  When the line is low, the light is off.  When the line is high, the light is on.  As you can see, in Medium, the light is on for much longer than it’s on in the Low graph.  Medium could be said to be the worse mode of those two, because the off time is greater.  High does not exhibit any PWM.

Other ways to visualize

There are many ways to visualize PWM.  Cameras can do it, with appropriate settings.  The following photos are just as above, but taken with the camera on my phone (iPhone SE 2020).  I used an app (VSCO) that gives me “full manual” (or close enough) to take these.  There are a couple of settings you need to control (and many others you don’t.  Most importantly, set the shutter speed to the fastest setting (on VSCO, that’s all the way to the left).  Then just put the ISO wherever makes it work (for me just taking it off “Auto” works).  All the other settings can be left on Auto.

I didn’t point the light straight at the camera.  The emitted light was just “visible” to the camera, which was otherwise in a mostly dark setting.

Also I’m not quite sure you can notice it but…. these images you see here taken with the phone camera match very exactly with what you see on the oscilloscope display!  It’s fairly coincidental that they lined up that way but it’s an interesting side note nonetheless.

There are even more ways!  One is by doing the same as above, but moving the light quickly in front of the camera.  Or do the same on video.

Should you worry about PWM?

Eh…. probably not.  Most major brands use a fast enough PWM that you won’t be bothered by it even if you do notice it.  So carry on enjoying all your lights!

Hope this post has helped a little with PWM.  At least you finally now know what I use as my reference light!

I didn’t actually start this post with the intention of being an exposition on PWM – just wanted to answer a question I got recently about the light I use for PWM comparison.  I might add or edit some info here, because almost certainly it could be expanded, and quite likely some needs to be corrected.  😀  Have a great weekend!


Notes

  • I bought this light myself.  I was not compensated in any way for writing this post.
  • This content originally appeared at zeroair.org.  Please visit there for the best experience!
  • For flashlight related patches, stickers, and gear, head over to PhotonPhreaks, another site where I write!
  • Use my amazon.com referral link if you’re willing to help support making more reviews like this one!

Author: zeroair

5 thoughts on “That PWM Reference Light… the Ultrafire WF-602C.

  1. Nice write up! The LED in the light is a Cree XR-E. The Q2 is the flux bin, and the WC has to do with color. There are many LEDs that have an ugly green when driven at low levels and an angry blue when driven hard. That’s why many people use PWM. Also, those circuits are WAY easier to build than a properly functioning constant current driver (which require you to check all your resistors individually as variance in the resistance bin matters for that).

      1. From experience and reading up on the topic, the actual color shift when using PWM or current control is negligible (PWM is slightly better, but both are within the limits of average human perception). The main benefit of PWM is significantly higher efficiency (less heat needs to be dissipated) and lower component count, both of which are very important in this use case, as you can imagine.

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