[time-nuts] Hi Power LED Light power supply...

[gary]

There are ways for the flicker to be more evident. Don't laugh, but 
chewing something hard like a pretzel can bring out the flicker. 
Basically you can get beat patterns between the vibration of your eye 
and the light flicker.

There is a common problem with DLP projectors that use color wheels. You 
will see reviewers shaking their heads and eat crunchy food in order to 
see "rainbows" on the screen.

A similar problem occurs with matrixed LED displays mounted on machinery 
that has vibration. Very common in industrial controls since they like 
LEDs for readability.

When I designed the 2nd generation LED display drivers, I bumped the 
refresh rate to 500Hz min. That was about 2x the frequency where I ran 
out of convoluted experiments to detect flicker.

On an analog scope, you can display a flat line and have it wiggle by 
eating something crunchy. I don't have an analog scope on the bench at 
the moment, otherwise I would figure out the right circumstances to make 
that happen.

The test pattern for flicker detection is to arrange LEDs where a group 
of them form a recognizable pattern. Take a plus sign as an example. Put 
the LEDs in an array. Illuminate the LEDs that are not in the symbol out 
of phase with those in the symbol. Vary the refresh rate. When the eye 
can see a pattern, the refresh rate is too low.


On 9/18/2012 12:06 PM, John Lofgren wrote:
> <snip>
> I would hence believe that a 50 Hz flicker must be pretty close to the edge of what can be perceived, so I'm having trouble believing that a flicker at more than twice that rate would be perceptible at all by anyone.
> <snip>
>
> Oh, but it is.  A couple of years ago I bought one of the Chinese 30 LED spot light bulbs for about $8 on ebay.  I thought I'd give it a try for a workbench light.  When I plugged it in at work (60 Hz power, here) the two guys standing behind me yelled "gaahhh" at the same time I did.  The flicker was horrendous.  The earlier comment about peripheral vision also applies, though.  It's worse in the periphery than in direct view.
>
> The "power supply" is nothing more than a bridge rectifier, two current limiting resistors, and a filter capacitor.  The capacitor obviously wasn't big enough, though, because it flcikered plenty.
>
>
> -John
>
>
>
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Dennis Ferguson
> Sent: Tuesday, September 18, 2012 1:52 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Hi Power LED Light power supply...
>
>
> On 18 Sep, 2012, at 12:42 , Chris Albertson wrote:
>> On Tue, Sep 18, 2012 at 5:21 AM, Bob Camp <lists at rtty.us> wrote:
>>> Hi
>>>
>>> I suspect those same 120Hz sensitive people would not be able to watch TV or
>>> a movie :)....
>>
>> In the old CRT type TV sets, the phosphor has some persistence.
>> Movies are modulated with a square waves, the frame blinks off and
>> goes dark then blinks on.   But the LED's brightness is fast enough to
>> track the sine wave and would be bright only for an instant with quick
>> pulses of light.
>
> Just to add to this...
>
> Ontario, Canada originally ran its power grid at 25 Hz.  When they
> switched the grid to 60 Hz in the 1930's some of the industrial power
> users, particularly in northern Ontario where private (usually
> hydroelectric) power generators were common, never got around to changing
> their plants over.  Mine and paper mills using 25 Hz power were common as
> recently as the 1980's, and might still be there for all I know.
>
> Standard incandescent light bulbs don't have a lot of persistence when
> run on 25 Hz power (I assume there might have been a time when you could
> buy incandescent bulbs designed for 25 Hz, but not in my lifetime).  They
> don't go entirely off, but they get significantly dimmer in the visible
> spectrum in the dips as the output red-shifts towards the infrared; they
> follow the sine pretty well.  In my teens, when visiting a place using
> 25 Hz power for lighting, I could initially see an incredibly annoying
> flicker when I first got there but after a minute or two this would fade
> and I'd no longer notice it.  Some other people would also see the flicker
> but others, including my parents, couldn't see it at all so there seemed
> to be variation (maybe age-related, maybe not) among individual abilities
> to see this.
>
>
> Dennis Ferguson
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>