RE: LED at 60 HZ? (was RE: Radio Shack Strobes)

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Original poster: "Philip Chalk" <phil-at-apsecurity-dot-com.au> 

Hi,

I think it's worth a try - and can't you get some NICE LEDs these days
(like Luxeon). I don't think there would be a persistence problem.

You should put a diode across the LED (with inverse polarity to the LED)
as they normally have a fairly low reverse breakdown voltage, maybe 5 or
6 V. Alternatively a diode in series.

Another thing which I know does work as a strobe is normal fluorescent
lights, in appropriately subdued ambient lighting conditions.  They
'flash' at 2x f mains - i.e. 120Hz, or 100 in my neck of the woods.

As usual, 'suck it & see' - no substitute for empirical data, & this one
is easy to try.

Just thinking, depending mainly on V & R values, it may result in quite
a large 'conduction angle', or duration of light pulse, 'smearing' the
line, mark, whatever that you're looking at.  At least it should turn on
quite consistently at the same point.

You could minimise conduction angle by closely matching the peak ac
voltage to the LED turn-on voltage, but would then tend to get
relatively feeble light output.  You want to turn it on hard, but
briefly.  A 555 or some other monostable, triggered by the mains
waveform would do it.

But do you realise how easy it is to make a strobe from scratch ?  Other
things which could be easily modified for the job (I'm assuming 'the
job' is looking at sync motor rotors ?) are engine ignition timing
lights, or camera flashes.

If I wanted a quick & dirty, single-purpose strobe for this, I reckon
I'd make one from a disposable (i.e. free) camera with flash.

So, now I've rather talked myself out of the idea, but still suggest
it's easy enough to try by experiment.

Regards,

Phil Chalk.



-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Wednesday, 25 February 2004 7:11 AM
To: tesla-at-pupman-dot-com
Subject: LED at 60 HZ? (was RE: Radio Shack Strobes)

Original poster: "Daniel Kline" <daniel_kline-at-med.unc.edu>

An engineering friend says a high-brightness LED on a
low-voltage transformer, like 6 Volts, connected through
a suitable resistance, should flash at 60Hz in sync with
the 60Hz mains. (50Hz for you all over there :)
It seems too easy to me...he thinks that the LED may have
a "persistence" issue. In other words, just because it
turns off quickly doesn't mean it stops emitting quickly.
Comments?
Thanks,
Dan K