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John Fields

On Tue, 29 Dec 2009 00:16:00 -0600, (Pete)
wrote:

I'm curious about the "Important" warning message I found on a wind-up
LED torch (flashlight) I just got for Christmas. It says:

"IMPORTANT: To extend the life of the LEDs allow 20 seconds between
turning ON & OFF. Do not turn ON & OFF repeatedly."

Hunh...? I can't imagine what effect that would have on LEDs. There's
no turn-on surge like an incandescent would have. How could the life
be shortened?

---
Since they want you to wait 20 seconds between turn-on and turn-off,
there will, ostensibly, be no turn-on surge from the supply and, since
LED's don't exhibit the same VI characteristics as incandescents, no
turn-on current spike through the LED.

What they're talking about is a _turn-off_ surge, so I suspect that 20
second wait will allow the thing to do whatever it needs to in order to
do a soft turn-off.
---

I searched the web for that message, and find a number of apparently
quite different and independent products with the same warning -- all,
interestingly from UK suppliers! (Though my unit is made in China.)
Could this be some kind of weirdly spreading myth?

---
I doubt it.

More than likely it's that the same (cheap) design has found its way
into many related products.

JF

John Fields

On Wed, 30 Dec 2009 14:28:58 -0600, (Pete)
wrote:

In article ,
John Fields wrote:

"IMPORTANT: To extend the life of the LEDs allow 20 seconds between
turning ON & OFF. Do not turn ON & OFF repeatedly."

What they're talking about is a _turn-off_ surge, so I suspect that 20
second wait will allow the thing to do whatever it needs to in order to
do a soft turn-off.
Turn *off* surge?? Where would that happen? I suppose there could be
thermal shock as another poster suggested but otherwise I can't see
what a sudden current cut-off could do.

---
L dI
E = ------
dt
---

Could this be some kind of weirdly spreading myth?

I doubt it.

More than likely it's that the same (cheap) design has found its way
into many related products.
Strange, though, that it covers such a range -- from my little 2-LED
torch to a 'super bright" 30-LED wand.

---
Scaling of the same bad design, probably.

Take one apart and post what you find in it?
---

And, as I said, all advertised
by UK sales outlets. None in the US that I can find.

---
OK

JF

John Fields

On Wed, 30 Dec 2009 20:24:29 -0600, (Pete)
wrote:

In article ,
John Fields wrote:
On Wed, 30 Dec 2009 14:28:58 -0600, (Pete)
wrote:

In article ,
John Fields wrote:
What they're talking about is a _turn-off_ surge, so I suspect that 20
second wait will allow the thing to do whatever it needs to in order to
do a soft turn-off.
Turn *off* surge?? Where would that happen? I suppose there could be
thermal shock as another poster suggested but otherwise I can't see
what a sudden current cut-off could do.

---
L dI
E = ------
dt
Yabbut... inductance? Where? Can't see any possible need for that
in such a circuit,

---
Not trying to be confrontational, but until you take one apart or
otherwise find out definitively how the thing works, your inability to
see why an inductance might be used is unimportant.

In a wind-up or a shake one, what do you think is generating the
electricity to drive the LEDs?
---

and any stray should be minuscule. (It's not even
dependent on being a 'wind-up' unit -- the others I see advertised are
all battery driven.)

---
Then I suspect that what might be happening is that, because of the
design, the miniscule inductance is generating a spike higher than the
reverse voltage of 7 volts or so allowed across the LED(s).
---

Take one apart and post what you find in it?

Maybe eventually (:-)) but I don't really want to offend the present
giver that soon! I may try to write the supplier and see if they can
refer me to a useful source. [Some hope...]

---
So buy one and take it apart...

JF

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