solarwind wrote:

> On Sun, Jul 5, 2009 at 10:00 AM, Gerhard Fiedler wrote:
>> solarwind wrote:
>>
>>> People say current kills. So what happens if you push 1 mA through
>>> an LED @ 1000 V? Will the LED blow?
>> =

>> [...] (It's called "Ohm's Law", which I think is kind of a misnomer.
>> It's not a law; there aren't any laws in science (which is a good
>> thing -- there are no lawyers either :). It's more something like
>> "Ohm's Definition", as it defines the relationship between voltage,
>> current and impedance.) =

>> =

>> As far as LEDs go, if you put 1000V directly across a LED, there
>> will flow much more current than 1mA (for a short time). If you push
>> 1mA through it, there will be much less voltage across the LED than
>> 1000V. Can't have it both; they are tied together by the LED's
>> impedance.

> So what happens if you have a 1000 V potential across the LED, but
> some sort of limiter that only allows a max of 1 mA to flow?

The limiter needs to be in series with the LED. So the 1000 V are not
across the LED, but across the LED and the limiter (in series). =


How much of the 1000 V will be across the limiter, and how much will be
across the LED?

Try to apply 1) Ohm's Definition (V =3D I =D7 R), 2) the fact that in a
series circuit the current is the same through all devices, and 3) a
data sheet of a LED of your choice (that gives you the voltage across
the LED when you push 1 mA through it).

For extra points, compare this to the standard circuit of a LED with a
series resistor, and suggest a suitable "limiter" circuit for this
situation.

Gerhard

-- =

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