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'[EE]:: Looking for an LED cionduction metaphor'
2007\10\06@214133 by

I'm looking for a "metaphor" / model / description that allows me to
easily explain a typical LED current/voltage curve to a layman in a
manner that isn't just understandable but which is so intuitively good
that it 'binds to the brain' as an explanation when we subsequently
discuss matters which depend on this characteristic.

An LED has an exponential increase of current flow with forward
voltage. Below a certain voltage it is for practical purposes non
conducting but apparently "rushes into conduction" exponentially above
a certain voltage.

The mechanical/hydraulic model need not be perfect - just model well
enough to allow intuitive grasp the *apparent* essentially zero
initial current and exponential current with voltage behaviour above a
certain threshold.

Russell

________________________________

For many years I've had good enough mechanical / hydraulic metaphors
for eg

Resistor - water pipe of given bore and roughness.

Capacitor - tank with variably elastic diaphragm separating input and
output sections.

Inductor - air dome

DC energy source - pump

AC energy source - double sided hydraulic piston driven appropriately
with no path between sides

Battery - dam with pipe outlet tapped off below surface

None of these metaphors is perfect but they are extremely useful in
explaining basic electrical concepts.

__

How about a canal/trough of water with water level representing
voltage and flow representing current. The diode would be a steep
(vertical) drop in the bottom of the canal but with a small threshold
just before. So until the water level reaches the threshold, nothing
flows, but once it hits it, there's significant flow corresponding to
the water level. It's not exponential, but it should do OK in modeling
the approximately constant voltage drop.
- Marcel

On 10/6/07, Russell McMahon <apptechparadise.net.nz> wrote:
{Quote hidden}

> -

> How about a canal/trough of water with water level representing
> voltage and flow representing current. The diode would be a steep
> (vertical) drop in the bottom of the canal but with a small
> threshold
> just before. So until the water level reaches the threshold, nothing
> flows, but once it hits it, there's significant flow corresponding
> to
> the water level. It's not exponential, but it should do OK in
> modeling
> the approximately constant voltage drop.

Very good.
The added feature is an "orifice-plate" which allows flow tailoring
Across the channel there is a "dam". At the top of the dam is a slit
which starts at zero width and widens in a curve such that flow rate

Thusly.

http://others.servebeer.com/misc/LED_hydraulic_model.jpg

This plate is inserted into a water channel.
Water level and thus head pressure will rise without flow until the
bottom of the "V" is encountered. As head continues to rise flow will
increase at a non linear rate with head. Over flow will occur if head
gets too large but that is not an essential part of this model.

Russell

On 7 Oct 2007 at 14:41, Russell McMahon wrote:
> I'm looking for a "metaphor" / model / description that allows me to
> easily explain a typical LED current/voltage curve to a layman in a
> manner that isn't just understandable but which is so intuitively good
> that it 'binds to the brain' as an explanation when we subsequently
> discuss matters which depend on this characteristic.
>
> An LED has an exponential increase of current flow with forward
> voltage. Below a certain voltage it is for practical purposes non
> conducting but apparently "rushes into conduction" exponentially above
> a certain voltage.

There is a shape to the knee of the voltage/current curve, but perhaps you could
ignore that and say that an LED draws little or no current until the voltage accross it
reaches the threshold voltage, then current rises more or less linearly with
increasing voltage (albeit rather fast). Same as a zener, TVS, transorb, whatever.

For some reason I always think of cars and driving for anologies. Not a great one
but: Voltage refers to speed, current is the fine you have to pay. Speed limit is
50km/h, so any speed below 50km/h there is no fine (zero current). But if you're
caught doing 60km/h then the fine is \$100, 70km/hr is \$200, etc, (current increases
linearly). P.S. In theory there is no maximum fine, but there is a limit at which which
your car bursts into flames and you can't go any faster :-)

--
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brownclear.net.nz

2007\10\07@074313 by
{Quote hidden}

The simplest I can think of that approximates this is a cylinder with the top cut at an ange
(45 degrees or so) being filled with water (marbles)
the water level is voltage
escaping water is current
to show a little leak current that is always there make a tiny hole in the bottom.
the hight of the cut is the knee voltage

Peter van Hoof

Russell

________________________________

For many years I've had good enough mechanical / hydraulic metaphors
for eg

Resistor - water pipe of given bore and roughness.

Capacitor - tank with variably elastic diaphragm separating input and
output sections.

Inductor - air dome

DC energy source - pump

AC energy source - double sided hydraulic piston driven appropriately
with no path between sides

Battery - dam with pipe outlet tapped off below surface

None of these metaphors is perfect but they are extremely useful in
explaining basic electrical concepts.

__

pilot operated valve?
with "gate" pressure the same source as mains pressure.

Russell McMahon wrote:
{Quote hidden}

I think of LEDs like a clicker.
Push a little and it moves a little.  More and it moves a little more.
Then at a certain pressure it **snaps** over to a new position.
At that point one is back to pushing linearly more or less.

Cedric

> I'm looking for a "metaphor" / model / description that allows me to
> easily explain a typical LED current/voltage curve to a layman in a
> manner that isn't just understandable but which is so intuitively
> good
> that it 'binds to the brain' as an explanation when we subsequently
> discuss matters which depend on this characteristic.
>
> An LED has an exponential increase of current flow with forward
> voltage. Below a certain voltage it is for practical purposes non
> conducting but apparently "rushes into conduction" exponentially
> above
How about a diode being a check valve and the reverse current being
leakage around said check valve.  The forward voltage would be the
pressure drop passing through the valve.

An LED really has no mechanical equivalent I can think of because it
generates light from a quantum drop in energy across the junction.  The
closest analogy I can think of is thermodynamic where the
thermoluminescence of thorium oxide is demonstrated in a Colemen lamp.
Perhaps a thermodynamic analogy but .. that's kind of stretching it as
thermoluminescence is again a property of quantum level changing.  Go
me there :D

Stephen

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>An LED has an exponential increase of current flow with forward
>voltage. Below a certain voltage it is for practical purposes non
>conducting but apparently "rushes into conduction" exponentially
>above a certain voltage.

water build up behind a dam or weir? Height of water represents the voltage,
and flow over the weir represents current? Possibly even more useful if the
top of the weir is not straight, but dished, so there is an identifiable
relationship between height of water, and rate of flow due to the increased
cross section of the water passing over the top.

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