Post by thread:[EE]:: Looking for an LED cionduction metaphor

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 <spam_OUTapptechTakeThisOuTparadise.net.nz> wrote: {Quote hidden}> 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. Very good. The added feature is an "orifice-plate" which allows flow tailoring with head. 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 increases exponentially as head increases. 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.brownKILLspam@spam@clear.net.nz

{Quote hidden}>----- Original Message ---- >From: Russell McMahon <apptechKILLspamparadise.net.nz> >To: PIC List <.....PICLISTKILLspam.....MIT.EDU> >Sent: Saturday, October 6, 2007 9:41:37 PM >Subject: [EE]:: Looking for an LED cionduction metaphor >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. 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'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. > > __ > > > >

--- Russell McMahon <EraseMEapptechspam_OUTTakeThisOuTparadise.net.nz> 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 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 ____________________________________________________________________________________ Moody friends. Drama queens. Your life? Nope! - their life, your story. Play Sims Stories at Yahoo! Games. http://sims.yahoo.com/

>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.