MadSci Network: Physics

Re: Help with circuit explanation

Date: Sun May 14 01:52:09 2000
Posted By: Karl Kolbus, Staff, Data processing, Mequon Consulting Corp.
Area of science: Physics
ID: 958017761.Ph

Hello Mr. Wulbert!
I am glad to be able to help you.
Having worked with LED's for many years, I can tell you, as I'm sure 
you've already discovered, that they are truly amazing devices. Unlike 
LED's of years past, they now come in a range of color output from the 
infrared (which, of course is invisible to the human eye) thru the reds,  
oranges, ambers, and yellows, up to the greens and blues. I use the 
term 'up' because a typical red LED emits light at a wavelength of about 
626 nanometers, whereas the blue LED's emit light at about 526 nanometers. 
Sound confusing? It's not... Really... Trust me! Wavelength (represented 
by the Greek letter Lambda; which looks like an upside-down "y") is 
defined as the distance between any two adjacent (look up the term if you 
don't know it) identical points on a 
waveform. For example, if you throw a pebble into a pond, it forms 
ripples. The ripples have peaks and troughs. The distance between any two 
adjacent peaks or troughs is the wavelength. 
Frequency (referred to as Hertz)is the number of peaks or troughs that 
pass a point in any given period of time; usually one second. You can see 
that if the space between the peaks or troughs is long (626 nm for red), a 
fewer number of them will pass that point than a wavelength that is 
shorter (526nm for blue). Thus, the longer the wavelength, the lower the 
frequency, and vice-versa. Relating it to the sound of a train horn, as it 
approaches you, the sound rises in pitch because the movement of the train 
compresses the ripples of sound and more peaks are reaching 
your ears in a given amount of time. If, instaed, the horn emitted light 
instead of sound, it would  start out as red, and gradually shift through 
orange, yellow, green and finally blue as it gets very close to you. When 
it passes you, the sound waves get stretched out and the horn falls in 
pitch - or the light goes from blue gradually back to red. Not only are 
they available in a wide range of colors, they can put out much more light 
that ever before. Some are so bright that you could damage your eyes by 
looking directly into one at a close distance. In fact, some are used for 
highway traffic signals, and even brake lights on cars.

But that doesn't answer your question. A light emitting diode is exactly 
that - a diode that emits light. A diode, any diode, has a cathode and an 
anode, and will pass current only when the cathode is negative with 
respect to the anode. This is called forward biasing. If you were to run 
an alternating current (AC) to the diode, it would conduct when the 
cathode is negative, and shut off when the cathode is positive. By doing 
so, it will 'rectify' AC and turn it into pulsating direct current (DC). 
It will pass current in only one direction. You can see this by placing an 
ohmmeter across the diode. When the black (negative) lead is on the 
cathode and the red (positive)is on the anode, you will read a low 
resistance. When you reverse the leads, you will read a much higher 
resistance. Try it! That is because an ohmmeter has a battery inside it 
that places a voltage on the leads; negative on the black and positive on 
the red. A diode has what is called a threshold voltage, below which no 
current flows - even if the cathode is negative with respect to the anode. 
Once this threshold voltage is reached or exceeded, the diode will 
conduct. The thing that makes a LED unique is that, when this point is 
reached, it emits PHOTONS (look it up!). The color of the light depends on 
the materials the diode is made from. For your project, I assume you took 
two LED's and soldered them together with the anode of one going to the 
cathode of the other, and the other anode going to the other cathode; in 
other words - in parallel, but backwards. The resistor would then be 
soldered to one of these junctions and power applied to the un-connected 
end of the resistor and the un-connected (but soldered together) leads of 
the LED pair. When the negative pole of the battery was connected to the 
LED's and the positive pole connected to the resistor, the diode that has 
negative voltage passing through its cathode would light and the other 
would remain dark. That's because you were FORWARD BIASING (cathode 
negative, anode positive) the lighted diode beyond its' THRESHOLD VOLTAGE 
(the point at which it starts to conduct), allowing current to flow, and 
emitting photons in the process. The other diode was REVERSE BIASED 
(cathode positive, anode negative), and a negligible amount of current 
flowed through it and no photons were emitted. When you reverse the 
battery, the other diode became forward biased and emitted photons, while 
the other became reverse biased and didn't. So, why do we need the 
resistor? Protection!! A LED, when it is forward biased, acts almost like 
a piece of solid wire and will conduct as much current (amperes) as the 
power supply can supply. The problem here is that a LED is a very small 
semiconductor device, and one of its' components is an anode wire, which 
is thinner than a human hair and unable to conduct very much current. The 
resistor limits the amount of current that can pass through the circuit 
and prevents the diode from burning itself up in one heck of a hurry. If 
you look at a specification sheet for a particular LED, it will show the 
absolute maximum ratings of it. The DC FORWARD CURRENT is the maximum 
current it can handle on a continuous basis. For a typical high-intensity 
LED like the Hewlett Packard HLMP series, this is 50 mA. Beyond that, it 
will burn out or have a severely shortened lifetime. You can figure out 
what size resister you need by applying Ohm's Law - I=E/R, where 
I=Amperes, E=Volts and R=Ohms. Plug the maximum current from the spec 
sheet and the voltage of your power supply into the formula, and Viola! 
Your resistance value jumps out. For a 3 volt supply, it should be in the 
range of 50-100 ohms. If not, re-check your calculations. Your teacher can 
help you if you get stuck. Don't forget that we are dealing with 
MILLIAMPERES and the formula uses amperes. Make sure you get your decimal 
ppoint in the right place! One last caveat: The spec sheet will also show 
a MAXIMUM REVERSE VOLTAGE (remember we talked about reverse biasing?). 
Even though we said that no current will flow in the reverse direction, 
exceeding this maximum voltage will also destroy the device. In the case 
of the HLMP series, the max. is listed as 5 Volts for a 100 microampere 
current flow. You can get the maximum amount of light from the unit by 
using a 3-Volt power supply and adjusting the resistor value for near-
maximum allowable FORWARD current flow, without having to worry about 
exceeding the maximum reverse voltage. One last interesting fact about 
LED's. When they 'burn out', they don't open up like a regular light bulb 
filament would; they usually short out, so don't try to use them like a 

If I can be of any further help, you may contact me directly at Karl-

Have fun, and keep experimenting - it's a great way to learn!

Your not-so-mad scientist,
Karl Kolbus     

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