"Perfect" LED current limiting challenge

[philba]

gorgon is right - he describes the most common usage of the term. That is how the term is used relating to PWM. You see this in motor control discussions.

[Chris Smith]

I dont recall that this is a discussion of PWM per se, with or without motors.

Its about Lasers and Leds with high speed switching. Those terms apply.

Terms used for one, do not necessarily apply to the other just because.

[philba]

this is about power control via PWM and the average current derived. however, you may choose to claim what ever you want but it doesn't make you right.

[dyarker]

Chris,

"If you apply duty cycle to time, then Time is divided up into time ON V.S... time OFF." Nope!

Try - "If you apply duty cycle to time, then Time is divided up into time ON V.S... time OF ENTIRE CYCLE."

Duty Cycle = ON / (ON + OFF)
Duty Cycle% = (ON / (ON + OFF)) * 100
--------------------------
How did you get to:
Duty Cycle = ON / OFF
from your quote “The product of pulse duration AND pulse repetition rate from a wave [radio] consisting of periodic pulses”.
"pulse repetition rate" = 1 / "time of one entire cycle". That includes both ON and OFF times in case you haven't heard, and don't even bother to read your own quotes.

-------------------------------

"For a 200 ns pulse at 1000 pulses per second
[.1% duty cycle] the following is true."
Any time you multiply a number containing only a 2 and zeros by a number containing only a 1 and zeros, you get a number containing a 2 and zeros.
Any time you divide a number containing only a 1 and zeros by a number containing only a 2 and zeros, you get a number containing a 5 and zeros.
--------------------------------
And, how does creating a 100A pulse help Ian squeeze 10 more minutes out of some D cells for a simple indicator??????????????

[jimandy]

This discussion reminds me of that line...

"If you can't bedazzle 'em with brilliance then baffle 'em with
bull sh-t."

[rshayes]

The terminology is basically the same for most pulsed techniques. It hasn't changed much since World War II radar, which was 60 years ago.

Duty cycle still means the fraction of the time that the pulse is present. This applies to radar, laser pulsers, switching power supplies, and motor controllers. Usually, it is easiest to compute the duty cycle by looking at one period of a recurring pulse train.

The duty cycle is simply the pulse period (seconds) divided by the cycle period (seconds). It can also be put in the form of pulse period (seconds) times repetition rate (hertz). The same relationships apply no matter what the application is.

It is obvious from the several posts that Chris has made that he does not understand basic arithmetic. He is trying to cover this up by claiming some special knowledge that applies only to pulsed diodes and that everyone else is ignorant of this "special knowledge".

A company called Laser Diode Labs was marketing pulsed diode arrays with outputs in the tens to hundreds of watts by 1968. These were mounted on copper blocks which needed to be used with a heat sink or a cold plate. Drive voltages were in the 50 to 100 volt range with currents in the 10 to 100 amp range. Multiple bond wires using heavy bonding wire were used to handle the large currents. This is the type of construction needed to withstand 100 volt and 100 amp pulses. The drivers for these would have used the same techniques that were used for radar with no basic difference in terminology.

Pulsing laser diodes and LEDs is not a new technique. It has been around for at least 40 years.

Modern laser diodes and LEDs have been improved to the point where this type of operation is unnecessary for most applications. LEDs reach their maximum efficiency in the 10 to 20 milliamp range. The threshold currents of solid state lasers have been brought down to the point where CW operation is possible with only moderate heatsinks, such as those used in the CD or DVD player.

[jimandy]

Oh, I get it. If my bathroom heater is on for 2 minutes and off for one minute, and then back on for 2....(etc.) the duty cycle is 2 (time on in minutes) divided by 3 (total cycle time in minutes) or a ratio of 2 to 3 which is same as 66.666.... percent.

and when its warm enough in there my doodie factor kicks in.

[Mike6158]

Originally posted by jimandy:
Oh, I get it. If my bathroom heater is on for 2 minutes and off for one minute, and then back on for 2....(etc.) the duty cycle is 2 (time on in minutes) divided by 3 (total cycle time in minutes) or a ratio of 2 to 3 which is same as 66.666.... percent.

and when its warm enough in there my doodie factor kicks in.

Ah... so that's what happend to this thread? The doodie factor kicked in?

[Chris Smith]

Duty cycle according to the "Communication Standard Dictionary" is......

"In Radio Transmission the daily schedule of a transmitting station" ......

.....showing that Duty cycle, only applies to WHAT subject your talking about.

You can insist its what you mean, or what you think it means, when some one else is talking but regardless of this, it applies to what they [not you] or the standard has assigned it to be interpreted as.

In radio its one thing, with DC its another, in lasers, its yet another.

But then again, name one detractor here that has actual experience in the field of lasers?

None. Gee, who could have guessed that one?

But as usual those with the least experience are ALWAYS the loudest detractors. Learning hurts.

Dazzle with ignorance and the crowd will follow.

Sheep must attack once in a while to feel their power? The power of ignorance.

And Im sure google has their own opinion.

Good luck making your semantic approach, diverting away from the main subject at every chance you get, just to pretend you can shoot ducks in a pond. Sorry, Still No sale once more.

Back to the subject before the morons deviated, got lost in their own semantics and suffered from a lack of literacy on the subject of English, technical words, and just electronics as a whole,....

..........The subject was Lasers and Leds under high pulsing rates.

And still unchallenged and unchanged......

“A laser running in a standard mode of 20 to 200 ns, using 100 amp pulses, still draws less current than a led running in a DC mode at 20 mills” and thus has less heat to rid it self of than the standard LED. Fact.

For those who cant learn or understand this simple fact and feel distraction some how makes their position appear stronger, My sympathies go out to your dyslexic and moronic approach. But dont feel bad, you have groupies that follow you every where.

PS,..Dyslexic, Doo Doo, Duty,.....all start with the letter D, and for some these are the same thing, hence their learning disorder.

And just for Steven who still doesn’t get it, its called a “Duty factor”.

Buy a dictionary of technical standards and avoid your future mistakes.

<small>[ January 06, 2006, 09:11 PM: Message edited by: Chris Smith ]</small>

[Jarhead]

DICTIONARY OF TECHNICAL TERMS FOR AEROSPACE USE
Web edition edited by Daniel R. Glover, Jr.
NASA Lewis Research Center
Cleveland, Ohio
INTRODUCTION
This dictionary was orignally published as NASA SP-7 in 1965. The editor for the first edition was William H. Allen. The introduction to the first edition describes how the original dictionary was prepared. The Web edition cannot hope to match the rigorous procedures followed by the original, but it is hoped that it will still be useful. The dictionary's accuracy will depend on the quality of feedback that we get from users. Please browse the dictionary and look for missing terms, out-of-date terms, typos, etc., and let us know if you have any constructive suggestions for improvement.


duty factor:
1. In computer operations, the ratio of active time to total time.
2. In a pulse carrier composed of pulses that recur at regular intervals, the product of the pulse duration and the pulse repetition frequency.
duty ratio
In a pulse radar or similar system the ratio of average to peak pulse power.
http://roland.lerc.nasa.gov/~dglover/dictionary/d.html

This section addresses safety requirements for practices most commonly undertaken, including some of requirements of the U.S. Department of Energy (DOE), Occupational Safety and Health Act (OSHA), and PPPL. The provisions of this section apply during the construction, alteration, modifications, moving, or demolition of any building or structures, systems and components at PPPL, and are applicable to both Laboratory personnel and outside subcontractors.
TCR-5008, Sect 4, R4-001
Duty Factor - The ratio of pulse duration to the pulse period of a periodic pulse train. A duty factor of 1.0 corresponds to continuous-wave (CW) operation.


http://www.microwaves101.com/encycloped ... efinitions
The pulse repetition frequency (PRF) is the same as the repetition rate (a.k.a. rep rate) and is the frequency that the signal is turned on and off through a full cycle, measured in Hertz. It is the reciprocal of the pulse period.

The pulse width is the length of time that the RF is switched on in one pulse period.

The duty factor is the time that the pulse is on, divided by the period. A 100% duty factor implies that you have a continuous wave signal. Duty factor can be expressed in decibels as well as percent; a 10% duty factor is -10 dB.


We at Power Technology, Inc. have been a driving force in the laser industry for over thirty-five years. Since 1969, we have been designing, developing, and manufacturing a comprehensive range of laser products for scientific, biomedical, and industrial applications.

Glossary Of Laser Diode & Photonics Terms:

Calculating Duty Factor
Duty factor (also known as duty cycle) is the ratio of pulse duration to pulse period. Duty factor (Df) is calculated as follows.

Df = pulse duration (sec) / pulse repetition period (sec)

Multiply the result by 100 to get your answer as a percentage.

Please note that as pulse repetition frequency increases, duty factor increases. As pulse repetition period increases, duty factor decreases. As pulse duration increases, duty factor increases.
http://www.powertechnology.com/calculations.asp


Though one would have to wonder whether the discussion participants understand how Laser Diodes are often rated. An example is what is commonly called a 50 W Laser Diode. It does not emit 50 W of power, continous, or on an averaged basis. It has a peak power of 50W, for a 50nS period of time, and the duty factor is 0.025% (please note the percent there...), for an average power of 0.0125 Watts. See further examples here:
http://www.mit-laser.cz/a/pulsed%20laser%20diodes.htm

MOCVD grown double heterostructure laser diodes at 1550nm with peak output powers of 4 W, 8 W, and 40 W are offered as standard products. The wavelength of these devices is centered at 1550nm to take advantage of an increase over A1GaAs and InGaAs lasers in the maximum permitted emission levels for eye-safe operation with respect to FDA requirements. Class 1 operation therefore should be possible with relatively high output powers
http://optoelectronics.perkinelmer.com/ ... D=PVGR4S12

Note the term peak output power.

Take a look at this datasheet to understand the concept:
http://optoelectronics.perkinelmer.com/ ... Diodes.pdf

[philba]

Originally posted by Chris Smith:
...
But as usual those with the least experience are ALWAYS the loudest detractors. Learning hurts.
...

I'm not sure how you measure loudness in a forum like this but I think the one with the most posts would be a good choice. Or possibly the most ad-hominum atacks.

[rshayes]

To quote Chris:

"A laser running in a standard mode of 20 to 200 ns, using 100 amp pulses, still draws less current than a led running in a DC mode at 20 mills? and thus has less heat to rid it self of than the standard LED. Fact."

This totally ingnores a basic property of semiconductors- They are not good conductors.

The active area of a diode, LED, laser diode, or other junction device is suprisingly enough, the junction. This is a boundary between two types of semiconductor or possibly a semiconductor and a metal. Junction effects occur within a very small distance of the junction, usually within a few microns. The current through a forward biased junction tends to be an exponential function of the voltge across the junction. The scale factor for the voltage is usually about 75 to 150 millivolts for each decade increase in current.

Current passing through the junction must also pass thrugh a mass of semiconductor material on each side of the junction to flow through an external circuit. This semiconductor material acts like ohmic resistors in series with each side of the junction. The value of this resistance depends on the area of the current path, the length of the path, and the dopant concentration in the semiconductor. As current flows through these resistances, there is a voltage drop which follows Ohm's Law which is added to the voltage drop across the actual junction.

On a linear plot of current vs. voltage, the resistive portion causes the overall characteristic to become a straight line at high currents. On a semi log plot, the exponential portion due to the junction becomes a straight line at low currents. This allow measuring the value of the series resistance. This has been a standard technique for over 60 years.

In the case of small laser diodes and LED's this resistance is in the neighborhood of 10 to 20 ohms. In a laser diode designed for pulsed operation, it may be a few tenths of an ohm. However, these are not the parts that we can buy without a major investment.

When operated at 20 milliamps, a 10 ohm series resistance will increase the diode voltage by about 200 millivolts. For a red LED, the junction voltage will be about 1.5 volts and the additional 200 millivolts is tolerable. The power dissipation will be 30 milliwatts across the junction plus about 4 milliwatts in the resistors for a total of 34 milliwatts. So far, so good.

If you increase the current to 200 milliamps, the junction voltage increases to somewhere around 1.575 volts, the junction power increases to 315 milliwatts, and the resistor power increases to 400 milliwatts, for a total of 715 milliwatts. The power in the device is nearly 20 times higher, so the duty cycle will have to be reduced to about 4.8 percent to maintain the same average die temperature. The total voltage required increases to around 3.6 volts from the original 1.7 volts. The average light output will be less than 48 percent of the original value.

Increasing the current to 2 amps increases the total power to 43.3 watts. The duty cycle must be reduced to about .08 percent and the drive voltage needed has increased to about 22 volts. The average light output is now about 8 percent of the original value.

At 20 amps, the power is 4036 watts, the duty cycle must be reduced to .0008 percent, and the drive voltage has increased to 202 volts. The average light output is now down to .8 percent.

At a repetition frequency of 1 KHz, the maximum pulse width would be limited to about 8 nanoseconds for the 20 amp case.

It is obvious that operating LEDs and lasers meant for continuous operation in a pulsed mode drastically reduces the average light output and efficiency. Unless you need a pulsed source, it only increases the circuit complexity with nothing gained to offset it.

As for this:

"But then again, name one detractor here that has actual experience in the field of lasers?

None. Gee, who could have guessed that one?"

As it happens, I have designed and built fiber optic links that used diode lasers modulated in the 100 MHz range. That's one. "Jarhead" is another. I am sure that there are other participants in this forum with similar experience. This statement demonstrates Chris's willingness to make absolute statements even when he has no knowledge whatsoever of their accuracy.

<small>[ January 07, 2006, 01:20 AM: Message edited by: stephen ]</small>

[Chris Smith]

1% duty cycle at 100 amps

200 ns on time [pulse duration]
19,800 ns off time [dwell]

Duty cycle for the above is 1% [99:1]

50,000 cps [pulse repetition]

10,000,000 ns total on time
990,000,000 ns total off time

Also the duty cycle for the above is 1% [99:1]

Add pulse duration + pulse repetition = Duty factor

[per each period of one second]

1% times 1% = 0.0001% duty factor
[not duty cycle]

as per Communication Standard Dictionary.


Yep, still less draw than a DC current.

And as usual, Steven speaks with ZERO experience in lasers or high speed switching for Leds.

Boundary layers and Plasma don’t mix.

But what the hell, theoretically and with out any working knowledge in the subject, any thing can sound possible if he tries hard enough? Just mix in two parts of this from some where else, and three parts of that, and it sounds good?

But its still irrelevant as he tries to rewrite old and ancient science, electronics, history, and the facts.

But there is a humor value watching others describe something that cant be done, that has already been done, and decades earlier at that.

<small>[ January 07, 2006, 02:21 PM: Message edited by: Chris Smith ]</small>

[ian]

Guys, I suggest you stop trying to refute Chris' nonsense, believe me, he can go on forever spewing this kind of crap. For every dopey thing you refute, Chris will add two more.

[Chris Smith]

Wassa matter Ian, you lose your argument because you had no legs to stand on in the first place.

It would be wiser that you dont engage in things you know nothing about, before you post.

But feel free to ask your groupies for advice here too?