# Impedance and LED on 120V.

Discussion in 'General Electronics Chat' started by AcousticBruce, Mar 25, 2014.

1. ### ronvWell-Known MemberMost Helpful Member

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Ratchit,

To answer your question: Yes Vf can be more than a few volts. If you look at the IV curves you will notice the voltage goes up slowly with current. So if you look at a curve (like below) you can see that in this diodes case it is about 6.25 volts per amp. From this we can draw a few conclusions:
Your 3500 amp number ain't gonna happen.
The number will be more in the order of 50 to 60 amps. Not that that is to important except it explains why the time constant is longer. It's still about 60 mj.
While the energy is still the same with the resistor in series, with the resistor most of the energy is in the resistor and not the diode.

PS. If you look at a few good LED datasheets you will find some with specs on maximum current vs time. IR seem to be the best, White the worst. Guess that makes sense.

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2. ### RatchitWell-Known Member

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You are confused. You have calculated the dynamic resistance at one particular point of the curve. It will change at another point of the curve. The dynamic resistance does not tell you what you want to know. You need to apply the Schockley diode equation to see what the voltage value will be at a high current. That diode voltage-current curve you posted will rise almost vertically after a few volts, and will reach very high currents in a short hurry from then on. The dynamic resistance at that point will be near zero. Of course the diode will not be able to accomodate a current that large for more than an infinitesimal amount of time, but my calculations indicate that both the energy absorbed and time are small enough for it to do so.

Ratch

Last edited: Mar 31, 2014
3. ### ronvWell-Known MemberMost Helpful Member

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You remind me of some Phds. that have all the formulas, but you should never let them build anything.
Go away and come back with a real scope picture.

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5. ### RatchitWell-Known Member

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I figure that it is all right for folks to be pedantic as long as they are correct. So far, you have not proven me to be wrong. By the way, the curriculum to obtain a PhD in EE does require a lot of hands on lab work. So you are not going to stump a PhD in EE by handing him a piece of test equipment and asking him to use it. That attitude of thinking is promulgated by techs who need an ego boost.

Ratch

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Never mind.

7. ### tvtechWell-Known MemberMost Helpful Member

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Hi Bruce

Something simple has become complicated again. Stay tuned and I will help as much as I can.

I just need to find the energy.

All the best,
tvtech

8. ### 4pyrosWell-Known MemberMost Helpful Member

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Anyone with real world experience knows who Cree is!

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9. ### tvtechWell-Known MemberMost Helpful Member

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Lol 4pryros. Don't start Buddy

Cree....http://www.cree.com/

As if we ALL don't know

Never mind,

tvtech

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10. ### ronvWell-Known MemberMost Helpful Member

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Dynamic resistance is one thing, but don't forget about bulk resistance either. What did you use in your calculations?
I have this picture in my mind of you bent over your bench with a roll of Romex to get .1 ohms, this really big battery a capacitor and at the very end of this a little 40 ma LED. Across the ground lead of the Romex at power on you measure 335 volts and 3350 amps and across the LED you measure a 5 volt drop. Amazing... And here I thought you were just funning me...

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11. ### RatchitWell-Known Member

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Yes, a PhD in the field would know that. But I could find out all by myself if I really wanted take the trouble to do so.

Ratch

12. ### ronvWell-Known MemberMost Helpful Member

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13. ### 4pyrosWell-Known MemberMost Helpful Member

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In the field???? Its not like they are hiding. Do read the trade magazines? Its like saying you don't know who Hienz is.

14. ### RatchitWell-Known Member

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Didn't you read post #33, where I said the value of 0.1 was a guesstimate? The bulk resistance is included within that figure. By the way, further down post #33 I state that changing the resistance does not change the value of 60 millijoules much because the time constant increases while the current decreases.

I don't know why your mind would conjure up such a fantasy. Where did you get a 5 volts? Below is a typical voltage-current curve of a diode using Schlockey's equation to plot current against voltage. Always keep in mind, however, that the pulse period is infinitesimal and the energy is only 60 millijoules.

Ratch

Last edited: Apr 1, 2014
15. ### RatchitWell-Known Member

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Since we are talking about a transient analysis, I have no idea what you are talking about with respect to phase shift.

Ratch

16. ### RatchitWell-Known Member

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Yes, in the field. I would not ask a history PhD, for instance. I read some mags, but there is not enough time in the day to read even a smathering selection. It is Heinz, not Hienz. It is permissible to misspell language words, but folks get insulted if you don't get their names correct.

Ratch

17. ### ronvWell-Known MemberMost Helpful Member

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I eagerly await the results of your lab experiment.
What is the scale on the left of your curve?

18. ### RatchitWell-Known Member

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Lab experiment? What lab experiment? That is a calculation using Schlockey's diode equation. The x-axis is voltage and the y-axis is current. You can see that before 2 volts is reached, the current is over 20 million amps. It rises so rapidly because it is an exponential equation.

Ratch

19. ### ronvWell-Known MemberMost Helpful Member

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So maybe it's 2 ohms in the led and .1 in the wiring?

Why does the Vf of an Led start out at 2 or 3 volts instead of .3 or so of a diode?

20. ### RatchitWell-Known Member

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Who knows? 16 millijoules is so small it doesn't matter too much. And, I already averred that changing the resistance total does not affect the amount of energy the cap stores very much..

http://en.wikipedia.org/wiki/Light-emitting_diode

Show me. Unless the diode package contains something else other than a plain LED, its voltage-current curve should be similiar to a regular diode. That is what the link implies.

Ratch

21. ### ronvWell-Known MemberMost Helpful Member

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