Several days ago I read this post:

http://forum.allaboutcircuits.com/threads/why-does-the-current-get-low.127064/page-2#post-1035886

This part is what I want to talk about:

LED's are not voltage devices they are current devices. If you had 120 volts and wanted 10 mA through it then (120 - 3) / 0.01 = 11,700 (ohms, or 11.7KΩ for a total of 0.01 amps.) The same LED would be just as bright in either circuit. But with such a high voltage, subtracting the forward voltage of the LED isn't going to make much difference. If I ignored the forward voltage of 3 volts then I'd be using a 120,000Ω (120KΩ) resistor. That'd give me 0.010256 amps, a difference of 256 micro-amps. Certainly nothing that's going to blow up anybody's circuit.

I said one of my friend that we can turn on an LED with 100V and we just need to keep current constant. even without resistor! I said him that it doesn't matter how much voltage you use. he said me you can't. I said him above example(I mean this part: (120 - 3) / 0.01 = 11,700) and he said you drop 117V when you use resistor. and he told me about something like gamma voltage and I cannot turn on an LED with each voltage. Is he right?

 

Can we trun on an LED without resistor by keeping current constant?

The current through an LED doesn't have to be kept constant; it just has to be kept within a safe range such that there is enough current to make the LED glow with adequate brightness, but not so much current that the LED gets damaged (e.g., through overheating).

That's all that matters. Exactly how you achieve that current control (via a series resistor, or a linear constant-current regulator circuit, or a constant-current switching regulator, or whatever) is completely irrelevant, as far as the LED is concerned.

 

Certainly. The whole point of using a resistor is to control the current through the LED. The main point of using a constant current source is to control the current through the LED. There is almost zero difference.

 

This answer made me confused:

http://electronics.stackexchange.com/a/177387/29617

What does he mean? e.g. we cannot make a source with 100V 30mA?

Edit: I have several LED with 1.6V and 3.5V Vf and a 9V battery and I turn them on without any problem while its output voltage(Battery) is 9V.

 

This answer made me confused:

http://electronics.stackexchange.com/a/177387/29617

I don't see what could possibly be confusing about that answer. It is straightforward, clear and correct.

 

This answer made me confused:

http://electronics.stackexchange.com/a/177387/29617

What does he mean? e.g. we cannot make a source with 100V 30mA?

Edit: I have several LED with 1.6V and 3.5V Vf and a 9V battery and I turn them on without any problem while its output voltage(Battery) is 9V.

It is not possible to have both a constant voltage source of 100V AND constant current of 30 mA across an LED. At 30 mA, the forward voltage across the LED will be what it says in the datasheet for that LED (generally 1.8 to 3.3 v depending on the LED). So, where do you expect the other 97 volts to go?

The whole point of a constant current source is that the source can adjust to always supply only 30 mA. If the load is a 10 ohm resistor, then the voltage drop from the constant current source is 0.030A x 10 ohms = 0.3 v. If the forward voltage of your LED is 3.3V at 30 mA, then the voltage from the constant current source to ground (or said differently, the voltage across your LED ) will be 3.3V.

 

You can think of a constant-current circuit as 'a sort of resistor which automatically adjusts itself so that the current through it stays the same regardless of the voltage across the circuit'.

 

It is not possible to have both a constant voltage source of 100V AND constant current of 30 mA across an LED. At 30 mA, the forward voltage across the LED will be what it says in the datasheet for that LED (generally 1.8 to 3.3 v depending on the LED). So, where do you expect the other 97 volts to go?...

...
That's the thing which is a question to me! we said it doesn't matter how much voltage we apply to LED but now you said "97 volts to go". you guys said LEDs are current devices.

 

...
That's the thing which is a question to me! we said it doesn't matter how much voltage we apply to LED but now you said "97 volts to go". you guys said LEDs are current devices.

The constant current device (circuitry) must absorb (dissipate) or avoid the development of the remaining 97 volts. Look up constant "current source" and you'll see that a heat sink will be required on the transistor that drops the 97V x 30 mA (3 watts). A constant current source must be made of active devices, it cannot be done with passive a (resistors) unless the load and voltage source are constant.

 

...
That's the thing which is a question to me! we said it doesn't matter how much voltage we apply to LED but now you said "97 volts to go". you guys said LEDs are current devices.

Yes, 100 V applied, as long as there is a resistor to drop the rest of the voltage. At the rated 30 mA it will drop a fairly consistent voltage across the LED. The rest of the voltage must be dropped across a resistor or a constant current source, or something.
The constant current source drops the rest of the voltage. It is not a constant voltage source always putting out 100 V. It puts out the 30 mA, 2 or 3 volts drop across the LED, the rest of the 100 V is dropped across the constant current source.
Get some LEDs, sit down and play with them. Theory is confusing you. Get some hands on experience with them.

 

The crux of the matter is that you can regulate current to a load or you can regulate voltage to a load, but you can't regulate both at the same time.
If you have a 100V supply and a 3V LED which needs 20mA of current, then you need something (resistor or constant-current source) to drop 97v from the 100v supply.

 

The LED cannot be allowed to "see" the 100V on its own. If it does, it will conduct a massive current until it explodes like a flashbulb. Whatever is in between the 100V and the LED must present only about 3V to the LED. At that level, the LED will be passingly only ~20mA of current. Even 1/2V more might destroy it. It's usually easier and more effective to control the current to that ~20mA than to regulate the voltage so precisely.

 

Is he right?

both of you are right, and you are talking about slightly different things.

1) you are right in that you don't need anything else to drive an led as long as you can maintain the current through it. That's absolutely true.
2) your friend is right in that in order to maintain the current through the led, you have to have other means to drop the excess voltage on something other than the led.

The thing is, if you apply a current through the led, it provides a deterministic voltage drop across the led. So if the led drops off 2v @ 10ma, and you want to apply 100v to the led, you will have to drop 98v on something other than the led.