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

Discussion in 'General Electronics Chat' started by booboo, Sep 11, 2016.

  1. booboo

    Thread Starter Member

    Apr 25, 2015
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    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:

    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?
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
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    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.
     
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  3. #12

    Expert

    Nov 30, 2010
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    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.
     
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  4. booboo

    Thread Starter Member

    Apr 25, 2015
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    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?:rolleyes:

    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.
     
  5. OBW0549

    Well-Known Member

    Mar 2, 2015
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  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    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.
     
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  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    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'.
     
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  8. booboo

    Thread Starter Member

    Apr 25, 2015
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    ...
    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.:rolleyes:
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    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.
     
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  10. hp1729

    Well-Known Member

    Nov 23, 2015
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    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.
     
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  11. crutschow

    Expert

    Mar 14, 2008
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    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.
     
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  12. wayneh

    Expert

    Sep 9, 2010
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    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.
     
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  13. dannyf

    Well-Known Member

    Sep 13, 2015
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    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.
     
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