thomas
Who needs headphones when there's the JVC MX-GT80?
- Joined
- Jun 22, 2001
- Posts
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- 11
Quote:
Actually, i realized my reply was partially wrong, the chart AOS linked to was what i meant to say...
The resistance of a diode shoud be infinate at low voltages and very close to zero at high voltages like you said, (rather than reaching a constant as a said),but the reason that never happens is because you can't supply enough voltage to completely forward bias the diode. Instead the resistance of the LED is ALWAYS in the non-linear range when it is producing light, if you supply enough voltage to completely forward bias it (ie zero resistance), it would probably burn it out.
And the diode's resistance? it varies depending on voltage, as AOS said- there's no way of finding out without measuring the current. I usually find it to be in the range of 100-1000 ohms when it is producing some light, but only a small change in voltage will take it outside that range, and different LED types/colours will also change it.
Quote:
Everyone posting so far has agreed on this point- the voltage measured is the source voltage, and its lower than the voltage measued on an open circuit because of the internal resistance of the battery...
Quote:
My calculation was about the total power dissipated by the LED, including heat and light. I wrote it in that form to clearly show that the resistance could not be zero (ie the diode is not fully biased) while the LED is emitting light.
I'm guessing that your equation is a convention of optoelectronics, which i have not studied, and that you've simply seperated the joule heating from the light production. I wrote the equation in that way to "prove" that the LED must have non-zero resistance when it is emitting light. Isquared* R may not have any practical applications (since you don't know R), but it is useful in explaining this subject.
I stand corrected. |
Actually, i realized my reply was partially wrong, the chart AOS linked to was what i meant to say...
The resistance of a diode shoud be infinate at low voltages and very close to zero at high voltages like you said, (rather than reaching a constant as a said),but the reason that never happens is because you can't supply enough voltage to completely forward bias the diode. Instead the resistance of the LED is ALWAYS in the non-linear range when it is producing light, if you supply enough voltage to completely forward bias it (ie zero resistance), it would probably burn it out.
And the diode's resistance? it varies depending on voltage, as AOS said- there's no way of finding out without measuring the current. I usually find it to be in the range of 100-1000 ohms when it is producing some light, but only a small change in voltage will take it outside that range, and different LED types/colours will also change it.
Quote:
All the explanation does not suit my knowledge. Except stereth's. You are not measuring the LED forward bias voltage but measuring the battery suply voltage. The voltage drop is due to the internal resistance of the battery when high current passed through. Remember that voltmeter has high resistance so that when you measure at no load will give you higher voltage |
Everyone posting so far has agreed on this point- the voltage measured is the source voltage, and its lower than the voltage measued on an open circuit because of the internal resistance of the battery...
Quote:
I square R is heat, not the energy to light up your LED. Power equation is IxIx(R_led+R_battery) + V(voltage for light)I |
My calculation was about the total power dissipated by the LED, including heat and light. I wrote it in that form to clearly show that the resistance could not be zero (ie the diode is not fully biased) while the LED is emitting light.
I'm guessing that your equation is a convention of optoelectronics, which i have not studied, and that you've simply seperated the joule heating from the light production. I wrote the equation in that way to "prove" that the LED must have non-zero resistance when it is emitting light. Isquared* R may not have any practical applications (since you don't know R), but it is useful in explaining this subject.