We argue over an 1-cent part? Probably 0.1¢ in high quantity.I've seen LED V-I curves with differences between "Ion" and Imax anywhere between 100-500mV.
You can probably get away with not using a resistor with higher voltage leds (green,blue,white etc) . Try it out , measure the current across a voltage range and see how much it varies across a few same type leds.
You might get away with it. Or, LED dissipation = increased temperature = lower Vf = greater current = greater heating = .......POP!Then, the supply can be used without a resistor.
What's going to limit the current at 10ma?If a led is rated -say- 1.88Vf, for a 10mA current; the led can be operated perfectly well with a fixed 1.88V supply with no need for any resistor, and will pass 10mA.
Check the data sheet for the particular led you want to operate with no resistor, apply that exact voltage Vf, and use an accurate voltmeter.
If no data sheet for it, measure Vf at the desired current. Then, the supply can be used without a resistor.
The characteristics of the particular led.What's going to limit the current at 10ma?
Not quite. If you you buy a 100 LEDs (same manufacturer and type), at exactly 3.6V some in the lot may draw 1mA and others may draw 100mA and go Poof!...
This is how I understand it:
An LED rated at 3.6V and 20mA will draw 20mA if supplied with 3.6V. ...
Thanks! I'm assuming you mean 20mA of current produces a different forward voltage for each LED.The batch of 100 is likely to be distributed along a bell curve, where most might draw closer to 20mA, but there will be outliers...
That is correct, but that is not what you postulated. You assumed that if you apply a fixed voltage of 3.600V, you can expect the LED to draw 20mA. I am telling you that you may get a wide range of currents, even in the same batch...Thanks! I'm assuming you mean 20mA of current produces a different forward voltage for each LED.
Yup, my statement was in reverse. Makes a lot more sense as to why it's a current driven device.That is correct, but that is not what you postulated. You assumed that if you apply a fixed voltage of 3.600V, you can expect the LED to draw 20mA. I am telling you that you may get a wide range of currents, even in the same batch...
If you apply exactly 20.00mA (using a constant current supply) to a batch of LEDs from some manufacturer, you will likely see a range of forward voltages from 3.3 to 4V...
Which is the cause and which is the effect??? Chicken or the Egg.
I think it was discussed here that button cell batteries have decent internal resistance.I'm a little late in this conversation, but would like to add that the little LED keychains that companies give away as advertising swag are usually a button cell battery and a white LED...and no resistor. An LED will light up to some extent if you're *close* to its turn-on voltage; perhaps this is how they safely eliminate the need for the resistor...or...the resistor may even be integrated (aside from the inherent impedance of the LED) INTO the LED...who knows.
So the answer to the OPs question is: it depends, at least in part, on...
supply voltage
LED forward voltage and current
LED impedance
Internal LED resistor?
maximum current out of supply
etc etc.
by Aaron Carman
by Jake Hertz
by Jake Hertz
by Aaron Carman