More current same voltage

Thread Starter

cjdelphi

Joined Mar 26, 2009
272
More current same voltage possible on an LED?

3.0v might consume 50ma is it possible to force 100ma but keep 3.0v? (I presume impossible....according to ohms law)
 

beenthere

Joined Apr 20, 2004
15,808
Not at all impossible. A LED is a special case of a diode, with a PN junction. It requires a certain voltage to bias the junction into conduction, but essentially has no resistance after that. Many people burn up LED's with too much current, as they do not understand how LED's work. There will be some level of current that will overheat the PN junction and cause the device to fail.

Here is our Ebook article on them - http://www.allaboutcircuits.com/vol_3/chpt_3/12.html
 

Audioguru

Joined Dec 20, 2007
11,251
The datasheet for the LED has a graph of typical forward voltage vs current.
If it is 3.0V at 10mA then it might be 3.2V at 30mA.

Most ordinary LEDs burn out if their current is more than 30mA.
The colour and temperature of the LED affects its voltage.
 

leftyretro

Joined Nov 25, 2008
394
Yes, forget about voltage when powering LED's, just limit or control current to the amount you desire that is still within the max specs of the LED. If you can PWM the drive then you can supply more current then the steady state value. Study the LED's spec sheet and learn the several ways to control or limit current, not voltage.

Lefty
 

David Bridgen

Joined Feb 10, 2005
278
For example

12v source.

12-3/0.020 = 450ohms

12v+ 450ohms +LED- Ground (Current will be 20ma)

Now if you look at the voltage across that LED it may read 2.8volts get the forward voltage reading of that LED now then increase the current without 2.8v moving ie 2.9v, 3.0v just increase current not voltage.
You clearly don't understand l.e.ds in particular, or diodes in general. Nor do you seem to understand Audioguru's post.

A diode is a non-linear device. The voltage developed across (not applied to) a diode is dependent on the current through it.

Increase the current through it and the voltage across it will increase too, however inconvenient this may be for you.

But, since the diode is non-linear, an increase in current by a certain percentage does not result in an a voltage increase of the same percentage.

Take a look at this, it might help you ..... http://www.davidbridgen.com/leds.htm
 

mik3

Joined Feb 4, 2008
4,846
More current same voltage possible on an LED?

3.0v might consume 50ma is it possible to force 100ma but keep 3.0v? (I presume impossible....according to ohms law)
It is impossible to force more current and keep the voltage across the diode exactly the same due to the bulk resistance of the diode and imperfections of the junction. However, if you force 100mA instead of 50mA the voltage across the diode will change slightly and won't double like a resistor due to the diode characteristics.
 

SgtWookie

Joined Jul 17, 2007
22,201
I'm surprised nobody brought up thermal considerations yet.

Assuming that you're supplying current to a PN junction (including LEDs) via a low-impedance regulated voltage source, you can increase the current through the diode without changing the voltage by increasing the temperature of the diode.

However, just past the point where the power being dissipated in the LED is greater than the ability of the PN junction and it's surroundings to dissipate that heat by conduction, convection and/or radiation, the junction will enter a "thermal runaway" condition.
 
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