Instructables

Reverse/forward voltage explained?

Hey,

i am looking to buy some LED's for an upcoming project but I have been confused by the terms reverse voltage and forward voltage.

Which one reffers to the voltage required if the LEd is corrected in the right way.

for example, buying some LED's from ebay it saisd this
"Forward Voltage typ/max: 2.2V/2.8V
Reverse Voltage: 5V "

Does that mean with the LED connected (anode positive and cathode negative) i would need 5volts of power? or would i need 2.2/2.8volts?


thanks

The FORWARD voltage is the volt drop through the LED, when the nominal current is flowing, when the ANODE is more positve than the CATHODE. Here, you will see betwen 2.2 and 2.8V in normal operation.

REVERSE voltage is the maximum the LED can stand the other way round without damage.

as someone who found this on a google search, this answer is flipping awesome and clear, thankyou!

Your welcome ;-)

recordmasta001 (author)  steveastrouk3 years ago
So how do i determine what voltage power supply to use for the LEDS?
As steveastrouk points out,
The Power supply needs to be greater then the forward voltage plus
an extra minimum of two more volts.   Six or nine volts are favorite values.
You must plan to put a resistor in series with the LED.
Pay attention, not to exceed the recommended LED current,
or the longevity will be cut down fast !
led9V.jpg
Take the FORWARD voltage, as the critical parameter and the rated forward current.

The rest depends on how many you want to deploy, and what voltage you have available to drive them.

Steve
You've seen this picture:
http://upload.wikimedia.org/wikipedia/commons/a/a5/Diode-IV-Curve.svg
right?

Because that's usually part of the story of the diode, a graph of current versus voltage, the so-called i-v characteristic.  Forward voltage and current are on the right hand side of this graph, and reverse voltage and breakdown are shown on the left hand side.

If you feel up to it,  you can even read the section of the article on diodes that refers to this graph, here:
http://en.wikipedia.org/wiki/Diode#Current.E2.80.93voltage_characteristic

Anyway, mostly I just wanted you to see the picture, because the folkore of diodes is really incomplete without it.  It's part of the story, and I'd feel like I wasn't really doing my job, as an educator, or storyteller,  if I didn't at least show you the picture.


diode-iv-characteristic.png