Here's something I'm curious about since Ohms law doesn't apply correctly to a "negative" voltage difference. What exactly happens if the led is powered with less than the voltage drop of the led? Will it just not light? But that can't be, I have tried to power leds with less V than the led's rated V, and they do light...
What happens if LED is powered w/less than the voltage drop?
- Thread starter Ethan
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The led's voltage rating is the voltage at which it lights up. It's not a sharp point, more like a soft-knee, where it starts lightning up. But don't expect to see anything below some 1.5V, depending a lot on specific led type...
Jakob E.
Jakob E.
No, as long as you don't go so low that you reverse the voltage :grin:
> Ohms law doesn't apply correctly to a "negative" voltage difference.
??? Sure it does.
However a silicon diode (or LED) does not have a simple number to plug into Ohm's Law.
I don't think you want the exact math. Anyway real LED's don't follow theory too exactly. But here are voltages and currents I would expect on a typical red LED:
2.0V 200mA (and death)
1.7V 20mA
1.5V 2mA
1.3V 0.2mA
The exact action at lower voltages is unclear, and varies in practice. It used to be common to find leaky LEDs: give them less than 1mA, they would not light at all, the 1mA of leakage just leaked around the junction. There are also some that act like several junctions, and will light weakly with less than 1.3V. But this is unpredictable.
Reverse voltage is not instant death to red or green LEDs. 5V is harmless. They "break down" around 7V reverse, but if the series resistor holds the current to not much more than 10mA they will not be harmed. They just don't light. A reversed LED without a resistor will burn up fast with more than about 7V, but remember that a forward LED without resistor will burn up fast with more than about 1.8 or 2.0 volts!
You might think you could just feed a constant 1.7V, but the current you get will vary a LOT with temperature, and once warm will probably go hot until it burns up.
??? Sure it does.
However a silicon diode (or LED) does not have a simple number to plug into Ohm's Law.
I don't think you want the exact math. Anyway real LED's don't follow theory too exactly. But here are voltages and currents I would expect on a typical red LED:
2.0V 200mA (and death)
1.7V 20mA
1.5V 2mA
1.3V 0.2mA
The exact action at lower voltages is unclear, and varies in practice. It used to be common to find leaky LEDs: give them less than 1mA, they would not light at all, the 1mA of leakage just leaked around the junction. There are also some that act like several junctions, and will light weakly with less than 1.3V. But this is unpredictable.
Reverse voltage is not instant death to red or green LEDs. 5V is harmless. They "break down" around 7V reverse, but if the series resistor holds the current to not much more than 10mA they will not be harmed. They just don't light. A reversed LED without a resistor will burn up fast with more than about 7V, but remember that a forward LED without resistor will burn up fast with more than about 1.8 or 2.0 volts!
You might think you could just feed a constant 1.7V, but the current you get will vary a LOT with temperature, and once warm will probably go hot until it burns up.
[quote author="PRR"]> Ohms law doesn't apply correctly to a "negative" voltage difference.
??? Sure it does.
[/quote]
What I meant was that when you take a supply voltage of say 5V and subtract the forward drop of 3, 20ma LEDs in series to be around 6V you get -1. Then proceeding with Ohms law you would get negative resistance.
Thanks for the info guys!
??? Sure it does.
[/quote]
What I meant was that when you take a supply voltage of say 5V and subtract the forward drop of 3, 20ma LEDs in series to be around 6V you get -1. Then proceeding with Ohms law you would get negative resistance.
Thanks for the info guys!
AMZ-FX
Well-known member
The rated voltage in an LED is the minimum at which it will produce the rated lumens.... they certainly will light up below the rated voltage but will be much dimmer as you noted. I have a guitar distortion pedal on my bench that I was repairing last night and it uses a pair of red LEDs as clipping diodes. The more you crank up the drive control, the brighter the LEDs get! At low drive levels you can hear distortion of the signal but the light from the LEDs is so dim you have to darken the room to see it.
regards, Jack
regards, Jack
