Resistor Voltage drop

[GatorGnet]

I know this has been beaten to death but I am messing something up on my math when calculating voltage drop with resistors. I have a 12v supply and I need to drop it to 3v to power a red led. What resistor should I use?

 

[Mike odom]

no no no... The red LED will drop 3V across it... it's the nature of semiconductors... what you need to do is calculate the resistor size to set the current in the LED...

(12V-3V) / desired current. If you use current in mA, the resistor will be in Kohms.

for example: (12V - 3V) / 20mA = 9 / 20 = .45Kohms = 450 ohms = 470 or 430 ohms.

The amount of desired current is dependent on the particular LED, and how it's used (if high temp. environments you have to lower the current to ease back on power dissipation).

 

[k7elp60]

Just because the LED's current is 20mA there is no need to illuminate it at the full current. Using half the current the LED is almost always plenty bright. I use a lot of LED indicators on approximately 12V DC. I use a 1k 1/4watt resistors. The extra resistance gives some latitude if the voltage is higher than 12V.

 

[Mike odom]

the D155 is sunlight readable at 2mA and will burn a hole in your retina at 20mA (spoken from experience)...

 

[GatorGnet]

The leds I have burn out on anything over 3 volts. Would using a 10k resistor be enough to limit this? Again the supply is 12v.

 

[mneary]

There will be 9V across your resistor. If you want 2mA in the LED, the resistor wants to be 9/.002 = 4500 ohms. (4700 is close enough).

 

[Hero999]

The leds I have burn out on anything over 3 volts.

You should always use a series resistor, even at 3V.

3V sounds too high for a red LED, it's normally 1.8V to 2V.

 

[GatorGnet]

Thats why I asked what value resistor I should use. I only have a 12v supply on the board I want to put it on.

 

[Hero999]

You need to know the forward voltage of the LED and the current required.

As I said above, I doubt your red LED is really 3V, just because you might've got away with connecting it to a 3V battery for a short period of time without it failing, it doesn't mean it's nominal voltage is 3V, it's far more likely a high current flowed and clamped the battery voltage to around 2V, probably slightly more.

[latex] R = \frac{V_{IN}-V_F}{I_F}[/latex]

 

[Mike odom]

You should always use a series resistor, even at 3V.

3V sounds too high for a red LED, it's normally 1.8V to 2V.

I've been seeing higher drops lately... 2.0-2.9 for red/yel and up to 3.2-4.0 for blue... damn pretty but expensive blue LEDs... even higher for the really bright ones

 

[Hero999]

Higher voltage drops are generally for shorter wavelengths and higher powered LEDs. I think the highest voltage drop red LED I've seen is 3V but it was a power LED rated at 350mA, not a little 5mm indicator.

Anyway with a power supply voltage of 12V it doesn't matter as long as the current isn't calculated for the maximum current rating.