LED Resistors

[tom_pay]

Hi All,

I have a LED with a forward voltage of 3.8v and a supply voltage of 3.8v (or just under).

Will this LED need a series resistor to stop it going POP!!?

If so, how do I work out its value?

Thanks

Tom

 

[DemonicSandwich]

3.8v is somewhat high. Usually a Pure Green, Blue, UV, or White LED would have a Vf of about 3.5v or so.

What model of LED is it?
What is the supply? If it's a battery, what chemistry is it?

 

[tom_pay]

Hi,

The LED is one of those high intensity ones.

Its going to run off 2 C batteries, but I'm going to boost it to 3.8v. The typical forward voltage as defined by the datasheet.


Thanks

Tom

 

[DemonicSandwich]

How are you going to boost the supply voltage? A boost converter?

If you are using a boost converter to boost the voltage, you could wire it in constant current mode and drive the LED directly.

 

[tom_pay]

I was going to use a DC-DC converter chip.

How would I wire it to give constant current?

Tom

 

[DemonicSandwich]

A DC-DC converter uses a voltage feedback to regulate the output.

Normally, a DC-DC converter is used in Constant Voltage mode. With negative side of the load connected to Ground and the positive side connected to the converter output.
Then a voltage divider connected across the load, connecting to the voltage sense of the chip so to maintain a constant output voltage.

For Constant Current mode, you do away with the voltage divider and use a single resistor for the feedback.
You do this by connecting one end to Ground, and the other end to the negative side of the load as well as the feedback of the chip.
Then the positive side of the load to the converter output.

The images below show the basic differences between Constant Voltage mode(left) and Constant Current mode(right).

 

[tom_pay]

Ok,

That is simple. I'll do that.

Thank you very much.

Happy Hollidays

Tom

 

[ben7]

I do think you would need a resistor in series with the LED-if you are not doing the constant current controller circuit to power the LED

Would a 3.2 volt LED work on 3.2volts w/o a resistor? Or would it draw excessive current and burn up?

 

[RMMM]

LEDs are current controlled devices. They should be current limited, either with a resistor, or a constant current regulator.

No need to blow a expensive LED because you didn't use a 2cent resistor.

 

[tom_pay]

Yes, that is the million dollar question.

I think that I'll do the constant current supply.

Though I'm still curious as to what would happen. To pop or not to pop?


Tom

 

[DemonicSandwich]

LEDs fade over time. Under normal opperating conditions, (rated forward current at rated temp) this is normally 50,000-150,000 hours (about 6-17 years continuous)

When driven with too much current, it will just fade more rapidly until it goes short circuit.

 

[tom_pay]

Yes, I don't want to have to replace the LED, they are very expensive.

I think I will use the constant current supply. I belive that it is the simplist and will ensure the LED lasts a while.

Thanks


Tom

 

[audioguru]

A cheap keychain flashlight does not use a resistor to limit the current in its LED. It uses the internal resistance of its tiny battery cells.

Half-decent (Name-Brand)LEDs have a detailed datasheet that shows the range of voltage of a certain LED part number (mine are from 1.5V to 2.4V when the current is 20mA) and show a graph of the typical current when the voltage source is varied.

When the voltage is increased from 1.6V to 1.8V then the current increases from 2mA to 40mA. The temperature also affects the current. The LED gets hot which causes its current to increase which makes it hotter and it goes POP.