Voltage specs for LED

Thread Starter

SPQR

Joined Nov 4, 2011
379
Hello all - simple question.

Recently I purchased THIS 7 segment LED display.
The company stated that the forward voltage was 1.8V.
Connected to output pins of an Arduino at 4.8V, the segments were all nice and bright.
I didn't believe the manufacturer's 1.8V spec so I used my simplistic power supply and made this Voltage/Current curve.


I'm happy to report that the manufacturer's specs are correct!:)
1.8V at 10mA gives a nice bright LED.

Questions:
1 - Why would an LED be manufactured for such a low voltage when CMOS and TTL are 3.3 and 5V respectively?
2 - Can I treat these just like any LED, use 5V and a 1k resistor to limit the current to 10mA?

Again, I thank you in advance.
 

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MrChips

Joined Oct 2, 2009
21,300
No LED should be connected to a voltage source without proper current limiting resistor or circuitry, which includes constant current sources and PWM circuits.
 

THE_RB

Joined Feb 11, 2008
5,438
Questions:
1 - Why would an LED be manufactured for such a low voltage when CMOS and TTL are 3.3 and 5V respectively?
The forward voltage of the LED diode is determined by it's chemical makeup in the silicon doping. That means it's colour, so a red LED will have a low forward voltage while a blue LED has a much higher forward voltage. They simply can't help that, it's the way the diode is.

...
2 - Can I treat these just like any LED, use 5V and a 1k resistor to limit the current to 10mA?
...
Yes, and as MrChips said it very important to use a resistor to limit the current to a safe value!

Normally with a 7 segment display the common pin is driven hard (no resistor) and the 7 or 8 pins for the segments each have their own resistor, as each segment is an individual LED.

There are some different ways of doing it, but as a general rule that is the best way.

You should probably have a google around for "7 segment display driving" and look at all the ways this is done. :)
 

Thread Starter

SPQR

Joined Nov 4, 2011
379
Excellent answers.

No LED should be connected to a voltage source without proper current limiting resistor or circuitry, which includes constant current sources and PWM circuits.
I had gotten that feeling in a variety of posts over the last few months.
It's good to see it written in perfect clarity.

The forward voltage of the LED diode is determined by it's chemical makeup in the silicon doping. That means it's colour, so a red LED will have a low forward voltage while a blue LED has a much higher forward voltage. They simply can't help that, it's the way the diode is.



Yes, and as MrChips said it very important to use a resistor to limit the current to a safe value!

Normally with a 7 segment display the common pin is driven hard (no resistor) and the 7 or 8 pins for the segments each have their own resistor, as each segment is an individual LED.

There are some different ways of doing it, but as a general rule that is the best way.

You should probably have a google around for "7 segment display driving" and look at all the ways this is done. :)
Interesting about the different voltage requirements as a function of color (doping).
I've many different types of LED - perhaps I'll take some time and do more of the V/I curves.
And I thank you for the electrical jargon - "driven hard" - it's good to know colloquialisms in any language.:)

Again, thank you both for your answers.
 
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