LED resistor value

Discussion in 'The Projects Forum' started by jamesxL, Mar 15, 2012.

1. jamesxL Thread Starter New Member

Mar 15, 2012
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I want to built a LED matrix to display text on. My question is what value from the forward voltage should I be looking at?

My source voltage would be 5v (coming from arduino)
The specifications on the LED say that the forward voltage is : 1.8v~2.9v
and the LED operates @ 20mA

I'm using http://led.linear1.org/1led.wiz to calculate the resistor I need for the LED.

When I look at the forward voltage should I expect the lowest? or should I take a middle value?

2. #12 Expert

Nov 30, 2010
16,690
7,330
Figure both voltages to find 2 resistor values, then pick the higher resistance...or even a bit more resistance. You don't need to be running LEDs at their absolute max current. It doesn't change the brightness very much and they burn out a lot quicker.

3. jamesxL Thread Starter New Member

Mar 15, 2012
4
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Thanks i just wanted to make sure i wasnt getting a resistor that was too powerful and would prevent the LEDs to light up

4. Audioguru New Member

Dec 20, 2007
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896
If the LED forward voltage is low then the voltage across the current-limiting resistor will be higher. Ohm's Law says the current will be higher which might burn out the LED or burn out the source.
If the forward voltage is high then the resistor will have a lower voltage which makes a lower current.

5. wayneh Expert

Sep 9, 2010
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3,244
Modern, high brightness LEDs will look fairly bright at even 5mA or less. Unless you're building a flashlight, I'd stay below 10mA and it should still be plenty bright. (Old LEDs are not nearly as efficient however.) Yes, more current is brighter.

Mar 15, 2012
4
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7. Audioguru New Member

Dec 20, 2007
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Just like our hearing's response to loudness (double the power sounds only a little louder, 10 times the power sounds twice as loud), our vision sees brightness logarithmically.
Double the power looks only a little brighter. 10 times the power looks twice as bright.

The same for dimming, which is why we can see in sunlight and in moonlight.

8. wayneh Expert

Sep 9, 2010
12,386
3,244
My experience exactly. It'd be great if that experiment had gone even lower in current. Folks don't normally think 1mA will light an LED, but it will be better than they think. It's a great way to save power if all you need is an indicator.

9. ishaan3731 Member

Jun 23, 2011
43
1
The voltage and resistor values ur talking of depends on the number of leds u will be using to display the text........ u can get the forward voltage to each led but the amount of current and resistor values wud be dependent on the number of leds......

So plz specify that.....

10. Audioguru New Member

Dec 20, 2007
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ur, u, wud and plz are not English words. Instead they are the garbage that is typed by little kids when texting on their cell phones.

#12 likes this.

Mar 15, 2012
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12. Audioguru New Member

Dec 20, 2007
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Your schematic does not show the value of the current-limiting resistors for the LEDs so we don't know if the LEDs will be dim or if the LEDs, 74HC595, transistors or all of them will burn out.

13. wayneh Expert

Sep 9, 2010
12,386
3,244
The Instructables link addresses that, leaving it to be determined by the particular LEDs:

Now to calculate the value of the 24 resistors you can use this site :
http://led.linear1.org/1led.wiz
You should first get some specs on your LEDs, you should know their forward voltage and forward current, you can get this info from the seller. The circuit operates on 5V so your Source voltage is 5V.