# Resistor size confirmation

Discussion in 'The Projects Forum' started by SouthernAtHeart, Jan 5, 2011.

1. ### SouthernAtHeart Thread Starter Active Member

Jan 2, 2011
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Hi, I'm going to use a 2 digit 7 segment display (fed from a 4511 IC), and would like a confirmation on the correct size resistors to use. I will be using a 12 volt supply. The data sheet says:

http://www.nteinc.com/specs/3000to3099/pdf/nte3075.pdf

I'm thinking a 680 ohm smt resistor, but I'd like to be sure before ordering these.
thanks,

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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220 to 330 ohm at 0.5W will do just fine

Jan 2, 2011
110
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Many thanks!

4. ### Wendy Moderator

Mar 24, 2008
20,772
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Rifaa, you got this one wrong, the OP was correct.

10VDC ÷ 680Ω = 14 ma (use a ¼W resistor)

10VDC ÷ 330Ω = 30ma

10VDC ÷ 220Ω = 45ma

Most LEDs require 10ma to 20 ma.

5. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Hmmm!! May be what you have requires 20mA...

The one's I have always goes between 30 to 40 mA. So I just implied with the practical apps I had.
I rarely go above 470ohms at 12V for Led's. Most of the time I end up at 220R.

Either way....I don't think it was wrong.

6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
You can never go wrong under driving, but you can shorten the units life quite a bit overdriving. 30ma is the max (exact wording "Absolute Maximum Ratings", read the data sheet for yourself), other ratings in the data sheet mention 20ma and 10ma for various specs. You should always use less than the specification of a component, this applies to resistors, transistors, and everything else (including LEDs).

If the OP follows your advice with 220Ω it will be around 40ma. The target should be 20ma.

7. ### t06afre AAC Fanatic!

May 11, 2009
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According to the datasheet the Absolute Maximum Ratings for the LEDs in the display is 30mA(pr segment). And the normal operation specs are at 20 mA. So in this case using 680 ohm sounds like a good idea. As I have said many times. Then it comes to electronics never assume. Read the datasheet and be sure

8. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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771
Sorry guys, It was not an assumption but rather what I have used.
I stand by what said for the fact I have done it. I do have the same type and I did use them at 470R. It gives me the desired brilliance for my needs. And I haven't had the misfortune of burning the segment.

As for life of it. I rarely believe what data says on life expectancy of light emitting devices. Cause they always fail before the said life duration. Mostly the current components, LED's being the most failed component before it's said duration.

But for SMD led's I prefer to be at 680 to 820 R.

Last edited: Jan 6, 2011
9. ### #12 Expert

Nov 30, 2010
16,704
7,351
You always run the LEDs above their rated maximum and find that "Led's being the most failed component before it's said duration".

Thanks for clearing that up, R!f@@
You make this easy to understand.

10. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,791
771
I know what u are implying .

Go on say it !! But that won't change what I have with me. I use them and they never run hot.

11. ### SouthernAtHeart Thread Starter Active Member

Jan 2, 2011
110
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Wow! I started quite a conversation here!

12. ### SgtWookie Expert

Jul 17, 2007
22,183
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Rifaa is being obstinate.

680 Ohms, 1/4 Watt in a 1206 package would be OK, if somewhat marginal. I'd be more inclined to go with a 330mW in a 1210 package. You will need one per anode. Use wide traces to help dissipate the heat.

[eta] If you want to stay with the smaller 1206 package, then increase the resistance to 820 Ohms. That will give you 12.2mA current through the LED segments, and be within the "rule of thumb" of doubling the wattage requirement of the resistors for reliability.

Last edited: Jan 6, 2011
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