# Resistance value for leds

Discussion in 'General Electronics Chat' started by Electro007, Jun 12, 2016.

1. ### Electro007 Thread Starter New Member

Apr 4, 2016
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Hi guys, I want to run 3 normal leds connected together from a 12vdc. 7ah battery to burn continously day and night. What value of resistor is needed to protect the leds and how long would the battery last if its on permanently?

2. ### dl324 Distinguished Member

Mar 30, 2015
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What is the forward voltage of the LEDs and what current do you want to operate them at?

3. ### eetech00 Active Member

Jun 8, 2013
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An LED part number would help...

4. ### dannyf Well-Known Member

Sep 13, 2015
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a resistor whose value puts the leds into the current that you desired for them.

5. ### EM Fields Member

Jun 8, 2016
238
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For the value of the current limiting resistor, subtract the sum of the forward voltages of the three series LEDs from the supply voltage and divide that difference by the current you want to allow through the LEDs.

That is: RS = 12V - 3 X VF(LED) / IF(LED) .

For the life of the battery, divide its capacity, in ampere-hours, by the current through the LED string.

For example, if your battery is rechargeable lead-acid, most manufacturers rate capacity at a discharge rate of 0.1C, which means that, in your case, if 700 milliamperes were taken from the battery, continuously, it would discharge to about 10 volts (check the data sheet for your battery) in about 10 hours.

Just for grins, assuming "normal" current through the LED string was 20 mA, the battery's life (time to discharge to 10 volts) would be:

t = 7ah / 0.02A = 350 hours.

However, since your discharge rate will be lower than C/10, you can expect more time.
Check the data sheet or contact the battery manufacturer for a definitive answer.

6. ### Electro007 Thread Starter New Member

Apr 4, 2016
23
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If They are connected in paralel would it make any difference? I will use normal superbright leds with operating voltage of 1,5-3vdc

7. ### ian field Distinguished Member

Oct 27, 2012
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The internet is full of pages of LED resistor calculation examples.

The people on this forum will help with any genuine difficulty - except being too lazy to Google it for yourself.

8. ### bushrat Member

Nov 29, 2014
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There is a link for that..
www.lmgtfy.com
Try it our, and click preview..

9. ### EM Fields Member

Jun 8, 2016
238
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Yes, it would.
Connecting LEDs in parallel without a series current limiting resistor between the voltage source and each LED is folly, since LEDs are diodes.

10. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
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They could "last longer", because at 9 V with 3 of them them in series they won't light at all. You can't really connect them in parallel, but you can connect each with current limiting resistor and those networks in parallel.

As the battery dies, the LEDs will dim unless driven by a current source.

11. ### shteii01 AAC Fanatic!

Feb 19, 2010
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OP said 12VDC. Where did you fish out the 9VDC?

GopherT likes this.
12. ### Electro007 Thread Starter New Member

Apr 4, 2016
23
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I did not google it because I dont believe everything on the net especially circuits! Thanx for all you guys that gave helpfull advice.

Apr 5, 2008
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Jun 8, 2016
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15. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
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3 LEDS * 3 V(Vf max)/LED = 9 V when you cancel units.

or KLV ; 12 = 3 + 3*3; The 3 volts gets dropped across the resistor, until there is less than 3V. When there is less than 3V across the resistor,the LEDS don't light. Two of the "3"'s have some slop in them.

e.g. Is this better?
12 = 3.6+ 3*2.8; 8.4=12-(3*2.8)

16. ### EM Fields Member

Jun 8, 2016
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If the LEDs are run in parallel, then the array will waste power equivalent to the product of the voltage dropped across each resistor times the current allowed through the number of LEDs in the circuit.

17. ### shteii01 AAC Fanatic!

Feb 19, 2010
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OP says they have 12VDC source.
OP says they have three 1.5-3V led. Let us assume that they are 3V led. Which gives us total of three*3V=9V.

I thought you said that they will not be able to light the three led. And that is the part I don't understand. They have 12VDC source to provide the required 9V, the led will light up just fine.

18. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
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It's a battery, the voltage is dependent on the discharge curve. See http://www.yuasabatteries.com/pdfs/NP_7_12_DataSheet.pdf

Once the terminal voltage goes below 9, the LEDs are off.
If you had 3V LEDs the battery could discharge further and the LEDS may still illuminate at reduced intensity until 3V. (give or take).

Granted, the discharge curve is steep.

19. ### Techno Tronix Member

Jan 10, 2015
140
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Here are some typical values,

Red LED: 2V 15mA
Green LED: 2.1V 20mA
Blue LED: 3.2V 25mA
While LED: 3.2V 25mA

20. ### Robin66 Member

Jan 5, 2016
104
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You can usually be tighter with those current values. A red LED passing 2mA is good enough if battery life is a concern