Powering 0402 smd LEDs

Discussion in 'LEDs and Optoelectronics' started by jerryb, Oct 26, 2020.

1. jerryb

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Feb 18, 2014
Hi, I want to light a model I'm building, I've inserted 5 led 0402 smd of different colours and while they will light up if I power them one at a time with a 3025 button cell I want then all on at the same time and I cannot figure out the resistance for the different colours. White, green, red, orange and yellow/green. If I connect all five to the cell only three, red, yellow/green and orange come on. I think I will use two 3025 in series as I have a on off bow for two cells. Can anyone point me to the info I need? Thanks.

2. Harald KappModeratorModerator

11,133
2,539
Nov 17, 2011
This is to be expected with the LEDs in parallel as the LED(s) with the lowest pass voltage will turn on first, the resulting current will drop the battery voltage so the LEDs with the highest voltage will not light up at all (or only very, very dimly).
You need to supply each LED with its own series resistor. Thus the voltage drop will occur across the resistor and less so across the battery.
The resistance can be calculated from the current drawn by the LEDs, the supply voltage and the pass voltage of the LEDs. See "Got a question about driving LEDs?"
You will need the parameters f the LED (operating current, pass voltage). If you don't have these parameters from a datasheet, here's an article which gives you the ranges of pass voltage to expect for each color. Note that for best match of brightness you will have to measure the pass voltage at a given current, e.g. supply 5 mA to the LED and measure the voltage drop.

3. ratstar

478
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Aug 20, 2018
Im good at doing this, voltage division and current division is my middle name.

4. jerryb

68
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Feb 18, 2014
Thanks for the info

5. jerryb

68
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Feb 18, 2014
How do I go about it for the above info? I meant to say I have a box for 2 x 3025 cells which sit one above the other.

6. ratstar

478
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Aug 20, 2018
So are they voltage controlled or current controlled colours? then i can draw you up a circuit if you want. (is that right? or am i not understanding) It takes alot of practice tho to understand resistors, I dont mean that its easy for a beginner, I bet even pro's actually stuff it up all the time as well. If you add capacitors you can make them change colours as the capacitors fill up, too I bet.

1,599
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Nov 8, 2019
8. Harald KappModeratorModerator

11,133
2,539
Nov 17, 2011
LEDs are current controlled, that's the reason for the current limiting resistor.
Nope. A resistor is one of the most basic and easy to understand components.
You lose your bet. Color of an LED depends not on the voltage or current (under normal operating conditions).

Martaine2005 likes this.
9. ratstar

478
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Aug 20, 2018
Yes.. sorry I think im being a bit confused..

10. jerryb

68
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Feb 18, 2014
OK guys, don't argue about it. I understand that each led will need it's own resistor, all I need is help working out the the resistors value. If I'm using 2 x 3V cells in series is that 3V or 6V, 50 years since I did basic electronics.

11,133
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Nov 17, 2011
12. bertusModerator

1,599
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Nov 8, 2019
Hello,

You can calculate the needed resistor as followed:
R=(Vsupply-Vled)/Iled

The supply voltage is the battery voltage in your case.

Bertus

13. jerryb

68
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Feb 18, 2014
OK, thanks all, I'll get back if I set light to the house.

14. Audioguru

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Sep 24, 2016
Nobody makes a CR3025 battery cell. Do you have CR2025 battery cells instead?

The Energizer datasheet for a CR2025 battery cells says its voltage is 3.0V when new with no load and is 2.9V with a load of only 0.186mA.
It is a small coin cell so it has 25 ohms to 100 ohms of resistance in it. Then when the battery cell is new with a 25 ohm internal resistance it can burn out a 2.0V red LED with a current of (3V - 2V)/25 ohms= 40mA for a few minutes with no external resistor.
20mA can be produced for a few minutes with a 25 ohms resistor in series with the red LED.
It can produce 6.8mA (fairly dim LED current) at 2.7V for about 14 hours or 20mA for 2.5 hours but with the battery voltage dropping the LED will appear to be slowly dimming.

Many LEDs like white, blue and bright green need a voltage of more than 2.7VDC

15. jerryb

68
2
Feb 18, 2014
You're right it's 2025, my mistake, sorry about that. My plan was to use 2 in series, I've been trying to work out the resistance needed for the leds listed in the first post and came to the conclusion that I need 150 and 200 ohm resistors or something like that. I think.

16. Audioguru

3,115
696
Sep 24, 2016
If the 2-cells battery is 6V with a 50 ohm internal resistance when new, a 150 ohm resistor in series with a 2.0V red LED results in a current of (6V - 2V)/50 ohms + 150 ohms= 20mA which is very bright and the battery will die soon. Is that what you want?

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17. jerryb

68
2
Feb 18, 2014
It's not going to be used a lot so that will probably be ok. It's getting the different coloured leds to be on at the same time with the same brightness that has been giving me a headache.

18. Audioguru

3,115
696
Sep 24, 2016
Your vision sees green light as being brighter than other colors.
The datasheets for LEDs show that each color has its own forward voltage.
The datasheets for LEDs shows that some are focused into a bright narrow angle beam (spotlight) and others appear not as bright but over a wide angle (floodlight).
The datasheet for an LED shows that its forward voltage and brightness have a range. Some are high and some are low.

19. jerryb

68
2
Feb 18, 2014
So if I put a different resistor on each led, can I achieve equal brightness? Some of the leds have a forward voltage of 2.2 and some 3.2 so is this possible.

1,599
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Nov 8, 2019