# how to determine the right voltage for led lamp?

Discussion in 'Troubleshooting and Repair' started by pharaon, Jun 3, 2019.

1. ### pharaon

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3
Oct 28, 2014
ok today i tried with 0.04 current and 2.6 voltage for 5 leds
1- connected for only one led and the 5 light up together

2- connect the first and last led and the 5 light up

then i tried to light up 10 leds same current at 0.04 but i had to rise up the voltage to 5 V

2. ### Alec_t

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Jul 7, 2015
That's a much clearer image and points to the LEDs being arranged as a series of 18 groups, each group having 5 LEDs in parallel. So I reckon you'd be looking at a power supply of at least 18 x 3V = 54V, capable of supplying a constant current up to 5 x 60mA = 300mA.

3. ### BobK

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1,633
Jan 5, 2010
Ah, this finally makes sense. At least I was right before I was wrong!

Bob

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4. ### Martaine2005

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May 12, 2015
Yes Bob, you guessed about 60v ages ago!. So you were right!.

Martin

5. ### pharaon

234
3
Oct 28, 2014
how did you calculate this part?

6. ### Alec_t

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Jul 7, 2015
See posts 10 and 12. If 66mA is the maximum rated current, it is always advisable to run things at less than maximum to prolong their life, so 60mA per LED seems reasonable.
I'm still puzzled by your assertion that "they all share the positive line but the negative is every 5 leds". If 18 groups of 5-parallel are indeed in series then they can't all share the positive line.

Last edited: Jun 11, 2019 at 11:37 AM
7. ### pharaon

234
3
Oct 28, 2014
yeah sorry about that, it's the opposite

8. ### Alec_t

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Jul 7, 2015
You mean they all share the negative line? That still leaves the puzzle.

9. ### pharaon

234
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Oct 28, 2014

see that top thin line that all each group of leds share

10. ### Alec_t

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Jul 7, 2015
I think you'll find that the light-coloured line is an insulating strip, not a power rail, and the darker areas are conducting paths connecting the groups in series. Can you confirm with a meter?

Last edited: Jun 12, 2019 at 11:52 AM
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11. ### pharaon

234
3
Oct 28, 2014
yes that's true

12. ### Harald KappModeratorModerator

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Nov 17, 2011
This is what the circuit looks like (the polarity of the diodes may be reversed, I can't see that from your photos):

Each of these meandered sections has 5 LEDs in parallel. The sections themselves are in series.
All else has been said.

13. ### Alec_t

2,607
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Jul 7, 2015
Conclusion: you'll need a ~300mA constant-current ~60VDC supply.

14. ### pharaon

234
3
Oct 28, 2014
thanks every one