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What LED drive do I need to power a 8x10 Watt LED in serie? Answered

Hi there,
I am building a 8x10 Watt 12v LED strip for my father in law with eyesight issue (an "ambitious" project considering my poor knowledge of the subject ;) ). Attached an image of the project. I though it was simple but I am now totally confused about how I have to power it, the more I read the less I understand.

8*10 = 80 Watt = 80/12 = 6.6A
So do I just need a 240AC to 12DC 7A driver or? But 7A seem pretty high for just 8 LED, somewhere I must be wrong, or not.

My LED have the following spec.

Model: 10W;
Forward Voltage (VF): DC 9-12V;
Forward current (IF): 1050MA;

Thanks!

31 Replies

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GautierA (author)2016-04-11

Hi Guys,

Thank you very much. So my circuit should looks like attached?

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steveastrouk (author)GautierA2016-04-11

Yes. The supply I selected will need to be adjusted to a slightly lower output drive - or you can stick another couple of LEDs in.

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GautierA (author)steveastrouk2016-04-20

Hi Steve,

Ok I have a working prototype. Juste few questions to evaluate my "success"

The power supply deliver 86.4V in standalone and a maximum of 60.6V when connected to the LED strip. The LEDs light-up at about 35V and are stable around 48V. Are these ranges Ok?

I use a pretty low grade potentiometer on the dimming interface, any recommendation for a smoother transition at 35V? Right now it is a bit brutal

At low voltage the LED don't light uniformly, see the photo (black one below, I used an old radiography sheet to dim the light and see the details). Is it usual or just some low quality LEDs?

Thanks!

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steveastrouk (author)GautierA2016-04-20

The voltage sounds like its being very heavily loaded. What are the supply specs ?

Brighness range ? You are using a 0..10V control signal ?

Variation: inevitable result of not using a single driver module for the string.

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GautierA (author)steveastrouk2016-04-20

Hi Steve,

The dimmer is the one you advised me, and I an using a 25k potentiometer, see the attached photos. I asked for a 0-10V dimmer but the guys here propose me something to to dim low voltage LED strip or some pretty sophisticated expensive stuff apparently not what I need. Maybe I asked the wrong question.

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steveastrouk (author)GautierA2016-04-20

It looks from the datasheet like the dimming resistor needs to be 100K, as I read the datasheet (page4). Minimum dimmer level is 8%

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GautierA (author)steveastrouk2016-04-21

Thanks Steve!

A 100K is working better. I still need 48V to get the LEDs stable and the transition is a bit brutal but I now reach 78.6V at maximum power. I will post the all stuff in Instructable

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steveastrouk (author)GautierA2016-04-21

That ties up with the LED data, Vf= 9--12V.

If you want to avoid the "brutal transition", turn the LEDs to a dimmer level that you like, then switch everything off, and measure the dimmer resistance.

Put a resistor in series with the pot, equal to that resistor: now you can't turn the pot down to less than the acceptable brightness. If you find say that the resistor needed is 10K, you won't notice the loss of travel on the pot at full brightness, if you need a 47K you will, and then I suggest a 47 K pot - total resistance now roughly 100K

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GautierA (author)steveastrouk2016-04-21

I get 6K, very low as far I understand. I may try with a 10K. Thanks for your patience and advices.

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steveastrouk (author)GautierA2016-04-21

I still think its OK though.

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GautierA (author)steveastrouk2016-04-22

Done, no resistance, useless as you said.

https://www.instructables.com/id/LED-Ramp-8x50-Watt...

Thanks for your help!

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GautierA (author)steveastrouk2016-04-21

indeed but I am in Information Security (and French), a bit a perfectionist I guess. Resistors are cheap, I will give a try.?

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GautierA (author)2016-04-22

Done, https://www.instructables.com/id/LED-Ramp-8x50-Watt-Dimmable/
Thanks for your help!

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steveastrouk (author)2016-04-11

The LEDs should be mounted in SERIES, not parallel, and run with an LED driver

They will need 120V @1A

You must NEVER run LEDs in parallel.

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Yonatan24 (author)steveastrouk2016-04-13

"You must NEVER run LEDs in parallel."

I believe that's true only when you use a constant current driver, RIght? With a regular power supply, It should work properly. Am I correct?

If he powers them @120V, He should cover the contacts because there's a risk of getting shocked...

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GautierA (author)Yonatan242016-04-13

this is why I am so confused, even the ones who know have a different opinion. My understanding is rhat a LED driver compared to a normal power supply, prevent thermal overrun. For the rest I am still looking for some lights. Thanks guys, thanks Yonatan

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steveastrouk (author)GautierA2016-04-13

One of the ones who knows is a professional engineer, with 30 years experience, and experience designing LED drivers. The other is 13 years old.

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Yonatan24 (author)steveastrouk2016-04-13

And the 13 year old one is asking questions to understand what is correct.

He is still confused

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GautierA (author)Yonatan242016-04-13

If you really are 13 Yonatan, thumb-up. And don't worry, learning always starts by feeling silly, guess how I feel now, and I am 54 ;)

I guess Steve is the pro, so LED in series. I can use one driver per LED and power these drivers in parallel, as show bellow. What are the consequences for the power supply (spec), the LED themselves and dimming fonction? I can get 10 pieces of 10W LED Driver DC8 26V AC/DC 12v for about 20 USD and the 96V for 60 USD. A bit more cabling but less expensive.

Thanks all!

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steveastrouk (author)GautierA2016-04-13

Which driver are you looking at ?

Now you need to add a 220-12V converter as well....And you may find the brightness of the LEDs is different from one another, because the current won't match.

There are other sources for LED drivers, what I highlighted was the one I found quickest.

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GautierA (author)steveastrouk2016-04-14

So I may try this also. A bit more soldering but 55% less expensive, if it works ;) and I already have a 12V 8.5A industrial power supply

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GautierA (author)steveastrouk2016-04-13

Hi Steve

220-12V standard converter is not an issue and is very cheap (well what Watt/Amp do I need for 8x10Watt LED?).

Driver like this http://www.aliexpress.com/item/10W-LED-Driver-DC8-...

Thanks

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steveastrouk (author)Yonatan242016-04-13

OK, look at any network of resistors in parallel with a perfect voltage source, ie, one with no output resistance, and make it 10 V

Let's put 4, perfectly 1 Ohms in parallel, what's the current in them ?

Now, make one of them 0.5 Ohms. What's the current in THAT, and what's the current in the other 3 ?

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steveastrouk (author)Yonatan242016-04-13

NO, it will NOT work in parallel with a constant voltage supply.

What controls the LED current ?

The supply, in your scheme would be 8A. One Led has a slightly different forward voltage characteristic, its temperature is higher than its friends. It pulls more current. It gets hotter. It pulls more current, it gets hotter. The current exceeds the device limit and it dies.

Another LED has a different forward voltage characteristic.....the cycle repeats.

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PS, in the un-useful case, of 8 LEDs fed with a single 1A supply, yes, it might not be fatal, but it would be a classic demonstration of current NOT sharing.

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Yonatan24 (author)Yonatan242016-04-13

Or 96V, Your correction...

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GautierA (author)steveastrouk2016-04-11

Thanks Steve, this is the info I was looking for! And I changed the wiring on the model https://3dwarehouse.sketchup.com/model.html?id=bd0ab066-922b-4a05-9982-cc676634eb21

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steveastrouk (author)2016-04-11
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Downunder35m (author)2016-04-11

As Steveastrouk said, LED's should not run in parallel.
One is a bit different or even fails and they all blow up.
Either use them in series on 120V with a suitable driver or to stick with 12V for the supply use a LED driver for every LED.
Proper drivers will also allow you to use a PWM signal to dim the LED's if required.

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