Author Topic: High power LEDs and laptop bricks  (Read 1293 times)

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Offline ema

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High power LEDs and laptop bricks
« on: November 02, 2016, 01:47:00 am »
Hi Guys,

I've got a few 10W SMD LEDs rated at 9-12V and 900-1100mA.

I know they require a constant current source, but I've also got a 12V 3A max laptop power supply and I was wondering if is possible to connect 3 LEDs in parallel directly to it. Is it a bad practice? What can go wrong?

Cheers
 

Offline Rick Law

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Re: High power LEDs and laptop bricks
« Reply #1 on: November 02, 2016, 06:16:27 am »
Hi Guys,

I've got a few 10W SMD LEDs rated at 9-12V and 900-1100mA.

I know they require a constant current source, but I've also got a 12V 3A max laptop power supply and I was wondering if is possible to connect 3 LEDs in parallel directly to it. Is it a bad practice? What can go wrong?

Cheers

Since this has been sitting for the better part of a day without a reply - I hate to see another newbie like me sitting around not knowing what to do.  So, until a better answer come along... (I am not an expert...  Just another newbie a step or so ahead). 

Give it a try!  It should work.  Do make sure your supply is 12V DC.  Measure it with a DMM to make sure it is 9-12V.  An unregulated 12V could be a good bit higher.

What can go wrong:
- your power supply may not be DC
- the power supply will drive it at 12V, and your 9-12V lights may be sub-par and not take it well
- your power supply may put a bit too much above 12V and your lights may not take it well

There is a chance that you may loose a bulb, but I think there is a good chance that it should work.

(I use a 12V power brick for an external disk drive - to drive my "supplemental desk lamp" : a 5050 smd led 9-12V, 2W.  I've been using that for a couple of years...)
« Last Edit: November 02, 2016, 06:18:47 am by Rick Law »
 

Offline CJay

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Re: High power LEDs and laptop bricks
« Reply #2 on: November 02, 2016, 07:13:02 am »
No, bad idea, you shouldn't put LEDs in parallel and you should always have some sort of current limiting to keep them within their specs.


LEDs are primarily current driven devices, they don't really care what voltage you feed them as long as it's over their forward voltage they will light and draw current, how much current and how long they will last depends on a few things but allow them too much current and they won't last long.

So, you need some way to limit the current and that can be as simple as a resistor with a suitable power rating.

You can parallel them but unfortuately even LEDs from the same batch will have slightly different current ratings and will draw slightly different amounts of current, the device that draws the most current at a given voltage will dissipate the most power and heat up more than the others which could/will kill it.

This is important, especially with high power devices, as it means you can't or shouldn't put them in parallel and use one single current limiting device, because the LED that dissipates the most power will fail.

When it fails that allows, in your case with three LEDs, the other two LEDs to draw more current which will cause them to dissipate even more power, which will cause them to heat up faster which will kill them even quicker.

So, best way to drive LEDs? That's a complicated subject but for a simple way, read on.

You need to know the forward voltage (Vf) and the maximum forward current (If), in your case let's say it's 9V and 900mA.

You've got a 12V supply (V) (we can safely ignore the current rating as it's in excess of what the LED needs and isn't figured into the calculations)

To calculate the resistor value we sutract the forward voltage of the LED from the supply voltage to work out what voltage needs to be dropped, so:

V - Vf

or

12-9=3

Call this Vdrop

Then using Ohm's law we work out the resistor value (R)

Vdrop/If

or

3/0.9 =  3.3 Ohms

And we also need to work out the power rating of the resistor or the magic smoke will escape so:

R x If

or

3.3 x 0.9 = 2.7 Watts (rounded up)

So, to drive your LEDs safely and prolong their life you'll need a 3.3 Ohm 3 Watt resistor connected in series with each LED

You'll also need a pretty substantial heatsink and ventilation to keep the LEDs and resistors cool.
M0UAW
 
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Online Zero999

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Re: High power LEDs and laptop bricks
« Reply #3 on: November 02, 2016, 09:05:00 am »
Oh dear, now we have conflicting advice: perhaps the worst thing for a beginner.

You may just be able to connect it to 12V and it might just work. I've done that in the past with LEDs. The total forward voltage drop was around 19V, for six in series, so I just connected them to a 19V laptop PSU and it worked perfectly. There was some risk in this: the forward voltage of LEDs declines, as the temperature increases, so it was possible the LEDs would draw progressively more current as they heated up, causing them to fail.

Using a resistor selected, assuming the lower current and forward voltage rating, as suggested above, will certainly avoid and disasters but it's unlikely to work at full brightness.
 

Offline Rick Law

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Re: High power LEDs and laptop bricks
« Reply #4 on: November 02, 2016, 05:30:17 pm »
Oh dear, now we have conflicting advice: perhaps the worst thing for a beginner.

You may just be able to connect it to 12V and it might just work. I've done that in the past with LEDs. The total forward voltage drop was around 19V, for six in series, so I just connected them to a 19V laptop PSU and it worked perfectly. There was some risk in this: the forward voltage of LEDs declines, as the temperature increases, so it was possible the LEDs would draw progressively more current as they heated up, causing them to fail.

Using a resistor selected, assuming the lower current and forward voltage rating, as suggested above, will certainly avoid and disasters but it's unlikely to work at full brightness.

As I said at the start of the post: "Since this has been sitting for the better part of a day without a reply - I hate to see another newbie like me sitting around not knowing what to do.  So, until a better answer come along... (I am not an expert...  Just another newbie a step or so ahead).  "

So, now that better answer came along, no conflicting advice.

Running it raw (no resistor) may work but some risk, running it with current limiting resistor will work but not at full brightness.

He has 12V3A (36Watt), and 10W bulbs x3 (parallel) at 30 Watts, so per nominal rating, the power brick is enough to over-drive the LEDs.  If his bare SMD is just 3-LED in series, that would be 4V per LED, it just may give him a shooting star LED (Bright-then-out).  A current limiting resister is a better answer.
 

Offline amyk

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Re: High power LEDs and laptop bricks
« Reply #5 on: November 03, 2016, 10:38:49 am »
At those power levels I would be inclined towards at least a simple constant-current regulator. LEDs are diodes after all, and their I-V characteristic is exponential. A small change in voltage can result in a huge change in current.
 


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