PSU for 100W COBs

[Nup]

Hey all,

 

So i have a 100w led CoB at work, but my power for it is a little shoddy (12v 3.5a laptop brick with a dc-dc booster).

 

I don't mind the dc-dc booster, but that little laptop brick gets hot! 

 

How feasible would it be to have the cob's be powered by a regular pc PSU. For a little more then the cost of laptop brick PSUs (that i can find locally), they give a lot a of power. Could even get a second light going. 

 

And if not, do you guys have any other suggestions for power? I know that led drivers/psu exist, but the tend to be expensive. 

 

Many thanks on advance  

[Spork829]

I have no idea what a CoB is, but if there's a safe adapter from a cable from a desktop PSU to the device, then it should work. But just be cautious with a desktop PSU outside of the case, as it can be dangerous or even deadly if someone were to somehow come in contact with the inside. Also make sure to let the fan intake and exhaust from/into open air.

[Nup]

3 minutes ago, Spork829 said:

I have no idea what a CoB is, but if there's a safe adapter from a cable from a desktop PSU to the device, then it should work. But just be cautious with a desktop PSU outside of the case, as it can be dangerous or even deadly if someone were to somehow come in contact with the inside. Also make sure to let the fan intake and exhaust from/into open air.

It stands for Chip on board, and its just 100 small led's together, about the size of a big cpu.

Spoiler

There are no adapters so far as i'm aware, and so was wondering as to how the wiring would work.

[Huntsman]

You could certainly use an ATX PSU, it's just another source of power anyway. BUT, be sure to regulate the current and voltage accordingly. Else you would just burn out all your LEDs in one shot.

 

It's not voltage that kills LEDs, it's current and subsequently heat.

[WEEEEEE]

It's pretty simple to power a LED off of a computer PSU. Look up "ATX PSU bench power supply" to get a rough idea of how to do get a single 12V connection out of an ATX PSU. The main thing that you want to be aware of is current consumption. As @Huntsman mentioned, you will need some form of current regulation. Look at the video below for an idea of how to do that.

Oh and one last note, you may want to be aware of the rails on the PSU. If the total wattage of a PSU 300W, it may only be able to provide 150W on the 12V rail.

 

[klh2000]

3 hours ago, Nup said:

snip

Assuming you have a proper heatsink with a fan attached to the LED, You can use this: 150w Boost Converter

It doesn't have current limiting, but this is what you can do:

 

1. Turn the blue trim pot all the way to the left

2. Hook up the LED to the output of the boost converter with an ampmeter in series with the led

3. Hook the input of the boost converter to the power supply ( Do not get the polarity wrong, it will fry the bosst converter)

4. Turn the trimpot to the right until you get a reading of 3A (3000mA)

5. You're done!

 

[W-L]

4 hours ago, Nup said:

-SNIP-

It would depend how hard your running the LED but that laptop PSU would only be capable of delivery around 42W max in terms of power to the LED, if you wanted to and had the cooling capability you can drive it harder. For powering it via a regular ATX PSU you can do that very easily using a LED driver but what specs is the COB chip exactly we can get a better idea of what it would need. 

 

Just an example this unit from Sure display will auto regulate the voltage down to the LED and supply a constant current that can be set with the two 2 pin shorting block:

http://www.ebay.ca/itm/300-3000mA-Buck-Regulator-LED-Driver-for-1-100W-High-Power-LED-/381469875795 

[mariushm]

It's not smart to use a plain boost dc-dc converter because like someone else said, LEDs are current driven devices.

 

If you set a fixed voltage, as the led heats up, the forward voltage of each tiny led in the big led drops a bit so the current will increase, and that has the potential of damaging weaker rows of leds inside the whole assembly. In rare cases, it can even damage the whole led completely.

 

Anyway, cheap 100w cob leds from eBay will be mostly factory rejects, or leds that don't have the proper CRI or the proper color temperature, and with uneven current on each strip of leds inside, so it's not even safe to push 100w in those leds with the best cooling you could have, because for example if there's 10 strips of leds inside, instead of each using 10w one may use 9w, other may use 12w, and if the leds are only rated for so much current, at 100w in total you could overdrive some leds.

 

[Nup]

14 hours ago, Huntsman said:

You could certainly use an ATX PSU, it's just another source of power anyway. BUT, be sure to regulate the current and voltage accordingly. Else you would just burn out all your LEDs in one shot.

 

It's not voltage that kills LEDs, it's current and subsequently heat.

 

13 hours ago, WEEEEEE said:

It's pretty simple to power a LED off of a computer PSU. Look up "ATX PSU bench power supply" to get a rough idea of how to do get a single 12V connection out of an ATX PSU. The main thing that you want to be aware of is current consumption. As @Huntsman mentioned, you will need some form of current regulation. Look at the video below for an idea of how to do that.

-video snip-

Oh and one last note, you may want to be aware of the rails on the PSU. If the total wattage of a PSU 300W, it may only be able to provide 150W on the 12V rail.

 

WEEEEEE and Huntsman; I had seen that video a good while ago! ATX PSUs do seem tricky, i was looking into them simply from the point of view that they seem comparatively cheap. ( as opposed to getting a laptop PSU per led, if i want 2-3. Also i now realise that voltage and current regulators/boosters are a thing. 

13 hours ago, klh2000 said:

Assuming you have a proper heatsink with a fan attached to the LED, You can use this: 150w Boost Converter

It doesn't have current limiting, but this is what you can do:

-snip-

This is the kind of setup i am using, actually. 

 

13 hours ago, W-L said:

It would depend how hard your running the LED but that laptop PSU would only be capable of delivery around 42W max in terms of power to the LED, if you wanted to and had the cooling capability you can drive it harder. For powering it via a regular ATX PSU you can do that very easily using a LED driver but what specs is the COB chip exactly we can get a better idea of what it would need. 

 

Just an example this unit from Sure display will auto regulate the voltage down to the LED and supply a constant current that can be set with the two 2 pin shorting block:

http://www.ebay.ca/itm/300-3000mA-Buck-Regulator-LED-Driver-for-1-100W-High-Power-LED-/381469875795 

I had looked at regular drivers, brick type ones (for attaching to mains), but most cheap ones are from china, and am less comfortable pushing those hard (compared to the CoB). Or would they be OK? I wasn't aware the kind you showed there exist, so good to know! A possibly option. I will likely choose what comes out as cheapest while still being reliable. 

12 hours ago, mariushm said:

It's not smart to use a plain boost dc-dc converter because like someone else said, LEDs are current driven devices.

 

If you set a fixed voltage, as the led heats up, the forward voltage of each tiny led in the big led drops a bit so the current will increase, and that has the potential of damaging weaker rows of leds inside the whole assembly. In rare cases, it can even damage the whole led completely.

 

Anyway, cheap 100w cob leds from eBay will be mostly factory rejects, or leds that don't have the proper CRI or the proper color temperature, and with uneven current on each strip of leds inside, so it's not even safe to push 100w in those leds with the best cooling you could have, because for example if there's 10 strips of leds inside, instead of each using 10w one may use 9w, other may use 12w, and if the leds are only rated for so much current, at 100w in total you could overdrive some leds.

 

The set up i have currently is with a voltage booster. So the current is constant (so far as i'm aware  ) . I am aware that the cheap CoBs are rejects, but they do the job fine for what i'm using them  

 

Thanks for the reply's all! 

oh, also, to help clear things up a little but this is a rough diagram of the current setup (easier then trying to explain it correctly  ) 

Spoiler

 

[Nup]

Or if yous want to give any ideas as to how best to do this (relatively cheaply) that'd be great too   

[mariushm]

2 hours ago, Nup said:

The set up i have currently is with a voltage booster. So the current is constant (so far as i'm aware  ) . I am aware that the cheap CoBs are rejects, but they do the job fine for what i'm using them

That's exactly the point, IT IS NOT.   A voltage booster (dc-dc step up converter, whatever you want to call it), varies the power output making sure the voltage always remains fixed.  You don't want this with LEDs because they're current driven devices.

LEDs work kind of like this :

 

[ 0v..... 2.7v] = region where LED doesn't turn on at all, produces no light .... ]

[ 2.7v ... 3.2v] = region where LED starts to conduct electricity and light up, but in this region it's like there's a resistor inside the led limiting the current going through the LED, even if the LED is advertised as up to 100mA, at voltage close to the lower limit it won't pass 100mA through it. The lower the voltage, the higher the internal resistance.

[3.2v ... ] = region where the LED is basically a short, it has virtually no internal resistance. Unless something else limits the current flowing through the LED (a resistor, a led driver), the LED will eventually blow up.

 

The 2.7v and 3.2v numbers are fictional, but they're typical for white or blue LED. Other colors (and leds made with other chemicals) have different threshold voltages. Also, these numbers DRIFT as the temperature changes. For example, a white LED at 25-30c (room temperature) may work starting from 2.7v, but when it runs at 60-80c as it happens on 100w COB LEDs, you may have forward voltage as low as 2.4v - 2.5v, which means that as it heats up the LED will use more power and this could cascade until the individual leds start to blow up.

 

So imagine you have 100w led made up of 10 strings of leds in parallel, each with 10 leds in series inside.. a 10x10 matrix and you power it with 28v and you measure it consuming exactly 100w.

To simplify things, let's just assume that each row of 10 leds uses the same amount of power, so we can just focus on one single row of 10 leds in series that use 10 watts.

 

When cold, each LED will have a forward voltage of 28v / 10 = 2.8v which is very close to the 2.7v figure I gave as an example as point where the LED actually starts producing light and pass current through the LED.  Since it's so close to the minimum value, it means that each LED will have a high "internal resistance" and allow just a limited amount of current to pass through.

Since in our example we simplified and assumed  that the row of leds uses 10 watts, that means the current going through each led will be 10 watts / 28v = 0.357 A or 357 mA.

 

Now, if you wait a few minutes to get the whole LED nice and hot at around 60-80c (which will happen even with a good heatsink), if you measure the actual power used by the LED you would be surprised to notice that the LED will actually use 105-110 watts maybe even more.

As each led inside warms up, the forward voltage of each led drifts down towards lower voltages and naturally, the "internal resistance" of each LED will be much smaller at the same 2.8v they see (since you power the led with a fixed 28v, so 28v divided by 10 leds equals 2.8v), which means more current can pass through the LEDs.

With 110w , you have 11 watts per row of 10 leds, so you have 11w / 28v = 0.39A or 392 mA per LED.

 

So you see, if each LED inside the big led was specified for only 350mA, by going with a fixed voltage you risk damaging individual leds because instead of maximum 350mA, they see 392 mA going through them when the assembly gets hot. Even if it works now with more current than they're rated for, some of the leds will weaken over time and eventually die.

 

A LED driver constantly monitors the current going through the LED and as the LED heats up and forwards voltage of each tiny led changes, the driver detects that more power is used by the assembly and lowers the output voltage of the regulator until the total power is again very close to 100 watts.

 

Without a LED driver with proper feedback, it's a smart idea to not drive leds at more than around 80-85% of their advertised power.

 

[iamdarkyoshi]

I've made a capacitive dropper that powers a 120w led fixture. Keep in mind that the LED ISN'T isolated from mains voltage.

 

My capacitive dropper circuit stays cool to the touch.

[W-L]

5 hours ago, Nup said:

-SNIP-

If you want a reliable mains type driver for the LED's Meanwell make some of the best ones out there. One thing to note in your current setup your being limited by the laptop power adapter so your only able to drive the LED at approx 70% max power.

 

This unit from meanwell would be able to drive the LED at approx 100.8W so to the full power rating of the LED without having to get a new adapter or DC to DC current limiting driver. 

http://www.ebay.ca/itm/Mean-Well-LPV-100-36-100W-36V-2-8A-Power-Supply-LED-Driver-Water-Dust-proof-/381131145261

 

[WEEEEEE]

10 hours ago, Nup said:

The set up i have currently is with a voltage booster. So the current is constant (so far as i'm aware  ) .

Not all buck/boost converters are constant current, only the ones with 2 trimmer pots (one for current and the other for voltage).

[Nup]

On 30/09/2016 at 4:48 PM, mariushm said:

-snip- 

Wow, thanks for taking the time to explain that. So a proper feedback driver will do a lot of good   

On 30/09/2016 at 7:43 PM, iamdarkyoshi said:

I've made a capacitive dropper that powers a 120w led fixture. Keep in mind that the LED ISN'T isolated from mains voltage.

 

My capacitive dropper circuit stays cool to the touch.

Oh noice. Do you have a build log of it?  

 

On 30/09/2016 at 9:00 PM, W-L said:

If you want a reliable mains type driver for the LED's Meanwell make some of the best ones out there. One thing to note in your current setup your being limited by the laptop power adapter so your only able to drive the LED at approx 70% max power.

 

This unit from meanwell would be able to drive the LED at approx 100.8W so to the full power rating of the LED without having to get a new adapter or DC to DC current limiting driver. 

http://www.ebay.ca/itm/Mean-Well-LPV-100-36-100W-36V-2-8A-Power-Supply-LED-Driver-Water-Dust-proof-/381131145261

 

Those seem good, but are quite expensive. May go for the other booster you mentioned no previously.  

On 01/10/2016 at 1:07 AM, WEEEEEE said:

Not all buck/boost converters are constant current, only the ones with 2 trimmer pots (one for current and the other for voltage).

Duly noted!! I'v leanered a lot here. Really thought I knew more... :L 

 

Thanks for the feedback and apologies for the late response. Weird week. 

 

If you guys guys had to power a set of 100w's which rout would you take, to try keep costs down?  

[iamdarkyoshi]

54 minutes ago, Nup said:

Wow, thanks for taking the time to explain that. So a proper feedback driver will do a lot of good   

Oh noice. Do you have a build log of it?  

 

Those seem good, but are quite expensive. May go for the other booster you mentioned no previously.  

Duly noted!! I'v leanered a lot here. Really thought I knew more... :L 

 

Thanks for the feedback and apologies for the late response. Weird week. 

 

If you guys guys had to power a set of 100w's which rout would you take, to try keep costs down?  

I don't have a build log. Look for Bigclive's capacitive dropper videos on youtube, and scale them up. Its really really simple.

[Nup]

2 hours ago, iamdarkyoshi said:

I don't have a build log. Look for Bigclive's capacitive dropper videos on youtube, and scale them up. Its really really simple.

Ayy I know that guy!!! His videos are fun. Little much for my level at times though. Thanks for the pointer  

[iamdarkyoshi]

Just now, Nup said:

Ayy I know that guy!!! His videos are fun. Little much for my level at times though. Thanks for the pointer  

No problem. For larger value AC capacitors, you can put two DC electrolytics in series, one backwards, with diodes across each one to prevent being charged negatively.

This schematic doesn't show the diodes, they are optional but reccomended.

[W-L]

5 hours ago, Nup said:

-SNIP-

Oh that other unit was a buck regulator so it would lower the incoming voltage but since you require a 36V input to the LED you will need a boost or Sepic buck unit that can do either or. While I don't see any that Sure-Display has for such high wattages there are others like theses from MingHe which would work while still being good quality units, a bit overkill on the wattages but they don't seem to have a unit for in between:

http://www.ebay.com/itm/CNC-Digital-LED-Driver-Display-Battery-Charger-400W-DC-DC-Boost-Converter-CC-CV/301047784982?_trksid=p2045573.c100033.m2042&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20131017132637%26meid%3D56dc0bcb7b4748b29a667ae2c3ec899f%26pid%3D100033%26rk%3D3%26rkt%3D8%26sd%3D400932315099