Cree LEDs: connecting multiple in a string

[jst3712]

Hi, I am new to high-power LED projects, and have done some research, but just need a bit of clarification.
I know this looks long, but please don't get put off because it's all really simple stuff, I promise!
What I want to do is connect 3x Cree XP-G LEDs in one series string. I will be driving the string using a 1000mA LuxDrive "BuckPuck". Basically I just want to clarify a few things before I start buying the parts, as I am finding constant current power supplies and the general operation of these types of LEDs a little overwhelming, especially connecting them together (so please bare with me!) -
My questions are as follows –
1)[FONT=&quot] [/FONT]Does the input power supply need to be regulated for the BuckPuck? The datasheet for the BuckPuck doesn't really say anything, so I'm guessing it doesn't.
2)[FONT=&quot] [/FONT]So the BuckPuck gives out 1A constant current at varying output voltages depending on the load – does this mean each of the 3 LEDs, in my application, will run at approximately 333mA, or 1A?!
3)[FONT=&quot] [/FONT](i) The minimum input power supply for the BuckPuck should be slightly higher than the total forward voltage of all connected LEDs, correct?
(ii) So for my application, I'm assuming 12 volts would be ok? (3.0V to 3.3V forward voltage range for this LED type, multipled by 3 equals 9.0 to 9.9V)… which leads me to my final question...
(iii) So say for example I was to run only 1 single LED and run it at 1000mA, according to the LED's datasheet, the forward voltage would be 3.3V. But because I am connecting 3 in series, the forward voltage would be approximately 3.0V per LED then, yeah?
When answering, please numbers to make it easier to follow.
Thanks in advance.

 

[Th232]

Can't help you on 1 having never used a BuckPuck before, but as for the rest of them:

2) All LEDs will get 1 A since they're in series. If they were in parallel you'd get 333 mA.
3i) Correct.
3ii) I personally think 12 volts would be a bit iffy. The datasheet gives the input margin at 350 mA as 2.0 V (assuming you're using DC), but it has a note that a using a greater current will require a higher margin. I note that figure 6 shows running 3 LEDs at 1 A off 12 VDC, but I would personally go with a 13 or 14 V supply as the actual Vf of the string may be higher than 9.9 V. But as I said, I haven't used a BuckPuck before, so 12 V may be perfectly fine. Wish they said how much the margin should increase by...
3iii) Given that you're running the LEDs in series, and as per my answer in Q2, each LED will be getting 1A, so from the datasheet, at 1 A the Vf for each LED in the string will be on average 3.3 V. The forward voltage of an LED doesn't depend on how many LEDs are in the string, only the current going through it (and some variance due to manufacturing).

Hope this helps.

 

[DM51]

Welcome to CPF, jst3712.

You have parked your thread in a rather obscure and sparsely-frequented area of the forum, so I'm moving it to the LED (emitters) section. I think someone there should be able to help you, but let me know if you have no luck and I will move it again to the Electronics section.

Edit: I see you've just had a reply while I was typing this! But I'll move it anyway.

 

[jst3712]

Can't help you on 1 having never used a BuckPuck before, but as for the rest of them:

2) All LEDs will get 1 A since they're in series. If they were in parallel you'd get 333 mA.
3i) Correct.
3ii) As per the datasheet for the BuckPuck, if you're running it using DC, 2.0V of overhead would be sufficient, so a 12 V power source should be just enough. A 13 or 14 V power source might be better though, but that'll depend on the exact Vf of your string.
3iii) Given that you're running the LEDs in series, and as per my answer in Q2, each LED will be getting 1A, so from the datasheet, at 1 A the Vf for each LED in the string will be on average 3.3 V. The forward voltage of an LED doesn't depend on how many LEDs are in the string, only the current going through it (and some variance due to manufacturing).

Hope this helps.

Oh right, so I was partially correct! Thanks for your help. Now that I know that all LEDs in the string will be getting 1A and not 333mA like I thought, I think I might change the BuckPuck to the 700mA version as the 1000mA is a bit high.

 

[Paul Baldwin]

Hi,
depending on your application they do do a dimming capable 1000mA buckpuck. I've bought a couple from Cutter Electronics and they have both been flawless. One has been powering the Cree Q5's in 2 light fittings I have retrofitted in my house. As the room is quite dark they tend to be on all day and all evening if I am at home, it's now 12+ months on
As for question 1 I know from experience that they seem to be ok with different power supplies, I've tried several. They are currently running off an old scalextrix transformer that kicks out 15+ volts. It's about 30 years old so I'm guessing it's not too well regulated, it says 12v output on the case.

Paul.

 

[jst3712]

Hi,
depending on your application they do do a dimming capable 1000mA buckpuck. I've bought a couple from Cutter Electronics and they have both been flawless. One has been powering the Cree Q5's in 2 light fittings I have retrofitted in my house. As the room is quite dark they tend to be on all day and all evening if I am at home, it's now 12+ months on
As for question 1 I know from experience that they seem to be ok with different power supplies, I've tried several. They are currently running off an old scalextrix transformer that kicks out 15+ volts. It's about 30 years old so I'm guessing it's not too well regulated, it says 12v output on the case.

Paul.

Cutter, yes, I'm familiar with that site.
Oh yes, the dimmable version, I almost forgot about that! Thanks.

The reason why I asked about the regulated as opposed to non-regulated input source is because I have a couple of spare non-regulated adaptors sitting in my drawer not doing anything, so thought I might as well make use of them. Switchmode wouldn't matter either I would imagine.

 

[Apollo Cree]

2) All LEDs will get 1 A since they're in series. If they were in parallel you'd get 333 mA.

Because of the voltage vs. current curves of LED's, they tend not to share equally. If they are driven in parallel, you may find one gets a lot more than its fair share of current. Batteries in parallel may well suffer similar problems.

 

[jst3712]

Thanks to everyone who has replied to this thread. I should be ok now :twothumbs
But feel free to add more info that may be of benefit.

jst3712.

 

[Th232]

Because of the voltage vs. current curves of LED's, they tend not to share equally. If they are driven in parallel, you may find one gets a lot more than its fair share of current. Batteries in parallel may well suffer similar problems.

D'oh! Very true, can't believe I missed that, especially after mentioning variance in my answer on 3iii.

Out of curiosity, what're the LEDs going to be used for? Given the original location lighting up part of the house?

 

[jst3712]

D'oh! Very true, can't believe I missed that, especially after mentioning variance in my answer on 3iii.

Out of curiosity, what're the LEDs going to be used for? Given the original location lighting up part of the house?

It's going in our shed actually, replacing the incandescent 240v globe. They'll be evenly spaced out by about 40cm, attached to the aluminium overhead support beam, so I've got myself a great heatsink!