How to power my XHP70 to 2.4amps


This is my first post here, I need some help with a small DIY project. I’m doing a powerfull floodlight for my room based on a XHP70

I bought a Cree XHP70 K2 from ebay with a 12v copper PCB from here :

I’ve fixed it on a copper heatsink to protect it from heat, and I’m powering the led with a constant current/voltage generator from banggood. On this generator, you can set the values you want with two potentiometers.

This is exactly this one :

So I put the values to 12 volts and 2.4amps (checked with my multimeter).

But I have a weird issue (anyway it’s weird for me) : When I add my led in my system, the current goes down to 0.65 amps and it’s impossible to set it higher. On the wattmeter, the system consumes 10 watts, so I’m sure my led isn’t fully powered.

Without the led, I can easily set more than 5 amps on this generator. I tried to replace the led by a resistor (5 ohm), and here, the generator was able to give 2.4 amps, and the system was using 30 watts.

So, I think I did everything as it should be, but as I’m out of ideas I’m requesting some help to see what’s wrong on what I did :slight_smile:

Thanks for reading, I hope my explainations are not that bad and understandable ! :slight_smile:

What is input PS? What voltage does it have?
You need to adjust output voltage over 12v. Far over, 12v for four-die led is just like 3v for regular one. Try to start your tests from 13v. With 2,4A current you will have big voltage drop trough this dc-dc converter, you input voltage should be at least 2v over (15v ot over).
Also dont forget that you have current regulated circuit. You dont need to adjust voltage precisiously, just set it over 13v and it should limit it when current will increase 2.4A.

Try to increase the voltage slightly.
The Vf of the led at that current is probably higher than 12v. Go to 13 or 13.5V and keep an eye on the current.

Thanks for these quick answers.

Unfortunatly, my PSU has an output of 12.3 volts (5 amps). I tried to test to go higher with my constant current circuit and it’s impossible to go higher than this value.

What do you mean by a voltage drop ? I tried to measure the voltage when the led was turned on, I had 12 volts too

I’ve seen on ebay a lot of led drivers for the 6volts version of the XHP70. They are supposed to give 4.8amps of output, so the 12 volts version should have 2.4amps like i’ve seen on the datasheet.

Isn’t it dangerous to go over than 12volts for this led ?

I’m newbie in electronics so sorry if these questions are maybe obvious for you :slight_smile:

Thank you.

When your dc-dc coverter has load of 2.4A, it will have output voltage not bigger than input minus 2v. This means you need psu with bigger output.
You can try to connect led wires to your psu directly (without dc-dc converter). This is not good idea for long run but you can test it this way safety. I suppose that you will have bigger current, but wont achieve 2.4A.
Your dc-dc converter have current regulation circuit. If you will find psu with bigger voltage (24v for example), you can set min current and max voltage on your dc-dc conv., and then adjust current until youll get required 2.4 Amps.

Thank you

I’ve tested directly connected to the 12V psu, the led is using 0.8 amps, indeed much less that what it should be.

I will try to find a laptop power supply, I know I have one with more than 12 volts.

I have a question, to what do I have to take care to keep the lifespan ? Because if I put 2.4 amps at more than 12 volts, the led will consume more than 30 watts.

Do I have to stay under the 30 watts and find a compromise between volts and amps ?

You should not manage voltage if you are trying to regulate led wattage. Just set any voltage bigger than 13.5-14v on you dc-dc conv. and set current that you need.
To keep lifespan you should not exceed datasheet wattage and die temperature (85 deg. C?). I suppose you need active cooling (heatsink with fan) to keep it cool.

Thanks for your help, I used my laptop power supply (almost 20 volts), i’ve been able to put the voltage to 12.9 volts and the power is at 30 watts now

so, can you confirm me this is not dangerous for the led to stay at 12.9 volts ? That’s fun to see how much the power used change between 12 volts and 12.9 volts :slight_smile:

I have some problems to understand why I need to set the voltage a bit higher, is it because the constant current generator uses a bit of the 12 volts (0.9 volts in my case) ?

Your dc-dc conv. have contant current output. You can limiy current and it will give as much voltage as led needs if voltage is too much.
Led have voltage-current curve. It requires more than 12V for 2.4A output.
You can set any voltage (18V for example) on your dc-dc conv., this is not dangerous if current limit circuit works.
There is much more problems with heat. You should measure pcb temperature after one hour or more.

Ok, thank you !

Indeed, it was a bit hot during my test, I set the voltage a bit down to stay at around 25 watts. And with heat, the led consumes more :stuck_out_tongue:

I added a fan outside the system, directly and now the temperature seems stabilized

Last question : I can’t always use my laptop psu to power the led, so I need to find another one. I would like to keep my 12v psu as I bought it for the led, I found this :

It seems like it can go up in voltage, do you think this could fit ? That would be a cheaper way than buying a new psu

Thanks again !

There is no need in boost adapter. Just use one that you already have together with AC 100V-240V DC 24V 4A 96W Power Supply Charger Converter Adapter

Ok, great find ! Thank you again.

According to the XHP70 datasheet I'm reading, such emitter is a 90+CRI beauty. I'm jealous! LoL.

As it has been said, voltage/current curves for leds are pretty flat. That's the reason you should use a constant current mode power supply to feed them. Don't mess with the voltage, a led's Vf varies with temperature too (diminishing), so leave the PSU's voltage potentiometer in full throttle, carefully turn the current's dial up to your desired value, and let the thing do its magic.

If you reflow the led on a 2S2P (6V) board, you'll be able to drive the led from the 12V CV supply + buck converter. I've seen buck supplies with as low as 0.6V (theoretical) voltage overhead, but even in such a case you may still need close to 14V of minimum output from the CV supply in order to drive the emitter on a 4S1P (12V) board.

Cheers ^:)

Haha, thank you, it’s been hard to find a led with this CRI. I searched for a colder one, but I didn’t find.

I prefer to invest in a new psu than trying to change the board, I’m already happy this works now, I will keep all like it is :slight_smile:

Thanks, without this forum, I would be still searching where is the issue, haha :slight_smile:

My work isn’t finished, now I must do the housing and it will be done.

Hmm, I will maybe try to ask one last thing. For my led, before I purchased the constant current generator, I bought a small pwm thing :

I don’t know if I can put this in my system, so currently I prefer don’t try. The constant current generator already works with a pwm chip, I don’t know if pwm + pwm is a great idea. What do you think ?

Put it away. Dont use it in this project.

Ok, that’s what I was mostly thinking :stuck_out_tongue: