# Thread: 2 XP-G and P7-driver: Low voltage!?

1. ## 2 XP-G and P7-driver: Low voltage!?

Hi,

I have two parallel wired XP-G R5 and a P7-driver from kaidomain http://kaidomain.com/ProductDetails.aspx?ProductId=1866. It gives the leds 2,88 A = 1,44 A each. The problem is that i get to low voltage over the leds. I'm only getting Vf 3,3V at max and cree says that it 'should' be ~3,55. Is that a problem for me?

2. what is the voltage of your battery pack?

3. Originally Posted by velman
Hi,

I have two parallel wired XP-G R5 and a P7-driver from kaidomain http://kaidomain.com/ProductDetails.aspx?ProductId=1866. It gives the leds 2,88 A = 1,44 A each. The problem is that i get to low voltage over the leds. I'm only getting Vf 2,3V at max and cree says that it 'should' be ~3,55. Is that a problem for me?
The LED wont light at 2.3V. Are they lighting?

4. Sorry, it should be 3,3V at 1,44A

Here are some values from yesterday:
Vin = 7,5 - 7,7V

Measured at the led:
Max 2,88A 3,20 V
Mid 0,87A 2,50 V
Min 0,27A 2,30 V

And yes, there is plenty of light

5. Originally Posted by velman
Sorry, it should be 3,3V at 1,44A

Here are some values from yesterday:
Vin = 7,5 - 7,7V

Measured at the led:
Max 2,88A 3,20 V
Mid 0,87A 2,50 V
Min 0,27A 2,30 V

And yes, there is plenty of light
Is there an AC voltage reading?
On the cheaper drivers the output is pulsed so 2.3 might be the DC average.
XPG turns on around 2.5V.

6. Yes, It is DC. I can try to measure with AC later this evening.

7. Try a battery with higher voltage. If it doesn't change, then the LED's are running more efficiently than CREE said. But you are running parallel, so the voltage should be high enough.

8. I have tried with two different models of drivers and I'm gettning almost identical values so I think the voltage is accurate. Now I only need a more efficient driver. At max it is about 76%. Has anybody tried http://www.candlepowerforums.com/vb/...d.php?t=201981?

9. Yes it's important to remember the Vf spec is only an average and varies from led to led. In terms of brightness, leds are binned by current, not voltage, if you have an R5 it's supposed to produce however many lumens at 350mA. Some led manufacturers do bin by Vf as well (largely to make design easier AFAIK) but Cree doesn't for whites. Provided the current is what you want you should really have to worry about the Vf if you have a constant current driver. The only issue is if the Vf is too high and the difference between Vf and power supply isn't enough buck drivers may drop out of regulation as the battery pack wears down and your light won't be so bright. (You do see this a fair amount with the cheap torches running off a single li-ion battery and it does vary from torch to torch.) Also you do have to be careful when using multiple leds in parallel since if the Vf varies too much one could easily be drawing most of the current sp overdriven and very bright and the other underdriven and dim but it doesn't sound like this is an issue for you.

10. Thanks, I don't think i'm having a problem with the Vf then.

The only problem is the driver. It has to low efficiency (75%) and i would really like to have mode with about 1600mA (800/led). I don't think there are so many drivers for XP-G @1400-1500 with 6-12V input and 3-4 modes yet?

11. Originally Posted by znomit
On the cheaper drivers the output is pulsed so 2.3 might be the DC average.
Don't most drivers use PWM for dimming, not just the cheaper ones.

12. Originally Posted by Nil Einne
Yes it's important to remember the Vf spec is only an average and varies from led to led.
This is true to an extent but I don't expect to see a voltage of 2.3V on a white LED with a reasonable output.
Here are some XPG numbers, see at 100mA you're at 2.8V

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