Direct drive Cree query.

[Jay R]

I burnt the board on my old Q3 by fiddling. If I fit one of those £7 Cree stars from dealextreme and direct drive it, what will happen ??

 

[Lighthouse one]

I'm running a Cree P4 direct drive on an 18650 Li-ion ( 3.7) full charge (4.2). Runs great...nice and bright...pulls about 850 ma. Does slowly drop off...doesn't bother me.

 

[Curious_character]

I'm running a Cree P4 direct drive on an 18650 Li-ion ( 3.7) full charge (4.2). Runs great...nice and bright...pulls about 850 ma. Does slowly drop off...doesn't bother me.

From the data sheets, it looks like the forward voltage varies considerably from LED to LED. Because of the highly nonlinear I-V characteristic, this means that for a given voltage, the current can vary all over the map. So you might get 300 mA, 800 mA, or 1.5 or more amps with direct drive, depending on the individual LED and battery voltage. The battery's internal resistance and external resistances will provide some leveling, but there could still be a great deal of variation. You might get lucky, you might not.

c_c

 

[EngrPaul]

Install it and measure the current. If it's below 1000 mA (likely) with a freshly charged 3.7V, you're good to go. If you get more, disconnect the ammeter and consider a resistor.

 

[LightBright]

Lighthouse - What kind of ammeter were you using (accuracy)? Hopefully the 10A scale. And you know that the ammeter has some series resistance too.

I was getting 1.5 AMPS at 3.7V driving a Cree, so at 4.2V - I don't want to try that!!

 

[Jay R]

Good suggestions. Thanks guys.

 

[EngrPaul]

DMM 10A scale.

I measure voltage and current at the same time using 2 DMM's for an accurate power reading. Then I disconnect the Ammeter and check that the voltage is the same across the emitter direct as it was in series with the Ammeter.

 

[Lighthouse one]

I have a good quality digital voltmeter... I've run this direct drive light for an hour with only a little warming of the light...so I figure the combo is fine for my Led and Battery set. I understand that another combination might produce different results.

 

[c0t0d0s0]

I am putting together a light that would be powered by a Downboy buck converter and either 2 CR123 cells or 1 17650 cell for flexibility. I wanted a low Vf emitter specifically to preserve full brightness longer when powered by a single li-ion cell, so I tested 3 different P4 Cree emitters I had on hand... At 400mA current, 1 emitter measured at 3.11V, another at 3.18V, the last one at 3.8V. Quite a bit of difference.... I imagine their performance would vary a lot in a direct drive application.

 

[Jay R]

All done. Current draw is 0.9 amp dropping to around 0.83. Nice and bright. Makes a good floody.


Cheers.