resistor question

[MtLuke]

I'm building a triple Cree Q5 helmet light in a small housing and am planning on running it @1000ma. My fear is it will get too hot, not a lot of heat sinking.
Does anyone know what kind of resistor to put in the unit to reduce the drivers output to only about 700-800 ma?
I'm planning on using a 3023 driver without a dimmer, I guess I could just order the lower rated driver but why spend more money.

thanks in advance, Eric S

 

[leew]

Is your buckpuck one of the dimmable types with a ref and a ctrl pin/wire? Are you looking for a resistor to put across these pins?

 

[Changchung]

What is the volts what you want to use?

 

[JWP_EE]

Are you using a separate driver for each CREE ?

 

[MtLuke]

I'm using one driver for all of them, driving them @ about 15.6 volts.
It's a 3023 non dimmable style, I didn't want to have to adjust it on my head while riding. I just want to set it and forget it.
I do have an older style 3021 that I could put a jumper on for a resistor if need be-it's a little beat up though..
I was just hoping to run it in line before the first LED.

Eric S

 

[Changchung]

I'm using one driver for all of them, driving them @ about 15.6 volts.
It's a 3023 non dimmable style, I didn't want to have to adjust it on my head while riding. I just want to set it and forget it.
I do have an older style 3021 that I could put a jumper on for a resistor if need be-it's a little beat up though..
I was just hoping to run it in line before the first LED.

Eric S

Hi, 3 Leds in serie 700ma and 3.4 volts for each Led you need a 8 ohm 4 or 5 watts resistor

3 Leds in serie 800ma and 3.4 volts for each Led you need a 7 ohm 4 or 5 watts resistor

3 Leds in serie 1000ma and 3.4 volts for each Led you need a 5.4 ohm (6 ohm) 6 watts resistor

 

[MtLuke]

Fantastic!!
Thanks Changchung, now do I mount it with the driver or instead of the driver?
Will I lose much in efficiency?

Eric S

 

[LukeA]

Fantastic!!
Thanks Changchung, now do I mount it with the driver or instead of the driver?
Will I lose much in efficiency?

Eric S

Running a resistor as part of the output of a constant current driver would be a Bad Thing. The driver varies the voltage out to keep the current out stable. The resistor fluctuates its drop based on voltage, so bad things will happen. Just spend the $13 and do it right the first time. Buy the 700mA buckpuck now or buy it later.

And the buckpuck will be much more efficient than a resistor alone.

 

[MtLuke]

your right, I hate doing things half assed.

I'll bite the bullet and spend the money...

Eric S

 

[Changchung]

You will have better results with the 700mA buckpuck... Go for it...

 

[2xTrinity]

Adding a resistor in series with a current regulator will NOT reduce output power, it will increase it. The LED + resistor will still be drawing 1000mA, and your resistor will be just be generating a bunch of waste heat.

Going with either the 700mA driver, or getting a variable driver and adding your own feedback resistor is how you'll need to do it.

 

[Hack On Wheels]

I'm using one driver for all of them, driving them @ about 15.6 volts.
It's a 3023 non dimmable style, I didn't want to have to adjust it on my head while riding. I just want to set it and forget it.
I do have an older style 3021 that I could put a jumper on for a resistor if need be-it's a little beat up though..
I was just hoping to run it in line before the first LED.

Eric S

Not sure where the 15.6V is coming from? 3 * 3.7V = 11.1V and if run at a lower current like 700 it might be more like 3 * 3.5V = 10.5V. Just wanted to clear that up in case it would affect anything for you.

 

[MtLuke]

Not sure where the 15.6V is coming from? 3 * 3.7V = 11.1V and if run at a lower current like 700 it might be more like 3 * 3.5V = 10.5V. Just wanted to clear that up in case it would affect anything for you.

The 15.6 is what the pack voltage is, not the vf of the LED's, it just gives me more run time without going to a larger battery- like a sub C.

Eric S