Question: Using resistors when powering Luxeon with 3 nimh cells

[JollyRoger]

I'm trying to find the best balance of "overdriving" the luxeon using 3 nimh cells (probably A or AA size) and a single luxeon with either a 1 ohm resistor or slightly greater. Here are my questions:

If my luxeons runs around 350ma at around 3.2 volts, I would be overdriving the luxeon to around 500ma when using a 1 ohm resistor and 3.6V.

However, as most of you know, nimh cells don't start out at 3.6V but more like 4.2V, etc. I am wondering if this 600-800+ma of current going to the luxeon during this period (until the voltage falls to a more stable 3.6V) will be ok for the luxeon. I will have the luxeon heatsinked very well...

I know some of you (lambda, Elektrolumens) have driven luxeons to >500ma with no problems. But since these leds are kind of expensive, I am wondering what the long term effects of this might be (assume that the luxeon will be very well heatsinked, of course).
Thanks.

 

[txwest]

Jolly,
I can't really answer your question, but I don't think you need the resistor for 3 NiMh's. The 3.6V is a no-load voltage. It's bound to drop some under aload. As far as the "extra" surface voltage after you charge the batteries, I switch my charger to discharge for a minute to bring them down to the normal 1.2V (if your charger has this function). TX

 

[ElektroLumens]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by txwest:
Jolly,
I can't really answer your question, but I don't think you need the resistor for 3 NiMh's. The 3.6V is a no-load voltage. It's bound to drop some under aload. As far as the "extra" surface voltage after you charge the batteries, I switch my charger to discharge for a minute to bring them down to the normal 1.2V (if your charger has this function). TX<HR></BLOCKQUOTE>

Hey JollyRoger,

I agree, I don't think you really need any resistance. Run a test: run the Luxeon Star with freshly charged batteries, and check the current and voltage.

I have driven the Luxeon Star direct with 3 Alkaline batteries, at 4.5 volts and 700mA, with no problem. I always use 1 ohm resistance just to be on the safe side (some Luxeon Star LED's might not be able to take this current), and to increase battery life.

I'd like to know how you come out with this? Perhaps it might be the best way to go, using rechargables, and no resistance at all?

Wayne www.elektrolumens.com

 

[dat2zip]

Based upon the first 3 LSs I got from ARC is that I've overdriven the living begeebies out of a couple of them. My original 4 Cell AA holder modified to use 3 AA cells and a couple of switches measured 750mA into the LSs with my NiMH batteries. Of course they got soo hot I burned myself a couple of times. I did not add any additional heat sinking on these (stupid me!). They still seem to be working great. Since then, I've learned that I need to add additional heat sinking if I want to overdrive them.

Also, the Voltage/current curve is more like 50mA/100mV change. So, if you have an LS that is 3.2V @ 350mA and your batteries are 4.2 Volts then the difference is 1V (ten 100mV steps) or another 50mA * 10 increase in current = 500mA aditional current (850mA).

I think the data sheet also concurs with the 50mA/100mV slope change.

I'm positive that I'm going to upgrade my battery holder with an aluminum plate and mount 4 LSs on them and drive them all direct drive from 3AA NiMH batteries. Should be loads of fun

 

[JollyRoger]

dat2zip, I hope you didn't permanently damage any of your luxeons....they got so hot to burn you?!!!! I don't think they were happy when that happened...

I'm not sure if only 3AA nimh could handle 4 luxeons!!!! let me know how this works out. I might try 2 luxeons and 3 "A" nimh. This might be ok...But I want atleast 1.5 hours of BRIGHT light from this (I don't want the voltage of each nimh battery dropping below 1V).

 

[dat2zip]

JollyRoger,

You are not going to believe this. I have yet (cross my fingers) to lose one of the LS yet. I can consider myself super lucky.

I have one battery holder that has the original mod. The other two I have dismantled to re-use the LS in some other application.

They seem to still work great. I'm totally amazed how well they still perform. I'm sure they are damaged in some way like reduced light. I don't have a light meter so I can't tell how much if any they have degraded.

As for the 3AA and 4 LS. That will be just for fun and for short pulse bursts. I think the runtime will be less than 15 minutes continious. Not bad for a flood light. It will be a good thing to impress those that think the single LS is just sooo sooo. I'm pretty sure the NiMH will do just fine.

I'm going to use 4 of the slight green variety for this fun mod. So, I'm not worried about killing one. That and the fact I plan on mounting the 4 to an aluminum plate should give me 1 minute or more before the plate starts heating up.

 

[**DONOTDELETE**]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by JollyRoger:
dat2zip, I hope you didn't permanently damage any of your luxeons....they got so hot to burn you?!!!! I don't think they were happy when that happened...

I'm not sure if only 3AA nimh could handle 4 luxeons!!!! let me know how this works out. I might try 2 luxeons and 3 "A" nimh. This might be ok...But I want atleast 1.5 hours of BRIGHT light from this (I don't want the voltage of each nimh battery dropping below 1V).<HR></BLOCKQUOTE>

My current application is using 30 heatsinked bare luxeon emitters. I driving them with a 24V 2A transformer & rectifier. My luxeons get warm and the transformer gets HOT.

I also have three linked in paralell that I have tested with a radio shack 12V 300mA power supply. I left them on to see how long the luxeons would last and the power supply lost!

 

[springnr]

****************************
I have driven the Luxeon Star direct with 3 Alkaline batteries, at 4.5 volts and 700mA, with no problem. I always use 1 ohm resistance just to be on the safe side (some Luxeon Star LED's might not be able to take this current), and to increase battery life.
****************************

Wayne, I was thinking about overdriving my maglight by putting a slightly lower voltage bulb in it and how to protect the bulb if I did the other day. A NTC Thermistor seems like a good way to go here but I have not got around to trying it yet. Seems like a NTC might be useful in y'alls Luxeon application also to protect from inital high voltage from battery then provide just a little resistance to help prevent runaway. Keith