# Eagletac MX25L3 XML2 resistor mod help

Resistor Mod help please! What resistor can I stick on top of that R050? An R030?
I need to get my head around resistors I still can’t do the math!

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Math isn’t too much fun without a calculator…

Parallel Resistance

First you need to know how much current is going to the LED in stock form. You can either measure it by yourself or refer to some reviews if there is any.

And what is your target (how much current do you want the XM-L2 to see)?

After that we can calculate and estimate the resistor value that you’re going to stack on the R050.

For example, let’s assume that the LED draws 3.0A in complete stock form, and there is only one R050 resistor. Therefore the controller voltage is about 0.15V.

Here is the estimation from my own Excel spreadsheet:
Add another R200 resistor, the LED sees 3.75A;
Add another R120 resistor, the LED sees 4.25A;
Add another R100 resistor, the LED sees 4.50A;
Add another R050 resistor, the LED sees 6A (you will probably kill the emitter);
Add another R030 resistor, the LED sees 8A, and now both of the LED and driver will explode (lol).

Again I am assuming the stock LED current is 3.0A, and if it isn’t then the above estimation would be meaningless. Just to give you an idea.

Yep got that… I get 0.01875ohm resistance, but still not got my head round how much current I can push through the driver before destroying it

From the Eagletac website:
XM-L2 U2 LED
LED lumen: 1505
ANSI FL-14 lumen: 1270

I’ve not got time to measure at the moment, but I’m assuming about 3/4A at the emitter, I’d be happy with 5A,

I really need to learn how to do the calculations, I can do everything else! Haha

If the LED lumens is 1505 (and let’s say it is accurate), the LED should be seeing 4.0-4.5A going into it… and you’re still not satisfied with that? :bigsmile:

No, more lumens!

After using the LD-2 driver everything seems dim!

Lol, ok. 8)

Well, if this is the case perhaps by adding another R500 resistor it will give you about 5.0A to the emitter.

EDIT: Opps, it should be R200! Keyed in the wrong thing in my spreadsheet just now…

Ok thanks!
I think I have some of them, I’ll get round to it when I have some spare time!

Can't answer your question about how much current the driver can take before frying, but I use 2 guidelines when unsure:

• I decrease resistance in steps (solder short wires to the driver so I can just change the resistor on the wire instead of repeated reflows) until the modes start to compress. What I mean by that is the distance between the modes current wise starts to decrease.
• Feel for heat generation in the driver. If something gets too hot to touch, I've gone too far and go back one step.

Not saying the above rules are good practice or fool proof.

Anyway it is just an estimation. Again I would suggest you to measure the LED current before trying to stack any value of resistor because too much current could possibly kill the LED.

If it were me I would do it in a more conservative way, for example start with R300 resistor and see how thing goes. We all know that these newer batch of XM-L2 can no longer handle >5.5A of current, and your goal (5.0A) is pretty close to the limit already.