Help with Current Sense Resistor Modding

Ugh! Do I really have to tear it down again? I figured the backside didn't look like it had anything important. Should have known I should have taken pics. Will try to get to it soon (like tonight).

-Garry

I like the way you think! All or nothing!

-Garry

BTW - am I okay gauging LED current by the input current on this driver? Or do I really need to check current at the LEDs? I guess I could pull that emitter board back out and screw it down on a big CPU heatsink while trying to tweak the driver. That way if I hit 4A/emitter it doesn't go "POOF".

-Garry

You should be ok in dd… I was thinking like this but someone might have a more elegant solution that involves giving you the 1.5a per emitter your looking for

Cut the trace at the red line, connect a wire from the fet to gnd, (blue line). That’ll keep everything like it was and put it in dd

*edit actually just unsolder the leg on the fet- probably a better idea before connecting the wire

GB wrote:

Ugh! Do I really have to tear it down again? I figured the backside didn't look like it had anything important. Should have known I should have taken pics. Will try to get to it soon (like tonight).

You don't have to for me bro. I am curious how you planned to resistor mod the light without opening it back up.

Unless you are a tinkerer and willing to risk failure it’s probably best to start by just stacking or bridging the resistors to see what that gives you. However, if like me, you can’t leave well enough alone I’m willing to lead you astray.

This is essentially an adaptation of the master/slave mod developed by Techjunkie and posted here by Oldlumens.

First a bit of identification.

On that FET the big tab at the top is called the drain and it’s connected to Led-.

The smaller pin bottom left is called the gate and is how the mcu turns the FET rapidly on and off(what’s called pwm). The amount of (on) time is called the duty cycle and gives you your modes.

The other small pin bottom right is called the source and is connected through those resistors to V-.

7135’s have the same 3 connections but they have different names. Don’t ask me why, maybe an EE could tell you.

The large tab is instead called ground and is equivalent to the source pin.

The bottom right pin is called Vdd and has the same function as the FET gate pin.

The bottom left pin is called Led out or Led -.

The middle pin is the same as ground and is often left unconnected without consequences.

All the 7135’s on a typical driver are connected in parallel so if you make those 3 connections between this board and any of the 7135’s on the slave board then all the chips are used.

What you would need to do is remove the FET and the two resistors and solder in 3 wires to connect the slave board there instead.

If you’re not comfortable soldering and desoldering smallish stuff then you shouldn’t try it but here’s why you should consider it. The difference between the two is in regulation and resistance. The FET is either fully on or fully off and will always pass as much current as the voltage allows during each (on) cycle with the maximum controlled by those resistors. As the battery voltage falls since the resistors are fixed value the current falls as well and the LEDs get dimmer in every mode. With the 7135’s, when they are on each one passes a fixed amount of current with the maximum controlled by the number of 7135’s. As the battery voltage drops the resistance of the 7135’s drops as well to maintain current level. The lights won’t begin to dim until the battery voltage drops down close to the forward voltage of the led and stays constant until then in all modes.

The FET/resistor combo would be more similar to the 7135’s if instead of fixed value resistors a pot were used instead that automatically lowered it’s resistance as the batteries drained.

If you’re interested I can post a sketch later showing the connections but you would need a slave board or a stack of 7135’s to do this.

Thanks for such a clear detailed description RBD! I understand how to wire a slave board up without a sketch. Now the question is, do I want to try adding resistors, or try the slave board method? Hmm . . . I think I'll try the slave board method, but I need to make an order. RMM stock those? I'll go have a look.

-Garry

Ok, seems Mountain is out of stock on them .

-Garry

subbed

Maybe buy a driver something like this http://www.fasttech.com/product/1186301-8-amc7135-2-group-25-modes-led-flashlight-driver-n all the wired up already

I wish we had that buck ck is working on… Use that for everything! xd

I do have some 105c's somewhere. You mean to use it as a slave board (removing everything else)? Or do you mean as a total driver replacement? I don't want to lose the battery voltage indicator if I can help it. (But then again, the 105c would have low voltage protection with it's switch to low & flash.)

Aren't you supposed to be getting those solar panels finished up fellfromtree?

-Garry

Yeah it’s raining man :)… Just wire it up like rufus was sayin

To use a 105C as a slave just clip the power pin and the pwm pin on the mcu or desolder and lift them up. You don’t need to remove anything.

If all you do is replace the FET/resistors with the 7135’s on the slave board you shouldn’t lose any other features on the stock board. The lvp on the 105C won’t be functional.

Ok, thanks. Not sure when I'll get back to this, but I'll try to post back.

-Garry

No worries Garry, just doing the sense resistor mod might satisfy your needs and doesn’t preclude the more complex slave mod so you could just take baby steps here. The leap of faith comes when you strip the FET and resistors and things start getting ugly. The good news is that there are firmwares available for mom switches to 105C drivers if it goes south on you.

Here is a pic of the backside of the driver. Not much going on as expected:

-Garry

Good thinking.

Could you post consecutive images of the top and bottom please. Hard to tell but if you measure the resistance between the bottom left pin of the FET and the top left pin of the mcu and get ~270 ohms then I think you have your pwm trace. The question then becomes is that 270 ohm resistor needed for use with 7135’s? I think some of that row of chips in the middle has to do with powering the indicator LEDs on the back side but the traces on the lower half are shadowed.

I don’t see why the 270 would be needed for that, I’d solder my PWM wire to the side of it which face the MCU.