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Old 08-04-2012, 01:29 PM #1
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Default Help with sense resistor mod

Hi LPF,

I ordered a buck driver from DX (SKU 106805) that was rated at 2.0A, but when I hooked it up with my test load, it outputted 2.3A, which is far too much for a M140.

Edit: Without any modification, this driver is awesome for a 9mm build.
So I've tested the driver right now, and the output is as follows:

Done with fresh 2x 18350s.

Med: 718mA 198Hz PWM
Low: 244mA 202Hz PWM
High: 2.35A 68.8KHz PWM (I'm not sure if that's a DMM error or something. May not be PWM)

Strobe: 1.1mA (Or whatever) 8.05Hz
SOS: N/A (Meaningless measurements)

Certainly perfect for a 9mm build.

BUT, my goal here is to drive a M140.

That's why I decided to mod it down to 1.8A by swapping the sense resistor.

However, I have hard time locating it on the PCB. Here are some pictures to help us out:

Edit: I've taken two more pics to show a better view.

Top, with the inductor coil repositioned:


Bottom, with useless PCB desoldered:


Old pics:
Top view:


Side view 1:


Side view 2:


Side View 3:


Side View 4:


Bottom of the PCB:


Moreover, what resistor should I swap it with?

Thanks guys, appreciate your help!

Cheers!


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Last edited by ryansoh3; 08-05-2012 at 08:38 AM. Reason: tested driver
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Old 08-04-2012, 06:21 PM #2
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Default Re: Help with sense resistor mod

Looking at this photo, I would assume the R10 (0.1 ohm) under the inductor is the sense resistor. I can't see if there are any in parallel with it.
Normally you would have to go larger to cut the current.

If it is the only Rxx resistor, I would replace it with a 0.12 ohm surface mount resistor & check current.



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Last edited by LarryDFW; 08-04-2012 at 06:23 PM.
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Old 08-04-2012, 08:30 PM #3
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Default Re: Help with sense resistor mod

Thanks Larry!

I was wondering if you had seen the last picture in the OP that shows the bottom of the PCB. I think there a couple more resistors.

I'll try to take a shot after I remove the bottom PCB.

Thanks again!

Edit: Yup, I nudged the inductor to let you guys have a better view and removed the useless PCB that was made for making contact with the batteries.

Thanks!
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Last edited by ryansoh3; 08-04-2012 at 08:44 PM. Reason: added pics
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Old 08-05-2012, 02:18 AM #4
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Default Re: Help with sense resistor mod

Sense resistors are usually well under 1 ohm.

They would be R10 or Rxx.

Look for those values.

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Old 08-05-2012, 05:30 AM #5
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Default Re: Help with sense resistor mod

Gotcha.

Unfortunately, I just noticed that the pic on DX shows a different circuit layout, as I can't find a R10 resistor anywhere.

The resistors I see on mine are labeled:
Front: R150, R200
Back: 103, 305, and 501.

After some research, I figured out that R150=0.15 Ohms and R200=0.2 Ohms.

The the questions is, which one should I swap?

Thanks for your help Larry. Appreciate it!

Cheers!
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Old 08-05-2012, 06:03 AM #6
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Default Re: Help with sense resistor mod

Quote:
Originally Posted by ryansoh3 View Post
Gotcha.

Unfortunately, I just noticed that the pic on DX shows a different circuit layout, as I can't find a R10 resistor anywhere.

The resistors I see on mine are labeled:
Front: R150, R200
Back: 103, 305, and 501.

After some research, I figured out that R150=0.15 Ohms and R200=0.2 Ohms.

The the questions is, which one should I swap?

Thanks for your help Larry. Appreciate it!

Cheers!
Havent seen these with anyone using in a build, have you done some basic testing on this driver?

i.e: What IC is it? , With a bench psu , how stable/constant current is with variable voltage? etc.

I think the R200 and R150 are paralleled, you can remove the R150 resistor you might get ~1.4A, IF you remove the R200 you'd get in the neigbourhood of 1.6-1.7Amps but i am not very sure.. just my 2c..
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Old 08-05-2012, 07:59 AM #7
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Default Re: Help with sense resistor mod

Quote:
Originally Posted by sinner View Post
Havent seen these with anyone using in a build, have you done some basic testing on this driver?

i.e: What IC is it? , With a bench psu , how stable/constant current is with variable voltage? etc.

I think the R200 and R150 are paralleled, you can remove the R150 resistor you might get ~1.4A, IF you remove the R200 you'd get in the neigbourhood of 1.6-1.7Amps but i am not very sure.. just my 2c..
Yeah, me neither. I'm using this buck driver just because it has 5-modes, which I'm desperately looking for.

I can't find an IC model; it must have been sanded out.
Does anyone recognize the chip?

I don't have a bench PSU, so I can't really test variable voltage input, but I'll hook it up with 2 x 18340's and drain them. I'm not sure though how to heatsink my DIY test load for that long.

I'll get back to you guys about that.

Thanks for the help on the sense resistors. I'll do some calculations and go for it.

Cheers!
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Old 08-05-2012, 06:52 PM #8
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Default Re: Help with sense resistor mod

I am almost certain thats the AX2003 IC. Try leaving only the R150 resistor on and see what current you will be getting.
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Old 08-06-2012, 04:20 AM #9
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Default Re: Help with sense resistor mod

Quote:
Originally Posted by foulmist View Post
I am almost certain thats the AX2003 IC. Try leaving only the R150 resistor on and see what current you will be getting.
Yup, I just did that and I got 1.35A, meaning that the voltage factor (?) is 0. 20143.

So in order to get the current I want, I need to solve this equation:
0.20143/(Rsense)=Output Current.

So if I want 1.8A, I should solve 0.20143/(Rsense)=1.8A, which brings me to
Rsense=0.112

I'll go out and buy some resistors to experiment with. Perhaps I can add a pot that spans from 0 to 0.5 Ohms and make it adjustable!

Thank you all for your support guys. I really appreciate it.

I'll add more info on the driver + build sooner or later.

Cheers!
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Old 08-06-2012, 04:35 AM #10
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Default Re: Help with sense resistor mod

Good luck finding any smd pot less than ten ohms

I think Foulmist is right about ax2003. You should use 0.25/rsense.
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Old 08-06-2012, 04:48 AM #11
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Default Re: Help with sense resistor mod

Quote:
Originally Posted by benmwv View Post
Good luck finding any smd pot less than ten ohms

I think Foulmist is right about ax2003. You should use 0.25/rsense.
Hahaha, yeah, I guess I should stick to resistors then.

Hmm.. I got 0.20/rsense.... Would that be a calculation error or perhaps something else?

Cheers!
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Old 08-06-2012, 05:33 AM #12
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Default Re: Help with sense resistor mod

No pots in that resistance range. Sorry I really would have loved to have some like that but they just don't exist.

There are some tolerances that might affect the equation. Give or take 5% of the final value. I=0.25V/R +- 5%

It might be 0.2V but most likely it's not.

Some experimenting will do If you go for 1.8A it will probably be better to put 2 resistors in parallel like the original idea. The heat will distribute better than just one at that current. If you are going to put 0.112ohm resistor. Put 2x ~0.224 oh whatever the closest value you can find.

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Old 08-06-2012, 06:55 AM #13
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Default Re: Help with sense resistor mod

Quote:
Originally Posted by foulmist View Post
No pots in that resistance range. Sorry I really would have loved to have some like that but they just don't exist.

There are some tolerances that might affect the equation. Give or take 5% of the final value. I=0.25V/R +- 5%

It might be 0.2V but most likely it's not.

Some experimenting will do If you go for 1.8A it will probably be better to put 2 resistors in parallel like the original idea. The heat will distribute better than just one at that current. If you are going to put 0.112ohm resistor. Put 2x ~0.224 oh whatever the closest value you can find.

Yeah, I gave up on the 0-0.5 Ohm pots even before I began to search for them because I had trouble finding SMD resistors on that range.

You're a psychic! They didn't have an 0.11 Ohm resistor, but they did have 0.22 Ohm ones.

Their minimum quantity was 100 resistors, so I begged for 5, and the shop owner agreed 5 0.22 Ohm SMD resistors for 1,000won = $1 buck.

The guy directed me to a soldering guy because I can't possibly solder flea sized SMD parts, and he stuck them on for free.

I'll update more info in two hours.
(Note to self: spikes, scope, and caps blah blah)

Edit: New problem presented here!
http://laserpointerforums.com/newrep...treply&t=76523


Thanks for your replies!
Cheers.
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Old 08-07-2012, 05:12 PM #14
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Default Re: Help with sense resistor mod

Ryan;

The R150 & R200 in parallel would be 0.09 ohms.

A larger resistor should decrese current.

A 0.1 ohm or R100 would be a good starting point.

LarryDFW
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Old 08-07-2012, 05:34 PM #15
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Default Re: Help with sense resistor mod

Quote:
Originally Posted by LarryDFW View Post
Ryan;

The R150 & R200 in parallel would be 0.09 ohms.

A larger resistor should decrese current.

A 0.1 ohm or R100 would be a good starting point.

LarryDFW
Yessir, I swapped both sense resistors a few days ago with two 0.22 Ohm resistors in parallel for a total value of 0.22 Ohm.

That set the output current to exactly 1.80A; I was very impressed because I didn't know it would be that exact.

Thanks for your help!

Cheers!
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