Help with circuit board please..

[Robocop]

I have several Arc LS boards that are all factory stock and in perfect working condition. I know that each one has a small SMD resistor labeled R15 attatched to pads on the boards. Ok there is another pad close to the R15 location and this pad is bare with no resistor. I know if I attatch another resistor(R15) to the bare pad it will bump the current to 667mAh. I have asked this a few times in other posts but it was never really answered directly so I will try again. Ok if I leave the R15 resistor where it is and solder a blob across the section where the bare pad is located what will happen when I power this circuit up? Will it blow my Lux?...not run at all?...or decrease in current? Has anyone actually done this before and if so what current did you get afterwards. I have a few resistors to play with and I really hate to destroy these perfectly good boards with my horrible soldering skills. These things are very small and I think it is much easier to simply solder across the entire pad rather than try to attatch another resistor. I was not sure to post this in the ARC forum or here but I think this is more tech. stuff so I put it here. Thanks for any help on this as I am not very good with the details of these boards.

 

[MrAl]

Hi Robo,

Wish i could help, but im not familiar with that circuit
board. I'd be careful modifying something where i didnt
know all of the side effects beforehand.
Perhaps you could experiment with the Luxeon simulator
circuit we built a while back (three diodes in series
with a 1 ohm resistor) so you dont blow any LED's.


Take care,
Al

 

[Doug Owen]

I bet the converter will sacrafice itself and protect you LED. Not a good alternative, IMO.

I'd recommend against it unless you have the resources to rebuild the light, don't care about cost and like smoke.......

Doug Owen

 

[pbarrette]

Hi Robocop,

I'd recommend against it as well.

It is my understanding that the resistor used in this circuit is a current sense resistor. It is used by the regulator IC to determine the amount of current flow and adjust accordingly. I also believe that the bare pads are run in parallel with the existing resistor.

This allows you to add a second resistor in parallel to the first, thus reducing the total resistance. Reducing the resistance means that more current must flow through the resistor to reach the IC's cutoff measurement point. This results in more current going to the LED.

If you short out the pads, you will be bypassing the resistor in the circuit. With the resistor bypassed, the IC will likely never reach its cutoff point.

In this state, I can think of at least two things that might happen with this setup. One is that the current to the LED would no longer be regulated and the circuit would pump out the maximum possible current to the LED. The other is that the switching IC would stop switching properly and would burn itself out rather quickly.

Later revisions of the ArcLS circuit are said to have thermal protection, which may prevent the IC from burning out. I recall that the thermal protection is actually being provided by the IC itself, which may or may not be at a low enough temperature to protect the LED from thermal damage.

A third possibility is that the IC may not have a stable operation with the sense resistor shorted out. If that is the case, then you may experience random, unusual operation of the circuit.

In any of the scenarios above, the regulation of the light will be negatively affected, assuming that the circuit still manages to power the LED.

Your best bet, if you wish to modify this circuit, is to find some resistors of the same size and practice soldering with them.

I used to use an old Radio Shack 15/30W soldering iron for work of this size. I first had to grind down the tip of the iron to a smaller diameter to make it easier to solder these small parts. I used my dremel tool to do this, but some large grit sandpaper, or a grinding wheel, or whatever else you have on hand will work. Just remember to heat up the iron and tin the tip with some solder after griding.

I tear apart old, dead electronic equipment (computers, cell phones, whatever) and salvage components from them. You can likely find resistors of the right size (though likely the wrong values) to practice with. For practice, you can usually cut the parts off the salvage boards with an X-acto knife and then practice soldering them back into place.

Be sure to stop into Radio Shack and pick yourself up some thin solder for this purpose. I'm currently using a spool of Cat. #65-035 Silver-Bearing Solder with a diameter of 0.015" and it works very well for these fine jobs.

Hope this helps,
pb

 

[Leeoniya]

pb, ditto on the solder choice. works great. also i like ot dip it in MG Chemicals' liquid rosin flux. flows like no other, very clean connections. make sure you heat the copper first for half a second or so to help the flow. I use an Antex 18W pencil soldering iron, heats up in about 30 seconds and has 1/64th" needle attachments i use. works well.

SMT components look small at first but when you get tools that are fine enough, you realize there is a whole lot of headroom.

the only components that get annoying are SOT-23, those are sorta hard to hold down when the solder wicks into the contacts it just pulles it off the pad. i found it useful to put solder on the pads prior and pressing down on the component while heating. once one leg is attached everything is smooth from there.

 

[greenLED]

It's never quite made sense to me, why is it that adding a resistor in parallel actually reduces the resistance of the circuit (and bumps current)?

Does this work equally for resistors in series?

 

[chimo]

Think of resistors in parallel like a plumbing system. Adding another pipe in parallel to the first one will increase the amount of water you get through (less resistance). A large resistance value is equivalent to a small pipe (restricts the flow of water) - a small resistance value is like a large diameter pipe (lots of water flow). Think of current as the volume of water flowing and voltage as the water pressure. Resistors in series means the "water" must travel through more pipe (more resistance). I hope that helps clear things up a bit.

Paul

 

[Leeoniya]

think of it this way, if you have one pathway for everything to flow, say like a pipe. and there is a half open valve in it. if you suddenly double the amount of pipes with half open valves, (granted that your pressure stays constant), you now have twice the amount of volume you can push through with the same pressure. so you effectively double the flow rate at the same pressure. in essance dropping the backpressure of the entire system.

in real life, you would normally see a sag in pressure if you double the pathways, given that the pressure source is not inexhaustable. this is why you see the voltage of a battery sag under a low resistence and a very high current draw.

the only thing i cant put an anaolgy to is why putting 2 half open valves one after hte other will cut the flow in half, lol. it makes sense if you think of it in electricity, but makes none if you try to make an analogy to pipes, cause the flow through valves is hte equivalent of the smallest valve.

i suppose you can think of it as making the valve one quarter open, instead of half open. that would work for the analogy. wow, thought i lost it there.

 

[Robocop]

A lot of interesting information here and many big words that I really do not understand...However they all sound bad and I appreciate the input and wisdom on this.It appears that the general idea is that this would likely end up destroying my board and with these boards (4 total) in such short supply these days I think I will just keep practicing my soldering and leave them intact. The curiousity is killing me but I am not willing to damage a circuit board just to find out what would happen. Can anyone tell me why this pad was left empty on this board in the first place?...Or even why there is even an additional pad at all?...I am curious why the maker would even need this pad if it was not intended to be used from the factory.

 

[Leeoniya]

wow, i didnt even notice chimo posting the hydraulic analogy 4 mins before me. i guess i deserve it for typing slow, or too much.

maybe the extra pad is there for modders! having 2 separate pads, after all, does reduce the effort of going from a modded light back to a stock light without touching any of the original components.

maybe the pad was there for circuit tuning of sorts?

 

[Robocop]

It just seemed a little strange to me to see this extra pad so convienently there as if it was intended to have another resistor. What I am having trouble understanding is do you have to use another R15 resistor to double the current or could I leave the extra pad alone and use an R30 resistor to get the same result?...I mean if you can add more resistance with 2 smaller values can you do the same with one resistor of bigger value?.....or maybe lower value...still kind of off on this topic at times....Anyway this is always interesting to me and I appreciate the input.....And along these same lines I salvaged some resistors from an old Peak 3Led AAA light. They were labeled 20R and I phoned Peak to see what this value was...anyway I learned these would not be very good for an extra resistor but I did chat a little with the man who helped me. Not sure who it was but he seemed nice and worked in the custom design department. He told me that if I soldered over the extra pad that the circuit would supply the maximum current available by the board and could likely damage the Luxeon,circuit or both. I guess doing this allows the board to operate with nothing to limit the current and he was not sure what that would be on this particuliar board. I am still curious what it would do however will not tempt it. I wonder if a 5 watt transplant would be safer to use this way??...or even a more robust 3 watt designed for more current. I was also thinking that a single 3 volt 123 cell could only supply a certain amount of power to this circuit and that it would be limited by this no matter what the circuit could do but I am still not the best at understanding this stuff. Thanks again for all the help on this.

 

[chimo]

The two pads are most likely there to allow the board creators to "fine tune" the output. The voltage drop across the resistors (which is governed by the current flow) feeds back into an input on an IC. The IC tries to maintain a constant voltage on that Feedback pin so it adjusts its output to allow more (or less) current to flow. It will continue to do this until the source input (battery) cannot maintain the power delivery and at that point the circuit goes out of regulation and just tries to "keep up". With a short circuit a great deal of current would have to pass through the solder blob to get the required voltage at the feedback pin. Remember that V=IR and if R is extremely small (as in a short), I would have to be extremely large to produce the same value of V. (that's when the magic smoke leaves the devices /ubbthreads/images/graemlins/yellowlaugh.gif)

There are a limited number of standard resistance values that can be obtained. By having two pads in parallel, you greatly increase the number of equivalent resistances that can be obtained. For an example of this, see the one of the Nexgen boards. I hope this clears it up some more. Cheers,

Paul

 

[greenLED]

Thank you chimo and Leeoniya, the concept is now clear in my mind. The analogy helped (although it doesn't quite work that way with water and pipes, it did help, thanks!)