Finally got around to playing with sense resistors on the stock S1100 driver yesterday and ran into a problem.
In stock form:
(opposite side of board, probably not relevant: http://oi47.tinypic.com/10gjuo0.jpg)
That's two R220s and two R240s, .22 ohms and .24 ohms. All are in parallel. Stock current is around 3.3A.
I have a collection of leftover resistors from who-knows-what. Some '0' and '000' (lower than my meter will measure), one that's .5 ohms (I don't remember the marking, but it reliably measured .5) and some 1R0s, which are 1 ohm. If I stack one of the '0' or '000' in parallel with the originals, at switch-on ZAP! - dead LED. OK, remove the low value resistor, switch to another test LED, and add one 1R0. Output increases from 3.3A to 3.5, LED survives. Add another 1R0, increases to 3.8A. Another 1R0, 4.1A. Then add the .5 ohm and ZAP! Another dead LED. Remove the .5, add a 1R0 in its place, and get 4.3A and no dead LED.
What's going on here? Anybody seen this kind of thing before? Has anyone successfully tweaked this stock driver yet?
Out of curiosity, what kind of LEDs are you killing?
Wire up the amount of test LEDs in parallel that the driver/battery combo will not be able to kill. Then measure amps going into all of those. Should give you and indication why you are killing LEDs, then you should be able to play around with various resistors values.
A trimpot could be helpful assuming its adjustable in the range you desire, some drivers get strange behavior when they get pushed to a certain level. A trimpot will more easily give you an indication where that level is.
You can check if current spike is the problem if you add that .5 ohm resistor but solder only one side, power ON and then, while the driver is working, make a contact with screwdriver or something similar to make that .5 ohm resistor active and part of the circuit!
Or maybe not, it’s just a thought!!!
In the past I have tested this driver with 3 XMLs in series and it does run at the rated current, and 3 18650 won't kill 3 XMLs. Thanks, never thought about that.
Only drawback is, one of these cells direct drive into a XML won't do more than 4.2-4.5A, and I'd like to end up around 6.5-7A.
One odd thing I noticed that may not be relevant, this driver (actually these drivers, I have two and they do the same thing) at turn-on is at a very low level for a very very short time, like 1/10 second, then bangs up to full output. When the LEDs died they came on for that short time rather dim, then flashed bright and died around the same time the driver usually goes up to full output.
Didn't Vinh or somebody over at The Bad Place build & sell off one of these with the output cranked way up? Was that with the stock driver?
Wanting to revisit this soon but I'm not sure what to rig up as a test that will 1: not explode if the driver goes direct drive, and 2: still allow enough current to tell if the output is scaling correctly as resistors are added. As I said above with 3 XMLs in series and 3 cells in series it would be hard to tell if the driver's giving the correct output or has given up and gone direct drive.
Will two XMLs in series survive direct drive from 3 cells? Or is there a safe way to connect something other than LEDs to the driver output to check the current?
The 0.5 Ohm resistor - is it really a 0.5 Ohm resistor? Is it marked, or are you relying on your DMM to determine it's a 0.5? The reason I ask is that with a regular 2 wire DMM, you can not reliably measure resistor values that low. It could be far less than 0.5 Ohms, and you're measuring resistance of resistor + two leads + 4 contact points + internal circuitry of DMM.
I forget what the markings were on that one but whatever it was it claimed to be 1/2 ohm and I measured 1/2 ohm, and the 1R0s are supposed to be 1 ohm and I measured 1 ohm. Not claiming I have a supernaturally enhanced meter, just that neither the markings nor the measurements gave me reason to suspect they were anything else. I understand it would be futile to try to accurately measure the difference between the R220 and R240.
What's different about this driver from the ones that use huge piles of '000's as the sense resistors? Any clue why it so abruptly goes from working fine to freaking out?
If the driver gives 3.3a with a parallel resistance of 0.057391 ohms. You should be able to figure the current when adding resistors or figure the resistor for the wanted current. 0.1893903 divide by the total parallel resistance for current.
To get the total parallel resistor value for wanted current, .1893903 divided by the current in amps gives the parallel resistor value needed. Assuming 3.3a was a close measurement at the led.
Does sound like your resistor value is quite low. If adding a resistor lower than .1 ohms the current will be well over 5 amps.
Buck drivers have a higher working voltage to play with, so its possible its putting out enough current to poof the led. More than direct drive from a single cell could.
A .5 ohm resistor should only increase the current to about 3.68a.
I know how resistors added in parallel reduce the total resistance and how that works in regards to drivers and setting the output current. This one does not behave like any other driver I have seen.
Why does it not increase more or less linearly with added resistors? First one increases output a tiny bit, second one adds a little more, third adds a little more, fourth adds a little more, fifth BANG it suddenly goes direct drive. If you were to plot this out on a graph it would be rising at a nice reasonable rate as resistors are added, then it shoots straight up like a rocket headed to Mars.
I think you are dealing with a large current without any protection from the reverse current that may be in the form of the spike creates a problem in the circuit. So, to protect your circuit from this back current the best method is the use of the diode because due to its unidirectional nature it will prevent the current to flow in the opposite direction. The other thing you can do is the use of opto-coupler that will also prevent the current to flow in reverse direction and provides isolation.
Utterly irrelevant to why the driver is fine with 7 current sense resistors and then stops all regulation and goes straight to direct drive with 8 current sense resistors.
Doesn’t matter what value you add at the 8th current sense resistor it still goes poof?
What is your total paralleled resistance value? Is it possible its so low it can’t regulate at such a low value, it just turns fully on.
Yes, it's the effective resistance value of the resistors that triggers the bad behavior, not the number of resistors. Still doesn't explain why it's perfectly fine with four 1 ohms added but totally gives up with three 1 ohm + one .5 ohm.
Total with 2x R220, 2x R240, 4x 1R0: 0.046676, all OK, output 4.3A
and with 2x R220, 2x R240, 3x 1R0, 1x 1/2 ohm (I forget what the code was on that one): 0.044595, instant 12v direct drive death
Four 1 ohm in parallel = .25 ohms. Three 1 ohm + one .5 ohm = .20 ohm.
Compared to the original pack of .22 & .24 ohm resistors, that difference of swapping a 1 ohm for a .5 ohm is tiny tiny tiny. It all goes about as expected adding them one at a time until it fails. Adding .25 ohms only raises the output by around 1 amp, if each 1R0 raised it by 3 amps I would understand why adding 5 would push the output up too far, but that is definitely not how it reacts.
Surface mount resistors with 0 or 000 on top of them are zero ohms. They do have some parasitic resistance, but you can’t depend on it being any bit consistent.
If you put one in a current sense resistor location, yeah, you’re pretty much at direct drive.
