Test 'rig' has just been rebuilt, it's a 4P 18650 box, with triple 22AWG wires attached to each end, neg side includes a typical forward clickie. Each cable is around 10" long. I've pulled 15 amps out of it testing other stuff so far...
I've built a p60 pill, MTG2, brainless zombie single mode 105C with only 4 380mA chips, just to see if the 16340s are up to it. The pill is assembled and functional, I just have to modify a reflector & make some kind of insulator/spacer, after that it'll be up and running in a L2P.
Does 2.21A at the tailcap at switch-on, falls to 2A at around 30 seconds (likely the fault of these particular cells, will get some more soon to see if they deal with it better). The 4 7135s are all on the backside of the board, MCU removed, jumper between Vcc & PB1 pads.
None of the reflectors I have are happy with it, any combination of emitter hole size and spacer thickness leaves a yellow area in the middle of the hotspot, which from other MTG2 experiments means the positioning isn't right yet... just can't get there with the parts I have currently. I need some SST50 and/or MC-E p60 reflectors, if such things exist....
LOL, how the hell should I know. The driver is doing something, as the same cells in direct drive do 3.6-3.7A. I haven't compared input current to current at the emitter, but I doubt the driver is eating the amount over the 1520mA it should be doing, as if it were, I expect it would have caught fire by now. Output is steady, unlike when I tried a similar setup with 2x 18650s, which got the driver VERY hot and it started complaining.
I still have the diode in place on the driver so that cuts the voltage down some (doesn't it?) that's being jumpered to the 7135's Vdd pins, but is that safe, or making it do something funny? Should I use a resistor instead of the jumper (and if so, what value)?
The current my be high if there is a semi-high resistance short. The emitter has contact pads the top of the pcb along the whole of two sides. Some current may be flowing through those pads into the reflector.
Nope. Nothing like that going on. It was checked every way possible before powering it up first time, and none of the 7-8 different reflectors come anywhere near the top of the LED package.
I think I'm slowly catching on. 1.5A @ 6v will require higher input current (tailcap numbers) than 1.5A @ 3v. So I think the tailcap numbers are correct for this little package of wrongness I've thrown together. Yes?
I don’t think so. 7135’s regulate current. The Vf of the emitter will determine voltage supplied. If more voltage is supplied, the 7135’s will convert the excess voltage to heat. Wattage will vary though. 1.5 amps of current at 3 volts will be 4.5 watts of power consumed. 1.5 amps at 6 volts will be 9 watts.
I was just guessing, based on the numbers I got from the S1100 before & after the MTG2 swap. Output current with the XML was 3.3A and tailcap numbers were ~1.2A. After the transplant output current was the same 3.3A but at tailcap with the same cells current roughly doubled, to 2.2-2.3A. Though, that's with a true buck driver so I don't know if any of that is relevant to a CC driver.
In a typical situation with a typical LED that would have a Vf around 3 volts, the answer to your question is yes. But some of the gurus in this forum have show the 7135 can handle higher voltages if the LED’s Vf is not too far from the supply voltage. Good heat sinking also increases capacity.
I’ve been playing with 6 ni-mh’s (7.2v nominal, 8.4+ fully charged) with MG-T2’s and can confirm that the 7135’s can handle it. I have them well heatsinked just in case.
The limit of 6V may not be a result of power dissipation, but could be the breakdown of the transistors inside the part. If it was strictly a thermal limitation they probably wouldn’t have a strict limit of 6V, but would have something related to Vf.
Yes, that's what he means.
Keep in mind you are burning a lot of power away though (more than 25%) and you'll need a very high power rating or several higher-order resistors in parallel with each other.
Since the current-carrying part of the 7135 is only in the negative path it only sees the difference between Vin and Vf. Stick to 6v max on the Vdd, and match the Vin to Vf within a tolerable range, and the chip will never know anything is any different than it was with a single 3v LED and a single LiPo.
Say you have a circuit with an input voltage of 25v DC, and a series string of LEDs with a total Vf of 23v. Difference is 2v. Vdd is controlled from some point in the circuit that only provides a max of 5v.
And another circuit with an input voltage of 5v DC, and a single LED with a Vf of 3v. Difference is 2v. Vdd is controlled by the 5v input voltage.
What's the fundamental difference there that will cause one of them to work, and the other one not?
Without a connection to both the + input voltage and the -, there will never be 25 volts applied across any part of the 7135. It's impossible, there is no connection to the +. It would be like trying to measure the voltage of a DC circuit using only one DVOM probe.
We use battery supplies that have a voltage swing of 1.2V per cell. That 25V supply would be 26.4V (6 cells full) but only 18V empty. One is too much, the other too little. I’ve gone as high as 3-XML and 3- cells in series but above that it’s not reliable. Try it with a power supply and let us know.
Stereo dude, his example has the Vdd pin controlled to 5V max which makes the question valid. It’s the power supply that is not germane.
Show me where the full 8.4v is connected to anything that could get to the Vdd pin. Through this whole thing, I have said, at least twice, that the Vdd pin does not get the full circuit voltage. The other two 7135 pins that control the current flow know nothing of what the total working voltage is. Nothing.
