My advice on string blades is all theory. Obi1's advice is from experience.
Take his advice under strong advisement, you should.
My advice on string blades is all theory. Obi1's advice is from experience.
Take his advice under strong advisement, you should.
We all have to start somewhere. The journey is all the more impressive by our humble beginnings.
http://led.linear1.org/1led.wiz for the lazy man's resistor calculator!
http://forums.thecustomsabershop.com...e-to-Ohm-s-Law for getting resistor values the right way!
I am actually having this problem as well, now that I've begun building a CF LS saber. I read the section about drive adjustments, did Class III Seriallel, etc, etc, and bench-tested one segment of LEDs. It's 5 red LEDs per section (2.1v @20mA), three sections in series. The segment instantly heats up to a temperature you can't touch without pain.
When I was wiring the diodes, I measured the forward voltage across the leads of the LEDs themselves. I noticed that the initial forward voltage began dropping almost immediately, then became more stable around 6.1v. I set the drive adjustment to ~6v equivalent and tested the segment. I do not believe I had the heat issue while the segment was connected to my PSU.
So...I'm kind of curious as to what I'm doing wrong. The LEDs were tested at an overdriven 40mA per (200 for the parallel sections). I did use rigid wire for the common anode, as suggested by the CF manual, but it didn't seem to help.
EDIT: I measured 120F with a non-contact thermometer, after about 30 seconds of run time. I am now noticing some dim spots in the segment. I'll try another segment at a lower amperage and see if that helps.
Last edited by Weaver; 10-06-2015 at 05:43 PM.
Did you check that all LED's in the segment light up? Maybe some of them carry more current due to others not having a proper contact.
Well...
Originally, the segment did light up evenly. I drove them at 40mA, under the advisement of the CF LS 6.5 manual. But then I encountered the heat problem, shortly before the dimming occurred. I decided that lowering the Drive Adjust calculation was a prudent move. During my study of the board, I discovered that the battery pack doesn't always output 7.4v. I measured ~8v from its leads, so I used that number for the new calculation.
The segment now reads correct current and voltage when I use my multimeter, and I no longer get the heat issue, but the LEDs in that segment are lost. Once you overheat a diode, it's done. I have to accept that. Of course...that's only 15 LEDs out of a batch of 250. I anticipated losses when I started this project, so I came prepared.
The new segment will be driven using 8v as the pack voltage, and 30mA as the desired current. If that doesn't work...then I know something's very, very wrong.
Side Note: The CF LS 6.5 really is a sweet board. I don't regret my purchase at all. It'll be a hard road, but so worth it.
EDIT: Hmm...you said something about improper contact. One of the LEDs (the last in line, actually) did have a broken lead that I bridged with solder. I didn't think much about it, since it was still conducting, but...could that have caused overheating in the other LEDs?
EDIT (Again): I've been thinking about this. It makes sense to me that the bridged connection might have a different resistance than the others. If that resistance is any higher, it could lead to overheating and overloading of the other diodes. So...with that in mind, should I try 40mA/LED again? Data sheet says 20, CF manual says 40 should be fine. I do have extra LEDs, but I'm not keen on blowing them out any more often than I have to.
Last edited by Weaver; 10-09-2015 at 05:37 AM.
Originally Posted by BlessedWrath
If you can get them, try better red LEDs with at least 50ma capacity maximum. Then, when 40-50 passes them there are no problems at all. 20ma is too low for use with CF LS without resistors IMO
Ancient, reliable and civilized.
Solder bridge is metallic contact and therefore low resistive, so should be no issue. But overdrive of the LED's is. I usually go by the voltage rating, the reds I used for my Class III are rated at 2.4V max, that gives you 7.2V for 3 in series. Close enough to the 7.4V of the supplies we usually use for the CF sabers.
But alas you are right, these batteries can be loaded up all the way to 8V, and that will be ~2.7V on the LED's. On the long run it can overstress the LEDs. So the drive adjustment shall be used to bring that down to an acceptable level. The drive adjustment works with voltage settings, so just ensure you assume 8.2V (be a bit pessimistic) and calculate the drive adjustment accordingly. The LEDs will be still bringt enough.
Actually when I started with the LS sabers, I tried the 5mm LED's (different ones, green, red, blue, UV, you name it). For short time they can all withstand higher voltages than the max rating, but interestingly driven slightly above the max they still gain a little brightness, but afterwards the light dims rapidly (until destruction). I do not have a physic explanation for this, but it shows that it does make sense to drive the LEDs in their safe operation area (around nominal condition, so 2V for reds).
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