Electronics, Lighting, & Gadgets - 1SNP and voltage doubler?
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02-03-11, 11:06 PM
A quick internet search shows that it's possible to fabricate a voltage doubling circuit with an efficiency of ~95% using off the shelf parts and no inductors. So....why are there no 1S packs with voltage multipliers? Seems like an easy way to avoid battery balancing problems, allow any capacity pack to be used and make pack charging easy. With variable input and output voltage settings it could even be used on various battery chemistries or LED's with different voltage requirements. I'm not saying it's easy but I've seen some pretty sophisticated DIY projects but never one of these. Are they out there?
Standard voltage multipliers work with AC input. Am I missing something here? No clue what a 1SNP is to stand for either.
02-04-11, 06:42 AM
If I were going to use a voltage doubler, it would be to use a dynohub with cheap 12V LED light bulbs. I find the current crop of dyno-specific lighting is ridiculously expensive.
02-04-11, 10:27 AM
There are several disadavantage of doing this. If you plan to run single or parallel 18650 and double the voltage with a inductorless DC to DC doubler, then you will have to get one that can handle 2.5 to 3 amps output. Those high lumen light such as MS or Gemini requires 10+ watts of power consumption. First off, you will have to find a converter that can double voltage with just 3.7v input and output 7.4v @ 3amps. Something like that will add cost to the package. If you go inductor/capacitor, then it will be a bulky battery.
Another disadvantage is the logic of boosting it at the battery than having it buck in the driver inside the light head. Probably not worth the total loss of efficiency. The extra cost would probably be better used to have better quality battery.
02-04-11, 12:42 PM
No clue what a 1SNP is to stand for either.
He is talking about pack config. MS packs are 2S2P (2 series, 2 parallel), so 1SNP means 1 in series (hence the need to double the 3.7V) with X number of cells in parallel: 3, 4, 5 whatever.
Probably cheaper/easier to design the light to work off 3.6/7V. All of EL34s light work that way, as does the DX Eastward apparently.
02-04-11, 02:08 PM
You still only have a fixed number of watt hours. A magicshine 4 cell pack gives 3 hours of runtime, so with a single cell, you'd get 45 minutes assuming a 100% efficent voltage doubler. Also it would be harder on the cell since you'd be drawing 4x as much current out of it; for a light that wanted 7.2 volts at 1 amp, if you ran it off a 3.6 volt cell you'd have to draw 2 amps, whereas with a 4 cell pack, each cell would only be delivering 1/2 amp.
That not only is not as good for the cell, but it means that you would not actually get the 45 minutes, because you must de-rate cells when you draw higher current from them. You may get 2 hours of runtime drawing 1 amp from a cell, but only 50 minutes if you're drawing 2 amps.
You still only have a fixed number of watt hours. A magicshine 4 cell pack gives 3 hours of runtime, so with a single cell, you'd get 45 minutes assuming a 100% efficent voltage doubler.All true, but I think he's talking about an arbitrary number of cells in parallel. If you made a 1s4p (one cell in series, 4 in parallel) pack with those same 18650 cells and had that magical 100% efficient voltage doubler, it would last just as long as the 2s2p pack and would be easier to charge (as you'd never have to worry about the cells becoming unbalanced.)
(Since the current would be doubled, the resistive losses in the wire would be quadrupled, but that's pretty small to begin with, and could be compensated for by making the wire thicker if you cared enough.)
The idea is sound, but I question the ability to make this voltage doubler quite so efficient (95%?) and have it work from 3v (a reasonable cutoff for 1s packs), and to do it cheaply enough to make this worthwhile. Really, I think the real answer is to include a balancer of some sort -- either include it in the pack, or have a separate charge plug with three contacts rather than just two and put it in the charger.
02-05-11, 12:01 PM
Going this route would be just a jury-rig deal really. The LED really doesn't even need 3.6 volts, the light head takes the 7.4 volts and regulates it down to what the LED actually wants, 2.8 or whatever it is. Ideally you'd build a light head that could regulate your voltage properly no matter what it was being fed.
Taking 3.6 volts and boosting it to 7.4 to feed into a lighthead that then cuts it back down to 2.8 volts is kind of silly if you're designing the whole system from scratch. If you're working with an existing lighthead, it might make sense, but if you're designing a new product it'd be an inefficient way to go, both in terms of building twice as much circuitry as necessary and in terms of power usage.
02-07-11, 12:20 AM
Good points all around. It looks like the forward voltage requirement for the white LED's is about 3.5 to 4 volts so I guess a doubler isn't really necessary although some sort of boost would be. And yes, it would be silly to boost a single cell to 2x the voltage and then regulate it down to whatever. However, a manufacturer could certainly build the required boost circuitry into the regulator but no one has. I'm just wondering why not.
The whole 2S concept just seems like a poor design to me. Maybe I'm over-reacting since there are obviously a large number packs out there operating with what appears to be an acceptable failure rate but it shouldn't be that hard to eliminate the need for 2S packs. Another option would be to incorporate balancing circuitry into the pack, maybe as part of the protection PCBA. I can understand why a mfg like MagicShine might skip this since they seem to be very price sensitive but it is surprising to me that no higher end mfg's have pursued this.
02-07-11, 08:20 AM
I think the reason nobody has is that a single cell isn't really very good from a runtime point of view with current high output LEDs, and if you're going to use two cells anyway, it makes sense to put them in series. It's pretty easy to make a buck regulator to drop the voltage down and to make it quite efficient.
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