question for Surefire U2 experts: boost... and buck circuit?

dabiscake

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Mar 10, 2003
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Hey all, I have a question for you U2 gurus/experts:

...So, I'm having this conversation last night with another fellow CPF'er about his Surefire U2. No doubt we're agreeing the U2 per design has a boost circuit. Whether Vin from the battery is 1x 18650 (4.2V) LiIon or 2x CR123s (6V), Vf at the led is typically always higher for even the lowest available xxxS (maybe -R?) bin Lux V.

But then he's also trying to convince me that the circuit can act in buck mode, at the lower levels.
I've always thought of the U2 as a boost circuit only, so I'm arguing back, and although it makes perfect sense to me in the 1x LiIon battery config (with Vin always lower than Vf, whatever the dropped-down level may be), I can't really prove my point with the 2x CR123s original battery config, since Vf on low setting can be lower than Vin (He's measured his Vf at the Led at the lowest setting (100mA?) to be around 5.2V < 6V).

"Vf is lower than Vin and the output is regulated, so it's a buck!", he says.

As I can't really disagree with him, I still don't get it then that you couldn't use 3x CR123s or maybe 2x LiIons, or heck, even maybe just swap the led for a LuxIII then?
Wouldn't a real boost/buck converter allow that much range? Or is the circuit in the U2 doing much more that meets the eye? Does somebody know how the circuit actually behaves in those lower levels, does it switch to buck mode if needed?
Maybe if PK could chime in... :grin2:

Please post your thoughts on this as you probably have a better clue than I do right now! Thanks.
 

wasBlinded

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Dec 14, 2004
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Its primarily a boost circuit. Once Vbatt gets a few tents of a volt above Vf of the Lux V (around 7 volts by my experiments), the regulation stops and you can't even turn down the output with the selector ring (which is why RCR123x2 is not good).

It does have very limited buck capability, in that on the low settings, current to the LuxV is less than you would have if you connected two series CR123 cells directly to the LED, but it only operates as a buck within a very narrow voltage range.

So call it what you will.

BTW, it doesn't work well with a Lux III, even using a single Li-ion cell. I'm not sure anyone has tried it with a Lux III and a single CR123 with a dummy cell, but even if that worked I don't think it would be any kind of improvement over a stock U2.
 

MikeRD03

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Jul 24, 2006
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Hi dabiscake,

A boost converter can´t do buck and around. If Vbatt is >= Vled there is no regulation possible.

A buck-boost converter is possible but the old way though an inverter is much less effective than any buck or boost driver! I won´t do that.

Therer is a new chip at the market that drives buck/boost with 4 switches and achieves around 90% effiicency but you will hardy find ready-to-go modules with that chip now..

greets
Mike
 

Luna

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Dec 27, 2004
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874
It is a buck/boost design. That is why you can run off of an RCR123 or 3 123A primaries and have multiple output levels. It just has a limited buck capability compared to our traditional LED buck circuits
 

dabiscake

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Mar 10, 2003
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Thanks for the replies, guys.
Luna said:
It is a buck/boost design. That is why you can run off of an RCR123 or 3 123A primaries and have multiple output levels. It just has a limited buck capability compared to our traditional LED buck circuits
You're right, it's all in the design, isn't it? I think I had the set idea that a buck converter would always give you a total range or something.
 
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