dc-dc converter circuits

[paulr]

I'm interested in running a 5 watt LS from one 123 cell, if necessary at reduced power like 3 watts. I'm not terribly concerned about runtime since it would be for intermittent use. I know that some Surefire lamps (P61, MN21, etc) run SF123's at high currents like that and work ok.

Any idea what kind of circuit to use? If there's a ready-made converter that would be great, but the BB700/750 need somewhat higher than 3V input to run a 5W LED. If not, I might like trying my hand at putting one together. I have reasonable electronics skills but am not familiar with flashlight dc/dc converters so would appreciate suggestions about circuits, where to get parts, etc.

It needs to be a pretty small circuit since the host light would be a 1x123 DIY McLux. Yes I know there'a a 2 cell battery tail but the 1 cell one just looks so much nicer.

Thanks for any advice.

 

[Doug S]

Paul since you are not much concerned about runtime, it is possible to do what you are considering. Be aware that the total energy available from a CR123 drops with increasing power. Study the datasheet at the link below. At 1.5W drain [this is about what an ARC LS draws] about 3 Whr is attainable but if you increase the drain to 4W less than 1.5Whr is attainable.
http://www.duracell.com/oem/Primary/Lithium/123.asp
If you use any of the BB boards be it the 400/500/or 700 the result will be the same. The board will deliver somewhere in the range of 300-400mA to the LED depending on Vf bin rating and variations of board components. The reason it doesn't matter which BB you use is that it will be running wide-open, unable to reach its setpoint current limit. Under these conditions the BB efficiency is on the order of 65-70% Expect to be drawing about 3.5W from the battery and delivering around 2.5W to the LED. Awhile back the ARC folks abandoned the idea of a single 123 powered "5W" version due [among other reasons] to the limitations of extracting and converting so much power from a single cell. If I were considering doing this [I am not] I would probably change the BB sense resistor to 0.20 ohm which converts it to a BB250. For only a slight reduction in brightness, converter efficiency and extracted battery energy is significantly increased. All else being equal, a 5W running at 250mA will put out more light than a 1W at 500mA since it is being underdriven.

 

[INRETECH]

Providing the power to run the LS5W is just half of the problem, you must be able to get rid of the excess heat that it generates, some small flashlights may not have the area to do this

 

[paulr]

Doug, thanks, that's interesting as it hadn't occurred to me that you could get enough voltage from a BB400/500/700 to make a 5W even glow on one cell.

Mike, the McLux head has very good thermal design (LED mounted directly to Al bulkhead, radiating fins) but not much mass. Sitting on a table running a 1W for a few minutes, it barely gets warm. It's designed to handle intermittent 5W but not continuous. I'd run a light like this for only short amounts of time. The usual application of a small bright light is to look for something nearby, normally for just a few seconds.

However, I don't know whether such a light really makes sense other than for nerd factor. A 5W light really wants a bigger reflector or lens than can fit in the regular McLux head.

 

[Doug S]

[ QUOTE ]
paulr said:
Doug, thanks, that's interesting as it hadn't occurred to me that you could get enough voltage from a BB400/500/700 to make a 5W even glow on one cell.



[/ QUOTE ]
The IC on the BB is rated to up to 18V output. The BB as built is best limited to 10V maximum as I believe this the rating of the output capacitor. Be aware that the way Wayne has configured this circuit it cannot tolerate being operated without a load. If operated without load the output voltage will rise until something fails.

[ QUOTE ]
paulr said:

However, I don't know whether such a light really makes sense other than for nerd factor. A 5W light really wants a bigger reflector or lens than can fit in the regular McLux head.

[/ QUOTE ]

Hey, lots of stuff gets done around here for no better reason than the "nerd factor". If you like the flood aspect of the Mclux, increasing the reflector size really isn't necessary.

 

[Entropy]

[ QUOTE ]
Doug S said:
[ QUOTE ]
paulr said:
Doug, thanks, that's interesting as it hadn't occurred to me that you could get enough voltage from a BB400/500/700 to make a 5W even glow on one cell.



[/ QUOTE ]
The IC on the BB is rated to up to 18V output. The BB as built is best limited to 10V maximum as I believe this the rating of the output capacitor. Be aware that the way Wayne has configured this circuit it cannot tolerate being operated without a load. If operated without load the output voltage will rise until something fails.

[ QUOTE ]
paulr said:

However, I don't know whether such a light really makes sense other than for nerd factor. A 5W light really wants a bigger reflector or lens than can fit in the regular McLux head.

[/ QUOTE ]

Hey, lots of stuff gets done around here for no better reason than the "nerd factor". If you like the flood aspect of the Mclux, increasing the reflector size really isn't necessary.

[/ QUOTE ]
Yup. The only real limit preventing the BB from powering a 5W with too low of an input voltage is the input current limitations of the switching regulator.

So if you underdrive the 5W, you might be able to get away with two-cell operation. I've heard of people running 5W emitters at 2W from 2 AAs.

To run a 5W at full power with only two cells, you would need to use a regulator that allowed for an external heatsunk switching transistor. With a decent switching transistor, 5W in a 2D Mag should be easy.

 

[star882]

"Providing the power to run the LS5W is just half of the problem, you must be able to get rid of the excess heat that it generates, some small flashlights may not have the area to do this"
The CPU voltage regulator in my PC has no heatsink, and it only gets warm(it probably has to handle 90w).
A 5w converter probably generates little heat.

 

[Entropy]

PC power supplies use VERY efficient switching regulators. (They can use larger inductors that have lower loss.)

A significant portion of the power dumped into a 5W Luxeon gets converted into heat. Not necessarily at the converter, though. You won't need too big a switching transistor to handle a 5W LS (Just a lower series resistance than the internal one in the swithcing regulator used in the BadBoy). But you DO need to heatsink the Luxeon itself. Even a 1W Luxeon can do a very good job of heating up a MiniMag. A 5W at full power in a Minimag will get VERY hot and will probably NOT ever be usable for continuous use unless you add heatsinking. (Possibly by using a lathe to add notches in the head, increasing surface area?)

 

[jtice]

Is there any limits, problems, things I should know, about a 5W LS + MM750mA board + 4=AA's ???

I am looking for maximum brightness from this. But still wanting over 1.5 hour runtime.

 

[Doug S]

See the thread below:
http://www.candlepowerforums.com/ubbthreads/showflat.php?Cat=&Board=UBB6&Number=255563&Forum=All_Forums&Words=&Match=Entire%20Phrase&Searchpage=0&Limit=100&Old=1day&Main=252465&Search=true#Post255563

As you can see, there is some question whether the MM can put out more than 5V. The datasheet for the IC says yes, but Wayne, designer of the MM says no.
Assuming that it can, this will work much better on NiMhs than alkalines. BTW, I assume that you mean MM and not BB. There is no such thing as a MM750 AFAIK. Note that to do this you will be running the IC on the MM beyond design output voltage rating. I would like to see you try this and report back your results.

 

[jtice]

datzips sitte is down right now,, but I think it was called the MM+ but it was 750mA.

I will be running the IC on the MM beyond output voltage ????

How, explain please. Yes I'm a newbie. /ubbthreads/images/graemlins/frown.gif

But everyone has told me that would be fine,, and BRIGHT. /ubbthreads/images/graemlins/smile.gif

The 4-AA's would only be 6V,,, the MM can take either 6.3V, or 7.2V.. i cant remember. But thats input rating.

Im not sure how this output rating goes.. please explain.

 

[star882]

"PC power supplies use VERY efficient switching regulators. (They can use larger inductors that have lower loss.)"
I don't mean the power supply(which does have heatsinks), but the core voltage regulator on the motherboard.
I have seen a 30w per channel class D audio amp that uses tiny SMD components for the output(no heatsinks)!

 

[Doug S]

John, the IC on the MM is the MAX1674. It has absolute maximum ratings for Vin and Vout of 6V per the datasheet. I speculate in the above referenced thread that these limits could be pushed a bit. There have been some additional posts in that thread, you may want to take another look. There is a pot on the MM that adjusts output voltage. There is some magic on the IC that causes the output to jump to fixed values of 3.3V or 5.0V when the pot is adjusted to either extreme of its travel. The topic Wayne and I have been discussing in the above referenced thread is whether the output can go beyond 5V as the pot is adjusted for increasing voltage before it jumps to the preset 5V value at the extreme of pot rotatation. The datasheet says that >5Vout is attainable but Waynes actual experience suggests not.

 

[jtice]

Ok, to clerify.....

Heres the MM700 I was refering to.

Heres the BB750 i was refering to.

Which board would be better with a 5W LS and 4-AA's?
And would this be overdriving the LS alot?

 

[Doug S]

[ QUOTE ]
jtice said:
Ok, to clerify.....

Heres the MM700 I was refering to.

Heres the BB750 i was refering to.

Which board would be better with a 5W LS and 4-AA's?
And would this be overdriving the LS alot?

[/ QUOTE ]

"Best" is very subjective. Since this is really too much of a load to efficiently use alkalines, I will give you estimates assuming 4 NiMh or NiCd cells giving a total voltage of 4.8V. Depending on the Vf characterists of your 5W led, the MM [if it can be made to work] will deliver 400-500 mA and the BB probably will be able to reach its design output of 750 mA. Overheating the BB board may be an issue. As always, keeping the LED from over heating will be an issue. The MM should be considerably more efficient than the BB.

 

[jtice]

I plan on using rechargables 95% of the time,, alkalines only in emergancy.

So, the MM would be more efficient and run cooler? But the means less bright,, right? How much dimmer? Im going for max brightness, but dont want to burn anything up. -- I havn't seen magic smoke,,, don't care to either. /ubbthreads/images/graemlins/smile.gif
This light will be well heatsinked, with the MM or BB mounted in the heatsink as well.

The BB750 is almost double the mA as the MM400... does that mean it will be about twice as bright?

Assumming I can keep the LS and the board from over heating,,, Will the BB750 be efficient enough to give me around 2 hours runtime?

With the BB750,,, would I be over driving the LS too much?

 

[Doug S]

[ QUOTE ]
jtice said:
The BB750 is almost double the mA as the MM400... does that mean it will be about twice as bright?


[/ QUOTE ]

No, not near twice as bright. This is because the LED is more efficient at lower currents and temperatures.

[ QUOTE ]
jtice said:

Assumming I can keep the LS and the board from over heating,,, Will the BB750 be efficient enough to give me around 2 hours runtime?



[/ QUOTE ]
No, perhaps 1 hour before dimming assuming 2000mAHr NiMh cells.

[ QUOTE ]
jtice said:
With the BB750,,, would I be over driving the LS too much?


[/ QUOTE ]

IMHO, yes. It is dependent on the thermal performance of your heatsinking arrangement. Here is a link that will keep you busy for awhile:

Thermal considerations

 

[jtice]

Lets say I have a BB750 turned down to 500mA.
Is that the same brightness, and runtime as a BB500 turned all the way up?

Im thinking if I want the 2 hour runtime, I should still get the BB750, and turn it down,, then if I want it BRIGHT, I can crank it back up to full.

 

[Rothrandir]

hey...don't worry about this guys heatsinking /ubbthreads/images/graemlins/grin.gif

ticey...you won't be able to adjust the pot...it will be potted in rtv.