DIY Light Build electronics questions

[jgoodstein]

I tried posting over and candlepower, but not getting the same responses that i get over here.

I want to DIY my own canister light. here are the parts I've chosen to date:

Cool-White Cree XP-G 3 Up Indus Star (at full power ~700 lumens)

Narrow 3-Up Carclo Optic ( 8 degree spread 87.3 efficiency)

3023-D-E-700PmA BuckPuck with Potentiometer

Nimh C cells 1.2V 6Ah with tabs. I plan on doing 10 in a series resulting in 12V 10Ah

Based on calculations here I come up with the following equation:

.7 Amp Draw X 6.5 Hours = 4.55AH battery requirement

In order to not drain said battery past 80%(20% remaining)

4.55Ah / .8 = 5.69AH this falls within the capacity of the batteries

This should give me 6.5 hours roughly at full power (700 or so lumens at) . I did not include Peukert's Number ( which calculates how quickly drop off is) but feel confident that the battery pack is sufficient for my needs ( my dive's don't exceed 1hr generally speaking)

I plan to heat sink with solid aluminum stock and use perhaps an otter box or self made canister, and modify a Mag lite that I own for the casing. I'm really concerened that I have the math right. What do you guys think of my increasing to 3 X 3UP of the lights in the single mag light head? battery requirements definetely change, but as far as brightness I'm pretty sure what I have designed above will meet my needs.

Where do most of you mount the Potentiometer (power adjustment) on the light?
Does all my math add up correctly?
This should fit in a mag lite head correct?
Any other recommendations to keep it cheap and functional?

 

[FWADIVER]

your buck puck is going to have an efficiency associated with it, I didnt see that in your calculations, the current draw on the batteries is not going to be a constant 700ma, it is going to depend on the forward voltage of the diode at 700ma and the current voltage of the battery pack, so depending on the packs discharge characteristic you are going to need to add some fudge or margin to the design.

 

[jgoodstein]

your buck puck is going to have an efficiency associated with it, I didnt see that in your calculations, the current draw on the batteries is not going to be a constant 700ma, it is going to depend on the forward voltage of the diode at 700ma and the current voltage of the battery pack, so depending on the packs discharge characteristic you are going to need to add some fudge or margin to the design.

Looks like efficiency on the buck puck at 12v is ~92%-95%.

Can you explain a tad more on the forward voltage and constant draw, and how I may calculate that. I don't see that on the data sheet for the light I selected. My real requirements is to mimic a retail canister light at a reduced price so battery time should be semi equivalent.

Edit here is the forward voltage according to cree


Forward voltage (@ 350 mA) V 3.0 3.75
Forward voltage (@ 700 mA) V 3.2
Forward voltage (@ 1000 mA) V 3.

Mine will be running at 700ma due to the buck puck. Not really sure how to add this into the calculations

 

[FWADIVER]

well the forward voltage is what gives you the power, i am assuming the LED's are in series so you have 3 LEDS = 3x3.2V=9.6V

9.6V x 700ma= 6.72 watts, apply worst case 92% efficiency:

=7.25W

7.25W/12V (nominal battery) = 604mA as the batter will probably start higher say 14V it will start off drawing about 520mA at 14V and will go up to 725ma as the battery drops to 10V the buckpuck will put out a constant power which is voltage of the LED times the current draw (700mA) the battery pack is also feeding constant power to the buck puck, but the current draw will go up as the batter voltage drops to maintain that power output discharging the battery more quickly.

 

[jgoodstein]

So the draw is 604ma running then? Should I then adjust my battery to have more then 12V and let the driver regulate it down?

 

[FWADIVER]

12V should be fine, but just be aware depending on the discharge of the pack, you may get less burn time that you expect as it goes over what we call the 'knee' and starts putting out more current than the light is drawing which accelerates its discharge.

 

[350xfire]

That 3 up module may be somewhat floody. I would recomment using the new Cree XML (1000 lumens at 3 amps) or the older Cree MCE (either 2.8 amps at 3.7 volt or .7 amps at 14.8 volts)

You would build a 7.4 volt LiIon pack and use a buck driver. You can use the LEDynamics at 700 mah for the MCE and get close to the 6 hours with a 2s4p 8-cell pack. I have had really good results with Cree MCEs.

 

[jgoodstein]

That 3 up module may be somewhat floody. I would recomment using the new Cree XML (1000 lumens at 3 amps) or the older Cree MCE (either 2.8 amps at 3.7 volt or .7 amps at 14.8 volts)

You would build a 7.4 volt LiIon pack and use a buck driver. You can use the LEDynamics at 700 mah for the MCE and get close to the 6 hours with a 2s4p 8-cell pack. I have had really good results with Cree MCEs.

Do you know if they have a three up for the xml? what driver would i switch to. Flood isn't really all that bad for what I use it on but options are always good.

 

[350xfire]

Do you know if they have a three up for the xml? what driver would i switch to. Flood isn't really all that bad for what I use it on but options are always good.

Cutter electronics out of Australia just sent email today that they are going to have a 3 xml. Try them first.

 

[jgoodstein]

the problem seems to be drivers, I can use resistors but I don't like that option. I will call them and see what they have as an option ( thank goodness for free VOIP calls to Aus)