What Wire to use?

[Klem]

Guys,

I am planning to convert a Maglite using the upcoming QP407 driver.
http://www.cutter.com.au/proddetail.php?prod=cut990

It has 7 XP-G's running on one of three settings; 350mA, 500mA and 1A. Voltage operating range is listed as 11-20V

I'm thinking it's capable of up to 90Watts.

What type of wiring is appropriate (12AWG)?

Is there a consensus on a heat resistant types (teflon coated etc)

What's a good one to use here?

Cheers

Klem

 

[Kandela]

I think your math is alittle off, not sure though, correct me if I'm wrong! Power in watts is voltage times current... we'll go over the top just to be safe, we've got a max of 7 leds running at 1amp. so we'll say 6v per led, 6Vx7=42v. Max 1 amp, so we'll say 1.3 to be safe, Power=VxI=42Vx1.3A= 54.6 watts. This was with CRAZY exaggerated numbers too...

So you're looking at a maximum of 55 watts... less than single standard filament bulb.. Some other things around the house use well over an amp of power, and most houses are supplied with 14 gauge wire.. So as a starting point i'd say you could EASILY use 14 gauge wire..

According to a AWG chart you could safely use 20 gauge wire. This will support 11amps, or a steady flow of 1.5amps. It does sounds scary skinny though, i'd go with some high quality gas/oil/sunlight resistant auto wire or outdoor wire, stranded, in 18 gauge..

If you have the space and if it makes you feel better (I'm like that too) then go with 16

But dude, 12, overkill. 14, still overkill.

 

[evilc66]

You're looking way too far into this, and in the wrong direction. All you need to worry about for low voltage DC is current and distance. Considering that there are three drivers on the board, you are likely to only be drawing a max of 3A (three 1000mA drivers in parallel). Typically in a flashlight application, your wire lengths will be under 6" in total length. With that being the max length, you only see a voltage drop of 0.02v (0.06W @3A) with 18AWG wire. 18AWG wire has a resistance of 0.006385 per foot, so the calculation is 3A * 0.006385 ohms = 0.02v (you don't divide the resistance in half to get your 6" wire length because the round trip distance is 12" if both wires are the same length). You should be able to safely go down to 22AWG if space is an issue.

You can use this page (calculator at the bottom) to see wire capacities and voltage drop based on your requirements. The calculator only has fixed voltage values, but the 24v DC option is close enough. Not that it matters here.

http://www.powerstream.com/Wire_Size.htm

 

[Kandela]

evilc66 has got it on the nose. Absolute max=16, Absolute min=22, average it out and go with 18 or 20.

If you chose the more robust gas/oil/outdoor wire you might HAVE to go to 20 or 22 to make it fit depending on space. I only suggested this because i know it's really tough, and I know my car gets really hot. Pretty simple to me... But yeah, you should get some other opinions on specific wire types, but like evilc66 said you're looking too hard into it. You're not dealing with a noise, low voltage or signal situation, just slap some fresh wire in there and you'll be fine

 

[Klem]

Kandela, Yes, my mistake. Now that you mention it, 7amps is twice the draw of a 35W HID at similar voltage...silly me, you could cook your dinner on the torch with that!

I read the max forward voltage of a Cree XP-G at 1A is 3.3V
http://www.cree.com/products/pdf/xlampxp-g.pdf

So, by your calc... 3.3V*7 emitters = 23.1V.

P=Vi, 23.1*1A, plus some inneficiency, 30Watts more like it?

Evil, I can see where you're coming from...we're talking extremely low voltage drops over such short wiring. I'm getting in the order of .02% by using your linked calculator on 20AWG copper wire.

Thanks guys, 'problemo solvered'!:thumbsup:

 

[Kandela]

Yep! I didn't look into the details of the driver as well as evil, but if there's 3 drivers, it could possibly be using 3 amps. I doubt it though, because with 7 led's it would be hard, (atleast more difficult) for a manufacturer to wire it up. (maybe the center is off in HIGH giving 6, so 2 per driver?) This would give you a total of 3 strings of 2 parrels, aprx 3.5v each max, totalling 7Vx3=21V @ a max circuit draw of 3.5 amps is around 75W. Still, less than a single bright 100W bulb, and maybe the draw of a high power blender, with a fraction of the distance. You're good to go!

*edit, I noticed there's a (3 on setting) so it probably uses one chain of 3 in series, and 2 in series for the others, either way its all good.

 

[Klem]

I phoned Mark Riley of Cutters Electronics (the designer/distributor of these things).

The way he described it is there will be 3 pin-outs for different current draws; 350mA, 500mA and 1A. You solder the power lead to the pin-out of one of those three choices.

Then when switching on/off it has 2 modes. All 7 on, or 3 in a line. So for example, all 7 emitters at 1Amp, or only 3 at 1Amp...

Unsure if this phone conversation helps explain anything?

 

[uk_caver]

Often the best way to work out current draw is to do most of your calculations using power.

The driver is a triple buck (voltage-reducing) driver.

Chances are that the driver efficiency is roughly 85% or so, and probably not worse than that, possibly a bit better.

If you take the maximum LED power (set to 1000mA, driving all 7LEDs, max Vf per LED of 3.3V) as being something like 23Watts, then allowing for driver losses, that's a total maximum power draw from the battery of 23/0.85 -> about 27Watts

From that, you can calculate the current draw, depending on your power source.
If you were running it from the minimum 11V supply, that's about 2.5Amps
If you were running it from the maximum 20V supply, that's about 1.4Amps.

Teflon-coated wire /can/ have advantages, but a lot depends on whether you end up having to buy much more than you really need.

 

[Kandela]

^
Caver guy took it up another notch. I'm not sure how much more precise you can get than this. Even factored in losses.
:thumbsup:

Thats cool you could get into touch with the builder, I must say I'm surprised, I figured you'd end up in some neverending loop between some callcenter here and some dusty phone in china!

With my initial sloppy estimate of 7 leds at one amp, evil factoring in distance with his suggestion and caver going purely on real world numbers, theres no doubting you're fine with the numbers here. I'm sticking to 18/20, and I don't think wire type will really matter in this situation; dc, low interference, low if any signal generations.

That's a nice module there too, you'll have to keep us up to date with how the install goes when you get to it!

 

[Klem]

Yes, wildo, although the build will probably end up on the dive torch thread.

7 XP-Gs at 1amp will require some serious heat-dissipation, which in turn means I'll have to sweet-talk my mate who has access to a lathe.

My original concern about the wire was that it would act like a filament and heat-up. I see now therre are more issues at stake.