opinions on ways to wire led series in "high/low" mode/ and cooling question

poolman966554

Newly Enlightened
Joined
Oct 20, 2010
Messages
77
opinions on ways to wire led series in "high/low" mode/ and cooling question

hello everyone. Ive been working on putting together an xml light bar for a while now.

Im still waiting for supplies, and with this 'wait' time, i keep thinking of more stuff i could try.

To get up to speed, im building a (now) 12 xm-l led lightbar running 4 xml in series with a .68 ohm resistor and a TVS(suggested by DIWdiver)

specs say xm-l2 @ 700lm is 3.2v @2300ma
math says that on a 14.4v supply, i need a .68ohm resistor.

how about this idea? Id like the 2300ma as a "low" mode and run them at 3A on "high" mode - like this-(pardon my fancy Mspaint skills)
ledschematic1_zps17686294.jpg


So orange being low, and yellow being high, this should work right?

If so, i still have no idea what diode i need to separate the .68ohm and the .3ohm resistors.. or will it be cheaper to just run a rocker with separate resistors? I could do that, but it will require more space inside the u channel,and id rather no make things tight. Also, Im just trying to work with what i have on hand..

Also i realize the voltage will increase to approx 3.3v with the amperage increase, so figured that in to make the resistor output at .4ohm

Another opinion if i may ask..
For cooling, i have 3 old P4 processor heatsinks (fins are 2" tall, 1/16"wide) i am planning to cut down middle and bolt to the entire length of my u channel (approx 16" x 2") with artic silver 5. (fins mounted vertically) Will this be sufficient?

btw, this circuit will also have a fuse as well, forgot to add it.

Thanks guys!

Edit: upon reviewing circuit, i realize my resistor values may change.. will this high/low separation diode have a resistance value to it? if so, how much will/might it throw off my calculations?
 
Last edited:

Revolvr

Newly Enlightened
Joined
Jul 20, 2013
Messages
29
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

I am not sure I see any need for that diode. Electricity will either flow thru the yellow wire or the orange wire.
 

poolman966554

Newly Enlightened
Joined
Oct 20, 2010
Messages
77
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

I am not sure I see any need for that diode. Electricity will either flow thru the yellow wire or the orange wire.

in theory, the orange will power the series @2300ma, but when i rock the switch over to yellow, it will power series with both resistors in parallel @3A

the diode (which im not sure how to rate) is to stop voltage from accessing .3 ohm resistor to keep series at 2.3a
 

DIWdiver

Flashlight Enthusiast
Joined
Jan 27, 2010
Messages
2,725
Location
Connecticut, USA
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

I think you'd be better off without the diode, running separate resistors. Why would this take more room? You have enough variables in the voltages without adding another one. You'd loose 0.4-0.7V in the diode, depending on temperature and selection. It's not a resistance, it's a forward voltage, like the LEDs.
 

Revolvr

Newly Enlightened
Joined
Jul 20, 2013
Messages
29
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

I agree. The easiest way to do it is to have the switch select between two different resistances. Then just worry about the right resistance on each branch.

So in your case you need about 0.68 for 2300 mA, and about 0.53 for 3000 mA. Except...

The LED voltage drop increases with increasing current. So the Vf at 3000 mA will be higher than at 2400. The XML2 increases to about 3.3V at 3A so the resistor would need to be around 0.4 instead of 0.53.

This will also make it easier to change the resistance to tweak the brightness at the two settings. You might find there isn't enough difference between 3A and 2.3A.

And also keep in mind the power going through these resistors is just under 4 watts.
 

poolman966554

Newly Enlightened
Joined
Oct 20, 2010
Messages
77
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

I agree. The easiest way to do it is to have the switch select between two different resistances. Then just worry about the right resistance on each branch.

So in your case you need about 0.68 for 2300 mA, and about 0.53 for 3000 mA. Except...

The LED voltage drop increases with increasing current. So the Vf at 3000 mA will be higher than at 2400. The XML2 increases to about 3.3V at 3A so the resistor would need to be around 0.4 instead of 0.53.

This will also make it easier to change the resistance to tweak the brightness at the two settings. You might find there isn't enough difference between 3A and 2.3A.

And also keep in mind the power going through these resistors is just under 4 watts.

After doing more research, i agree with both you and DWIdiver..My current resistors on hand are 10w and will be strapped to my heatsink as well. i was just hoping to use what i had on hand instead of purchasing more resistors. I dont have very good electronic shops here, and everything must be ordered via website which includes $7 shipping..
I did figure for 3.3 voltage, as i wouldve used .3ohm+.69ohm in parallel = .39 ohm , but would expect a more significant rise in lumens as @2.3a = 700 vs 3A=1000 lumens.. thats roughly 300 more lumen per led, X 12!

considering i have 3 thermal switches to mount to heatsink,tvs, fuse, switch, and approx 9' total wire.

8400 lumen vs 12,000lumen (according to specs) not including reflector loss

Thanks guys. any thoughts on the cooling factor? Again, i have p4 processor heat sinks, approx 4"x4"x 2" tall. i want to cut then in half, and bolt to rear portion( directly opposing leds) with thermal compound.
Any thoughts?
 
Last edited:

DIWdiver

Flashlight Enthusiast
Joined
Jan 27, 2010
Messages
2,725
Location
Connecticut, USA
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

A couple of things.

First, I apologize for no carriage returns. Something has happened and about 10% of my characters typed don't show up on the screen, and I can no longer get a carriage return, even using the cut-and-paste method that's been working for a year. (edit: Fixed!)

Second, a "low" that's more than 75% of "high" will barely look like a different setting. I would make low no more than 50% of high.

Third, you're probably going to have over 100W of heat on high, including resistors and LEDs, but subtracting light output. That's a lot of heat to get rid of using passive cooling, even with good heatsinks. Since it's going to be cold and you have good heatsinks, you might be okay, but don't be surprised if the thermal cutouts activate after a while. I'd take a try-it-and-see approach.

Fourth, I don't know where you are, but if in the US, you can get small packages shipped from Digikey First Class Mail for under $3.

Lastly, I just found the answer to a problem that's been annoying me since I switched to Win7 almost a year ago, and got unbearable when IE updated itself to version 11 without my permission: The enter key didn't work. With IE11, it started missing characters randomly. I googled the problem and found that if I click the left most button on the toolbar to "switch editor to source mode", everything works great! What a relief!!!
 
Last edited:

znomit

Enlightened
Joined
Aug 1, 2007
Messages
979
Location
New Zealand
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

Where is the 14.4V coming from and what is the light for?
 

samgab

Flashlight Enthusiast
Joined
Aug 23, 2010
Messages
1,259
Location
Auckland, New Zealand
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

Don't forget the effect that temperature has on forward voltage:
Fullscreen%2520capture%252025112013%252083435%2520p.m..bmp.jpg

For a given voltage, as the temp increases, the current increases, which causes the temp to further increase, which causes the current to increase more... and so on. For a hypothetical example from the above table, imagine you had a fixed 13.0V running through the 4 LEDs. At 30DegC you would be running at 2.1A. Then as the temp increases to 50DegC, you will be running at 2.3A, and at 70DegC you'll be at 2.45A. But that is what the TVS is for I suppose, to prevent a runaway (on the voltage at least, to prevent voltage fluctuations). But would a PTC thermistor be a possible useful addition here for thermal management? As the temp rises, it would increase resistance to prevent harmful overheating. It could be mounted onto the heatsink at an appropriate place. Just an idea. But of course, PTC's have their own resistance, even at normal temps that you'd also have to calculate for... Just thought I'd throw another idea into the arena anyway, it might be pointless if you have excellent heat dissipation.

Edit: Found THIS... Quite interesting stuff on this topic.
Edit2: More recommended reading HERE... Explains nicely the benefits of driving LEDs with constant current rather than CV.

(Oh, and by the way, I've gone through the whole process you're going through now in the past, and documented the learning curve I went on, with heat sinks, drivers, circuit design, etc.
I'm running an XML and an XPG light bar (still in experimental stage), but indoors on a 240V AC supply, so I use a meanwell CC driver for each (mild current to keep heatsinking requirements minimal: 700mA for the XPG string and 1750mA for the XML string. This (running moderate current) keeps the efficiency of the circuit up too.).
If I was going to use a 12/14.4V vehicle supply for the same sort of design now, rather than trying to reinvent the wheel, I'd probably be inclined to search for CC LED drivers that are designed to run on vehicle's power supplies, with that range of DC input V, and with the CC output required. But I'm not sure if what you're doing is for vehicle mounting or not. (Edit again: just read in a previous post that it's for a quad.))
 
Last edited:

mds82

Enlightened
Joined
May 1, 2006
Messages
622
Location
Connecticut
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

I',m suprised no one else has said this already, but i would definitely recommend using a LED Driver versus just a resistor. that resistor will get HOT!!!!!!! I did something similar a while back and the resistor got to hot it melted the solder connections right off.
 

Steve K

Flashlight Enthusiast
Joined
Jun 10, 2002
Messages
2,786
Location
Peoria, IL
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

I',m suprised no one else has said this already, but i would definitely recommend using a LED Driver versus just a resistor. that resistor will get HOT!!!!!!! I did something similar a while back and the resistor got to hot it melted the solder connections right off.

and the resistor was rated for the power that was being dissipated, and provided with the correct airflow or heatsinking?
 

poolman966554

Newly Enlightened
Joined
Oct 20, 2010
Messages
77
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

A couple of things.



Second, a "low" that's more than 75% of "high" will barely look like a different setting. I would make low no more than 50% of high.

Third, you're probably going to have over 100W of heat on high, including resistors and LEDs, but subtracting light output. That's a lot of heat to get rid of using passive cooling, even with good heatsinks. Since it's going to be cold and you have good heatsinks, you might be okay, but don't be surprised if the thermal cutouts activate after a while. I'd take a try-it-and-see approach.

Fourth, I don't know where you are, but if in the US, you can get small packages shipped from Digikey First Class Mail for under $3.

Lastly, I just found the answer to a problem that's been annoying me since I switched to Win7 almost a year ago, and got unbearable when IE updated itself to version 11 without my permission: The enter key didn't work. With IE11, it started missing characters randomly. I googled the problem and found that if I click the left most button on the toolbar to "switch editor to source mode", everything works great! What a relief!!!

Thanks for the good info. I live in Northern PA.. i did miss the shipping method of upsp on digikey. As a matter of fact i ordered a more suitable spdt switch, and some 1.8ohm 10w resistors for $3 (to justify the $3 shipping ;))
That should get me around 3v @1.3A per led for a low.

As for the heat, i can lengthen my u channel by spacing out the leds a bit more. that will create another few inches to mount more heatsink. i have a bunch of old videocard heatsinks aswell, approx 2"x2"x 1" tall i could mount to the top portion of u channel. Thoughts? not sure what else i can do here, as being out in the open subjected to the elements, installing cooling fans might not be feasible. i might have to live with less lumens instead..

should i have some kind of "vent" in my u channel? i read on a few posts about having to let the hot air out. is this really necessary? Id prefer not to have a potential water problem, with blowing snow getting in and melting.

Also, glad you got your browser figured out! sounds like it was extremely frustrating!!

Don't forget the effect that temperature has on forward voltage

Yes i am aware of voltage increase with temp. i have a small 5amp meter going in series with one of the strings just to keep an eye on things during the first few runs. also have a k type thermistor to keep an eye on the temps. was planning to install directly next to an led to get best reading.


I',m suprised no one else has said this already, but i would definitely recommend using a LED Driver versus just a resistor. that resistor will get HOT!!!!!!! I did something similar a while back and the resistor got to hot it melted the solder connections right off.

By the math, i should be using 2 watt resistors. they will get hot, but should survive. For the extra .50, i upgraded to 10 watt resistors. Also i agree on using a 14v auto driver, but all this is really costing me is my time, which i dont mind as im learning something new.. and the $6 in resistors.
 

poolman966554

Newly Enlightened
Joined
Oct 20, 2010
Messages
77
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

you're probably going to have over 100W of heat on high, including resistors and LEDs, but subtracting light output. That's a lot of heat to get rid of using passive cooling, even with good heatsinks. Since it's going to be cold and you have good heatsinks, you might be okay, but don't be surprised if the thermal cutouts activate after a while

i read that i should have 1 square inch for every watt.. how many would approx be here(passive)? i was planning on cutting down the pencil line to make two halves, and mount side by side.. all the way across the back.
20mm star for scale



I have 3 of these (3.75" x 3.75"x 2" Tall) Im not sure what the square inch as surface area is..

do i measure both sides of the same fin? if so, 3.75"x 2" = 7.5 PER SIDE of 1 fin.15sq in per both sides of 1 fin. There are 29 fins.. is there something about distance between fins im not applying here?

i cant imagine this sink is good for 217 Watts, counting only 1 side of each fin..

Edit: wiki CPU_power_dissipation_figures says this heatsink was designed for 115w of dissipation..WITH a 80mm fan im assuming.. how would i calculate this number to passive?
 
Last edited:

DIWdiver

Flashlight Enthusiast
Joined
Jan 27, 2010
Messages
2,725
Location
Connecticut, USA
Re: opinions on ways to wire led series in "high/low" mode/ and cooling question

The simple answer is you can't calculate it. It depends on so many factors it's simply not practical to try to make such calculations. Face it up, face it down, mount it sideways with fins vertical, with fins horizontal, cut it in half, mount it near a wall, near the floor, everything will change its performance.

(ah, the simple pleasure of using the enter key!)

In fact, you do use both sides of the fin. Unfortunately, in this case the proximity of the fins to each other reduces their effectiveness substantially. Heat loss is by both convection and radiation. Since most of the radiated energy would impinge upon the next fin, that effect is substantially nullified. That's why they didn't bother to anodize it black (which improves radiation). The restriction presented by the long narrow passages also reduces the airflow due to convection. So packing the fins in tighter reaches a point of diminishing returns. Also, at low flow rates, the exiting air is hot, so it's not cooling very well in the region near where it exits. The point of forced air (fans) is to cause cool air to reach all surfaces. This dramatically improves the performance.

What you CAN do is test it. Bolt a string of LEDs to it and run it for a while and see how hot it gets. There's plenty of mass there, so it won't heat up quickly. You'll be able to monitor it and turn it off if it's getting too hot.

Thermocouple based thermometers are best for that. Infrared ones can't read bare aluminum for crap (because it doesn't radiate well at IR), but put a piece of black electrical tape on it and you'll get reasonable readings. Biological ones are cheap and easy to come by, but calibration is an issue. I've been touching hot things for decades, and still only count on getting +/- 5C. Too hot to handle without real discomfort is around 60-70C, depending on the person.

By the way, that's quite a bit bigger than I pictured. It might actually do 115W, but the temperature rise might be higher than you want for LEDs. Did they say what the temp rise would be?
 
Top