Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Led selection

Status
Not open for further replies.

Dr_Doggy

Well-Known Member
I want a rgb led , i hope 3 watt is enough brightness, something fairly bright and narrow angle is best,
and smaller = better ~1/2"-1",
+ cheap!
...fast shipping (2week)
i just want it all!~

think i found this:
https://ca.mouser.com/ProductDetail...EpiMZZMtKo6xK7%2bVI1pEltFN%2brIPGi24lrIjqzGU=

but i wonder do i need heat sink for this one, how big?
also it always gets me with the different volt inputs, 2.4 V, 7.2 V, 3.5 V, I will need to start with 12v battery, how should I apply voltage step down per channel? at cathode i will have mosfet for microcontroller~still selecting
~have lots of these:
https://www.digikey.ca/product-deta...ies-americas-corp/IRFU3704/IRFU3704-ND/446741

what is path of least resistance!?
 
For LEDs, you DO NOT feed them voltages because each LED has a different range voltages. Instead you limit the current. a resistor can be used to limit the current.
The datasheet shows that the green is TWO LEDs (in series) so of course their voltage is higher than the red or the blue that are singles.
The datasheet has a graph of maximum temperature at various currents with three different heatsink spec's.
 
I've been using the LZ1 in white, and yes, they do get really HOT when run on the higher amps, soldering them is fun also :woot:
 
You would also get hot if you were the tiny size of those LEDs and had a lot of current flowing through you.
 
What I've seen when using LED's, mostly It's pointless to use them at rated current, much lower will do...see constant-current circuits, those tend to be much more stable than resistor current limiters, scpecially when voltage fluctuates like with battery systems. About heatsink I dunno, but some leds get ridiculously hot which is also other reason I don't use them at rated current.
 
thanks aG! wouldn't have seen it without hint,

I once seen a thread on how to calculate resistance for led , but cant find it now,
isnt the equation something like:

(total volts- led volts)/ forward current = ohms
9.6/.7amp = 15 ohms for red?

plus thats 7 watts lost on the resistor, can we get them that big?

maybe a constant current circuit for each channel is really the way to go?
 
I once seen a thread on how to calculate resistance for led , but cant find it now,
isnt the equation something like:

(total volts- led volts)/ forward current = ohms
9.6/.7amp = 15 ohms for red?
Almost. 9.6V/15 ohms= 640mA.

Thats 7 watts lost on the resistor, can we get them that big?
A 10W resistor is common. There is a lot of heat because the 12V supply voltage is too high for the 2.4V red LED. If the supply is 6V then the resistor heats with only 2.5W. If the supply is regulated at 3V then the resistor heats with only 0.4W. But most LEDs have a range of forward voltage so you should calculate the resistor for the lowest forward voltage.

maybe a constant current circuit for each channel is really the way to go?
The constant current circuit will produce a little more heat than the resistor if it has the same 12V supply.

I wonder why the multi-color LED has TWO green LEDs but only a single red and blue LED? One green LED will be bright enough then a lower supply voltage can be used for less heat in the resistors.
Did you find a suitable heatsink for the LED? If all 4 LEDs have 0.7A then it heats with 9.2W.
 
plus thats 7 watts lost on the resistor, can we get them that big?
there are up too 100w resistors, even bigger but you can see how big those are then :D
LDO constant current sink controlled with opamp and transistor or even better, mosfet, or even better still, buck-converter ....but bucks are hard to design. With Opamp constant current driver you sniff current with 0.1 ohms or 1 ohms resistor that then controls driver accordingly, however this is linear fashion so not that efficient than buck converter, but still better than resistor only, only bit more complicated...there's many ways to skin a cat :)
 
ya, it all sounds like fun and $$ , did i mention i dont mind pre-built driver!

i think i found the heat sink, if I can read datasheet i get one that is 5°C/W
will this heat sink work, looks kinda small, but then again i need small, if this does work i can go with the 1/2"pvc:

https://www.digikey.ca/product-detail/en/assmann-wsw-components/V2017B/AE10837-ND/3511410
+
https://www.digikey.ca/product-detail/en/led-engin-inc/LZ4-00MD09-0000/1537-1098-1-ND/4976788

it is too bad, this led going to be sealed in to a chunk of pipe surrounded by water, if i trusted my gluing skills better I would try to sink right to the water through the pipe, but I don't , so i wont!

maybe it would be best to buck it down to the 3.6v mark and have less loss in each resistor?
 
did i mention i dont mind pre-built driver!
And where's the fun in that? :p Why buy brand new when you can make half features double the money?
 
The LED chip needs a MCPCB "star" to be bolted on a heatsink. The part number of the chip mounted on a star is shown on P2 of the datasheet . The LED plus star have a thermal resistance of 3.0 degrees C per Watt and if the ambient is 30 degrees C then with 10W the chip will be 60 degrees C if the heatsink is perfect (impossible) and does not even get warm. The max temp for the chip is 125 degrees C but try to aim for 120 degrees C or less then the heatsink and thermal grease must have a thermal resistance of 120 - 60= 60 degrees C for 10W or 6 degrees C per Watt. The thermal grease adds about 0.5 degrees C per Watt so the heatsink should have a thermal resistance of 5.5 degrees C per Watt when it is in free air and is not enclosed. Pipe?? Water??

The tiny heatsink you found will glow red hot with 10W since its thermal resistance is 31 degrees C when it is in free air and is not enclosed. You need 5.5 degrees C per Watt.
 
And where's the fun in that? :p Why buy brand new when you can make half features double the money?

..... what, now theres pcb involved!!? where does it end!

I see now i got that value a little backwards, plus another little problem, proper heat sink is way too big for pipe (only sealed at led side, still not much airflow),
a little mickey mouse, but i think my best option may be to flatten a piece of copper pipe and some thermal padding between, and glue it in to pipe so it gets water flow in pipe, but not in led chip. and skip on the pcb, also not sure how i will clamp it down, maybe some cable ties??

Also how does this sound for driver, more fun too is that it is smt:
https://www.digikey.ca/product-detail/en/texas-instruments/LMZ22005TZE-NOPB/296-40330-1-ND/5178207
and i will use circuit labeled: Simplified Application Schematic , is that ok?

then i can use buck to step down to 5v and equate my resistors as such:

2.5/0.7 = 3.5ohms
and brings my power loss on resistor to <2watt on Red LED.

does this sound better?

...ohya.. this is why:
 
i can use buck to step down to 5v and equate my resistors as such:

2.5/0.7 = 3.5ohms
and brings my power loss on resistor to <2watt on Red LED.

does this sound better?
But you did not read the datasheet. Only typical LEDs are 2.5V but some are 2.1V and some are 2.9V. If yours is 2.1V then its current will be (5V - 2.1V)/3.5 ohms (that is not a standard value)= 829mA. Its current will rise as it heats which causes its voltage to drop which causes its current to rise more until it melts, unless the heatsink has a fan blowing away the heat.
 
again many thanks for the watchful eye AG!
Does this mean I should spec out my resistors to the 2.1v ~~ 2.9v @700ma=4.14ohm , therefore using a 4.3 ohm resistance(vendor didn't have 4.1/4.2 ohms, and i like the safe side!)?
 
Does this mean I should spec out my resistors to the 2.1v ~~ 2.9v @700ma=4.14ohm , therefore using a 4.3 ohm resistance(vendor didn't have 4.1/4.2 ohms, and i like the safe side!)?
If the 4.3 ohm resistor is 5% high at 4.515 and the LED is 2.9V then the current is only 465mA which is still very bright and might look the same unless you had one at 700mA beside it.
 
well soldering smt was a total fail! I got 5/6 legs on then one would pop off, eventually the whole thing split in half

so i resorted back to my standard rgb's i was able to fit about 5 or 6 in there so this adds up enough to match the current specs, not brave enough yet to run water through though:

 
SMT's need a good heatsink on alum substrate. with 1 sq in per watt.
if relying on water cooling, then add thermistor or PTC for safety feedback to heatsink.
Give a good silicone spray coating with several layers.
solder connections need to be solid and done quickly.<5s
 
Why are you not brave enough yet to run water through? If it isolated, low voltage, you have nothing to be scared of! The worst that can happen is some corrosion. Actually no, I tell al lie, the WORST thing that can happen is you end up splitting the water and get a build up of hydrogen and oxygen, and then a spark...
But that's extremely unlikely ;). Good luck with your fountain!
 
Sounds like you are trying to do your smt soldering onto something that isn't flat???
Usually all you need to do is tin one pad, position your smd, tack one leg down to the pad you tinned (you don't need more solder for this, there is enough already), then solder the remaining legs. Finally solder the first leg properly with a bit more solder.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top