# 3528 White SMD LEDs

Hi,

I wanna make a multy led circuit, like 50~100 smd leds (probably 50) and I wanna power them up from a 3.7~4v Lithium Ion battery 800mAh, now i know that each led should get a resistor to act as normal led.

The smd leds im using are 3528, 3 x 2.8mm, 3~3.3v, 20mA. I have already done a circuit with 12 leds in a array and they get preaty hot of curse of the high drawing power they get but i was thinking is there a possible way to make a array/parallel/series matrix of leds without or at least one resistor?

By putting a few in series, you can reduce the number of current limiting Rs required. The offset though is you need to boost the voltage. You will need to work out if the boost circuit uses more or less power than the limiting Rs.

Weedpharma

Use a boost converter, get your voltage to 12V for 3 LEDs in series, or 15V for 4 LEDs in series.
Each string can then share the same 20mA of current to turn on.
25 4-LED strings then only need 500mA.
Need a current limit resistor per string.

Just connect 50 of them in parallel.
Use a 1ohm/3watt (cement) current limiting resistor.
Put some shades on before you power it up.
She'll be right mate.
Leo..

Wawa:
Just connect 50 of them in parallel.
Use a 1ohm/3watt (cement) current limiting resistor.
Put some shades on before you power it up.
She'll be right mate.
Leo..

Leo don't joke, leave that for the Bar Sports section.

Sure, it's supposed to be bad to put LEDs in parallel.
Uneven current, hottest LED gets the most current, and gets hotter. etc.
Practically, it isn't that bad. As long as you use the same batch of LEDs, and keep them all on a the same heatsink.

The factory made downlights in my house use six SMD LEDs in parallel.
Ten of those sets in a string. 60 LEDs in total.
They are all mounted on the same heatsink (same temperature).
Running several hours a day for the past year.
No failures yet.
Leo..

What is it about LEDs that turn a normally knowledgeable person to over stretch themselves?

Yes it is.

As long as you use the same batch of LEDs, and keep them all on a the same heatsink.

That is a way oversimplification. It is the chips that have to be at the same temperature, once you put them into packages that makes it impossible to achieve.

The factory made downlights in my house use six SMD LEDs in parallel.
Ten of those sets in a string. 60 LEDs in total.
They are all mounted on the same heatsink (same temperature).

Yes I know, and do you know that even then it doesn't work all the time. This video is from someone who was sold LEDs on eBay from the reject bin. They are made using that very same technique.

So why do you think those LEDs ended up in the reject bin. They were from the same batch weren't they?

So advising someone to mount LEDs that are already in surface mount packages on a heat sink is not going to work. If you want to try it yourself then I would suggest you try testing it like the guy tests it in that video.

I like to tinker with LEDs, and a have a large number of those COB lights.
Never seen it so uneven as in that video.

You can't measure the forward voltage of the individual LEDs in those cob lights.
The forward voltage might be ok though, just uneven light output.
ESD could be the cause of that.
I once soldered a few 3watt RGB LEDs with a cheap soldering iron.
After that, half of the blue LEDs were dim, but they still measured ok.

I would hesitate to put two or three LEDs in parallel.
If one is different, the other two get the full current.
Maybe not so much of a problem with 50 LEDs in parallel.

Don't know how OP is going to mount them.
And why 50 LEDs on one small LiPo battery.
It would have been much easier to use one 3watt white LED on a star base.
Leo..

I'm with Grumpy on this one.

It is far safer (yes I'm a conservative) in the long run.

Weedpharma

If I had to drive 50 LEDs, I would use a boost converter, as CrossRoads sugested in post#2
10 strings of 5, or even 5 strings of 10.

But read post#0 again. OP has already put 12 LEDs in parallel on his battery.
nuff said.
Leo..

That's a really nice example of that kind of leds.

Sadly I don't have a boost converter (I saw many people talking about that). Yes I tried 12 leds in parallel
and they got really hot after a while.

I wanna power them up directly from a battery a 3.7~4.2v Lithium Ion 800mAh battery (I powered up with the same battery the 12leds above) so how could i power them without boost converter?

You can't power them any other way than individually, with each its own current limiting resistor.
The source is 3.7 volts, and you need 3 volts per unit.
If you stack units, you also need to "stack" the supply voltage.
So it's either a boost converter, or individual setup.

So it's either a boost converter, or individual setup.

Wait, so you tell me that a boost converter give exact current for all leds? how's that?

No it doesn't.
It gives you a higher voltage so you can put your modules in series.
You still need to limit current, and using a resistor is the simplest way to do so.
Each string of modules needs one current limiting resistor.
The modules in series will share the same current.
Modules in parallel will each use that same current, so you'd need to add up all currents to know the total current drawn by your setup.
The same goes for the strings you'll put in parallel.
So if you can put enough modules in a string, then you could be able to save total current drawn.
This can prove to be more efficient, even though you'd be using a boost converter which isn't lossless.

Hmm, do you know any super mini boost converter to convert the 3.7v~4.2v to higher voltage?
I have a battery 5.5cm x 3.5cm so i need to make the circuit exact as the battery in this way i need a small boost converter to fit in that "module" to make it thin as possible. Battery thickness 4mm.

It's max power output is 20 W, so it might not do the trick for you.

It isn't that hard to get Google results for "boost converter".

I don't know how to use google

This is big enough for 50 LEDs.
Adjust to ~18volt, and connect series strings of 5 LEDs and a 100ohm resitor.
Leo..

Input 3V Output 12V 0.4A 4.8W

that's not what i expected, at 12v it's about 4 leds in series and i need to put a resistor to each series
that will be 12 paralel of 4 leds and one resistor each series, 12 resistors.
48 leds x 20mA = 960mA * 12v = 11.5 Watts. each mini smd led has 4LM * 48 = 192 LM
PCB dimension 4cm x 1.3cm.

A actual 10W led works with 900mA and has 900LM and works at 9~12v dimensions 2cm x 2cm.