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How do i or is it ok to connect 100 LEDs (BLUE) with 12V DC 2amp supply in series? Answered

Hello Everyone,
I bought 1000 5mm LEDs ( Blue) pack and now i want to make a string of 100 LEDs in series. Is that ok if i connect all 100 in series and connect them with the ac to dc adapter i bought ( 12V DC , 2 amp)? if not, then how much LEDs i should use in a series with this adapter ?

What I have :--
1. Soldering iron
2. 12V DC 2 amp power supply.
3. 1000 5mm LEDs ( blue), IF = 25mA, VF = 3.0~3.4V.
4. Thin insulated wire.


You can't, At 3V forward drop, you could JUST run 4 in series.

then what do i exactly need to run 100 around leds ? what voltage and ampere of current adapter?

At the risk of telling you more than you wanted to know, I am going give you some more details about this technique of putting a resistor in series with several LEDs all in series.

Basically, I am going to show you some the same math used by the various LED-calculators, and how that works.

For a voltage supply, Vsupply, in series with N LEDs and 1 resistor, R:

Vsupply = (voltage drop across N LEDs) + (voltage drop across resistor)

Vsupply = N*Vforward + I*R

Solving for I gives:

I = (Vsupply-N*Vforward)/R

The first important thing I mention is, I>0, meaning forward current, only happens when Vsupply is greater than N*Vforward.

The second important thing, is the goal of this circuit is to make current, I, constant, and that requires all the variables on the right side, i.e. {Vsupply, Vforward, N, R} be constant. In other words:

If {Vsupply, Vforward, N, R} are all constant, then I = (Vsupply-N*Vforward)/R is constant.

Of those variables, {Vsupply, Vforward, N, R}, usually the variable you need to worry about the most is Vsupply. That is to say you need to worry about the uncertainty in Vsupply, in words, how well regulated is your voltage supply.

As an example, suppose I have a string of N=14 LEDs, each with : Vforward=3.25, in series with a 100 ohm resistor, and all those numbers are essentially constant. In contrast, Vsupply is supposed to be 48 V, but in truth sometimes it is 1 volt higher, sometimes 1 volt lower.

Using the formula above for I,

Vsupply=47 volts, then I=(47−(14*3.25))/100 = 0.015 A = 15 mA
Vsupply=48 volts, then I=(48−(14*3.25))/100 = 0.025 A = 25 mA
Vsupply=49 volts, then I=(49−(14*3.25))/100 = 0.035 A = 35 mA

The error in I is essentially the error in Vsupply divided by R. A way I could make this error smaller, is by using larger R and also fewer LEDs in the string, like maybe N=12 and R=470 ohms.

Vsupply=47 volts, then I=(47−(12*3.25))/470 = 0.017 A = 17 mA
Vsupply=48 volts, then I=(48−(12*3.25))/470 = 0.019 A = 19 mA
Vsupply=49 volts, then I=(49−(12*3.25))/470 = 0.021 A = 21 mA

Anyway, this is part of the reason why people on this forum keep recommending you use some sort of black-brick power converter, since those will give you good voltage regulation; i.e. an output voltage with small error; i.e output voltage close to what it is promised to be.

Although probably the main reason they're recommending a power brick, is because lower voltage is inherently safer. Also the power brick provides galvanic isolation
from the mains supply, and that makes it safer too.

For the sake of completeness, the URL for this LED calculator tool is:


I mention this as a courtesy to anyone didn't already know this. I'm not sure if that is the link, to the original, or the best, LED calculator out there, but I've seen it before, and it's the one linked first when I ask Google(r) about "LED calculator".


For the love of whatever Deity you believe is up there, **Do Not** use a 300V power supply.

That led.linear1.org Calculator up there will help you find what solution works best for you. If I was in your shoes I'd use a 12V supply and wire 3 in series and 100 sets in parallel. On the higher voltage rating like Iceng suggested you have a 48V supply and wire 14 in series, 7 groups in parallel.

Doing the whole thing in series just isn't a great Idea :P

Still, I'm curious what your making with this !

Sounds like this Illuminated shot glass shelf I'm working on. 65 slots and over 750 solder connections, and counting >.<

That's the one.

I sure wish i didn't spend an hour drawing that up in paint ;-)


2 years ago

No. 100 blue LEDs in series will require 100 * 3.0v = 300V power supply. This might be achievable with a voltage doubler off of 240 mains or a voltage multiplier if you have 120V mains.

With a 12V supply you can only really connect 3 LEDs in series with a small current limiting resistor.

so i need a 300v dc adapter? with how much ampere?

-Max- will tell you 25ma and it will work But


Because when you disconnect any led from another led, as you certainly will positioning the string you will immediately have a lethal 300 vdc arcing to your hands and through your chest stopping (called fibrillating) your heart to room temperature.

Sorry redtulip, but even 48vdc may be too dangerous for you...

I now recommend you buy this 19 vdc adapter for only $4


and wire twenty strings of 5 leds in series with a 120 ohm 1/4W resistor each as it will be much safer for you..

Keep in mind the led strings are called Parallel circuit strings...

And every parallel string is using 25ma...

So a 300 volt Dangerous LED single string uses only 25ma...

The 48 volt seven parallel strings uses 7x25 => 175ma...

The 24 volt twenty parallel strings uses 20x25 => 500ma...

Icing already beat me to it lol! Like he said, 300V is quite a lot of volts, and you can seriously hurt yourself. Find ElectroBOOM on youtube, he has a LOT of videos about high voltage and it's dangers. If you learn about the dangers involved, and how to be safe, and respect the power, then it is not a big deal.


HOWEVER, to avoid dealing with a ballast that can potentially kill you, I recommend instead wiring the LEDs in a series-parallel matrix. That way you can use your 12V power supply to power 100 strings of LEDs in parallel. (what you would need to do is wire 3 LEDs in series, and add a 60 ohm resistor in series with them, after making 100 of those, wire them all in series. You will have some significant losses in all the resistors, but with a 12V that is the best you can do if you want to drive the LEDS as bright as possible.)

I think you may benefit from watching my tutorials on Volts, Ohms, Amps, and Watts, and Series and Parallel circuits: I even cover lots of practical examples in both videos, including LEDs!

The only way to run that many LEDs on a 12V 2A supply is to run them in series parallel array. You'll need to break them up into groups of 4 LEDs wired in series with a 1 ohm resistor wired in series with each group. Then wire all 25 groups together in Parallel. If you need them in a long string you'll have to get creative with the wires and lengths so you can line the groups up in a line.