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How many LEDs i can connect to 240v AC supply using resistance? Answered

hello everyone,
I just want to figure out how many 5mm super bright blue LEDs of  VF 3V , IF 25mA  i can connect directly to our 240v AC supply in series, only using a resistance in between.

I google some circuits what i got :-

1) Android App named "ElectroDroid" can calculate Resistance for my 240v ac for 76 or 77 or 78 or 79 LEDs in series as 480 or 360 or 240 or 120 ohms respectively. I attached the screenshot with this question..

2) A link http://led.linear1.org/led.wiz can calculate also but highest amount of supply voltage can be used 170V  and number of LEDs i can use is 56 in series with 82 ohm resistance in it.

so i think option 1 can be more useful because i dont want to use extra thing ( transformer i think ) in my LED string to step down voltage in series.

OR if anyone can simply tell me how many of LEDs 5mm blue VF 3 IF 25mA  i can use in a series directly to 240V AC supply with less complicated circuit



3 years ago

Ok, I will just point out a few things that you will need to know if you plan on making this:

1) Dealing with mains voltage can be really dangerous if you do not know what you are doing. Research some high voltage safety procedures and follow them religiously. Make sure to use protection like low current fast acting fuses and such for safety.

2) AC power can be quite a bit more complicated than DC, and the calculations can get pretty hairy. 240V AC refers to the RMS voltage, not the peak to peak voltage of the AC. RMS stands for Root Mean Square, and it is just a special type of average. Like the average voltage of the AC is 240V, but the peak voltage is more like 340V.

3) Resistors can be quite lossy, but since you want to use AC mains, you can actually avoid using them and instead use capacitors to limit the current! But you do need to first calculate what resistance you will need, and then figure out what capacitor value has the same capacitive reactance at the mains frequency (50Hz or 60Hz?)

4) There are lots of things VERY wrong with that first picture with the electrodroid app, I do not know how you managed to make it show that you need a 120 ohm resistor when the voltage drop across the resistor needs to be 237V!

5) When you connect LEDs in series, the diode voltage drops add up. If you have 3 LEDs in series, and each diode has a forward voltage of 3V, then the resulting series will need 9V. If you connect 100 LEDs all in series, the resulting LED series will need 300V, so you can pretend that you have one giant LED with a voltage drop of 300V in the electrodroid app, then calculate the resistor needed with that!

6) I HIGHLY recommend you AVOID using these stupid calculators for the very reason that they make assumptions that may not be true, and, like the screenshot of electrodroid shows, for whatever reason, the result is REALLY super wrong!!!


Answer 3 years ago

So here is how you should go about the formulas and math to figure this out. Dont worry, it is not rocket science!

First, remember that 240V ac RMS is actually sqrt(2) times less than the peak voltage, which happens to be about 339V.

Next, remember that LEDs do not like reverse voltage, there is a maximum reverse voltage on LEDs, which is generally 5V. Since AC constantly reverses direction, you do not want the negative half to kill all the LEDs, so you need at least 68 LEDs in series, because 68*5v = 340V. But what about the forward voltage, which is only 3V?

Next, we need to figure out how many LEDs we can power in the forward direction. The absolute maximum current you would want the LEDs to draw is 25mA. You say the LEDs will draw that much current when the voltage across them is 3V, so 113 LEDs in series will draw 25mA exactly at the very peak of the positive half of the mains cycle. Technically no resistor would be needed, but that would be bad practice. The LED voltage drop (voltage-current curve) changes with temperature, LED life, type, etc etc etc. 100 is a nice round even number, so let's go with that!

So now, with a peak voltage of 339V, and a LED drop voltage of 3V*100leds = 300V, we need to get rid of as much as 39V, and we don't want our LEDs to ever draw more than 25mA, so now just use ohms law! V/I = R! 39V / 0.025A = 1560 ohms, or 1.56k.

BUT if you plan on using a resistor, be aware that it is going to be dissipating potentially a lot of power. if we constantly had 39V across it with 25mA going through the resistor, we would be dissipating 39v*0.025mA = almost 1W! however, we only see this maximum voltage and current at the very peak of the waveform, not constantly. The calculation to really figure out what resistor size you will need will probably require some integral calculus, IDK of any simple formulas.

However, if you want greater efficiency, take some time and calculate what capacitor you will need to achieve the same "capacitive reactance." There is a formula you can use, XC=1/(2πfC), or you could use electrodroid for this. Rearrange the formula to solve for C instead of Xc, because we know that Xc should be about 1560 ohms. C should be about 1.5-2 microfarads, depending on your mains frequency.

But MAKE SURE YOU ONLY USE HIGH VOLTAGE 1000V FILM or MKP CAPACITORS AND A 100mA FUSE IN SERIES! The reason why you want to use such a high voltage capacitor is both for engineering margin, but also because there is a phase shift across the capacitor. If that phase shift, and the voltage on one side of it will be different than on the other side of it. So you want to make sure that the maximum instantaneous voltage across it never gets close to the maximum rating. Also because capacitors can fail short-circuit, you want some fuse protection so that the fuse burns out, and not your house.


3 years ago

yeah it's basically the same question that you asked last time around; it's just not safe. (like the calculator says)

Besides, if one burns out then the whole string will go out leaving you checking nearly 100 leds to find the bad one.

**It would really help** if you told us what you are trying to accomplish with this. then we can all offer some different solutions that may just work out for you !

like will they be arranged in a strip like 12V led strips?

you looking for a fiber optic flower like whats in the bottom of this thing?

maybe lighting a garden path so you can see at night?

If you give us some more details; You may just get a reasonable solution :)


Answer 3 years ago

look at the pic i just uploaded with the message..i want to make such strings of leds for my house..These are ready-made and not of leds what i am using for now...


Answer 3 years ago

Perfect! Now we have something to go on !

you can use some simple 12V led strips that can change the color of the entire strip, Check the link in my last post.

Or if you wanna a more Elaborate setup, you can use Neo-pixles and change each led to a different color and even animate them ! you just need a controller like this

Is that what you were thinking of? Because this is much simpler and quicker to put together then soldering the lights your self :)


Answer 3 years ago

Btw make sure the controllers are compatible with the lights before you buy. Just ask the seller if the controller is good to go before you buy it.


3 years ago

I'm sorry to say, anyone who asks such questions shows that she/he should connect exactly zero (0) LEDs to 240V AC. In fact it shows that you don't have the formal training (technical/engineering degree) - which may be ignored, nor the knowledge to connect ANYTHING to 240V AC.

Do you know about the regulations to handle mains power in your country? Regarding your question, no. I mean, if you only kill yourself - hey, that's your problem. If you hurt/kill someone of your family, a friend, a customer, start a fire ... that means putting other people in trouble.

Get a decent power supply, do your LED experiments with that and learn more. Once you learned enough to know why this question was wrong, you will know enough not to have to ask it.