Run a string of LEDs from the mains

[mackrackit]

Now for a dumb sounding question. How do i run the LEDs at 30mA instead of 50mA. Im lead to believe that if i use, for example, 5 LEDs on 12V instead of 4 then obviously the voltage drops but the current is supposed to drop too.

No dumb questions

Some of the confusion might go back to folks calling the resistor a current limiting device. It is and is not...

In this case look at it as a voltage limiter. But being volts and amps kinda go together...

Look at the math Ioannis gave you.

1. Have a 4x1N4004 or 4007 diodes as a rectifier, or even better a bridge rectifier (4 pins) for 500Volts/2-5Amps.

2. On the AC line, fuse and a small resistor for the in-rush currents, say 2-5 ohms.

3. After rectification a capacitor of 220uF/500 Volts.

4. Calculate maximum voltage as 250 Vac x 1,41=353 Volts DC on te capacitor.

5. Calculate the total drop of LEDs say 332.5 Volts. Calculate the number of LEDs 332.5/3,5= 95 LEDs.

6. Calculate the resistor to be in series as (353 - 332.5)/.03 = 680 Ohms

7. Calculate the power of the resistor .03 x .03 x 680= 620mW. I would go x4 to set it at 2,5Watts

6. Calculate the resistor to be in series as (353 - 332.5)/.03 = 680 Ohms
Change .03 to .05 and the result is 410~ Ohms.

So lets say you are running 5 LEDs from 12 volts and each LED uses 3.5 volts and you want to run at 0.03 amps.
Well guess what... 5 LEDs at that voltage adds up to 17.5 volts.
4 LEDs? = 14 volts...
Do not need to worry here. Could be a bit dim...

3 LEDs? = 10.5 volts. Yup, now we have a problem. There is 1.5 volts to much.
12 - 10.5 = 1.5
1.5 / 0.03 = 50 Ohm resistor.
1.5 / 0.05 = 30 Ohm resistor.

[timmers]

... or use a constant current regulator.

Leds in series with a current sense resistor at the bottom. Regulator will up the supply voltage until enough current flows through the resistor to generate the devices feedback voltage. As the Leds age they go dim because their resistance increases. No problem, regulator will up the voltage to compensate giving you a constant brightness.
Connect the enable function to a PWM output, and hey presto PWM dimming!

[The Master]

Ioannis: I am trying to take all of this in but i dont understand all of it.

These LEDs do work fine if i put 4 in series on 12V (3V per LED). Thats the min value specified in the datasheet. They are pleanty bright enough at that.

Sorry but i dont understand what that pic means.

mackrackit: This confuses me a bit. you say theres no need to worry with 5 LEDs on 12V but they could be a bit dim. Are you saying theres no need for any resistor? I wouldnt normally use a resistor in this case anyway.

I may have misunderstood but i think some previous posts said that 80 LEDs (3V each at 240V) would blow up because of the current. If i use a transformer that supplies 12V 6A then isnt this the same problem?