How do i power leds from a 240v ac line?
Answer:
Use a transformer to step the AC voltage down to something manageable such as a standard 6.3 VAC, rectify it to get DC, filter it through a capcitor to eliminate ripple and then put a resistor in series with the LED. The value of the resistor will depend on the particular LED in question as specs vary. For that matter, there are LEDs that can operate on 12 or 15 VDC, so the transformer may need to be adjusted accordingly.
You don't use a transformer..
Those Christmas light chains you can now buy also don't use a transformer.
You need to know how a LED works, it doesn't behave like a incandescent light bulb.
1) It is a diode, as such it already uses only half of the sine wave of the AC. But you can add a 250V >/=50mA bridge rectifier to achieve true DC. You end up with Plus and Minus 250VDC.
2) Because it is a diode, its polarity is important, Anode to plus, Cathode to minus.
3) You do not need a capacitor to smoothen the DC, your eyes will do that job.
4) You do need to limit the current in LEDs with a resistor, they don't have a filament, hence they do not have an internal resistance to limit the current.
5) You connect the LEDs in series, but watch the polarity, Anode to Plus, Cathode to the Anode of the next LED, that Cathode to the Anode of the next LED, and so on.
LEDs come in various specification, but the most common ones use 3Volt and need a current of 25mA.
Here comes the math:
Say you use 20 LEDs in series, then those "eat-up" 20X3=60 Volt. So you still have to "kill" the remaining 240V-60V=180V with a resistor. R=U/I R=180/0.025=7.2kOhm
5Watt.
If you use 80 LEDs in series, you have 80X3=240 Volt, and you don't need a resistor. This is what those Christmas light chains would do.
What Marianna said is correct, particularly for a string of LEDs, but to that I would add that if you use the max current rating of the LEDs as your design current, at least some of the LEDs are likely to have a finite life. Most are almost as bright at 2/3s to maybe half current as they are at full rated current, and if ran in that range, may have a virtually infinite life barring high temperture environment. So I would initially use a resistor that would limit the current to 1/2 the LED rating, if the brightness was sufficient, I'd leave it, if not, then parallel the resistor with another one of 1/2 the value and wattage of the first (yielding 3/4 current). You can add one more like the second resistor in parallel to get full rated current for comparision, or if you need absolute max brightness without regard for LED life expectancy.
But if you are only going to have a string of fewer than about 40, I would put a high voltage 1-amp diode in series (like a 1N4007) instead of a bridge, thus cutting your supply voltage in half, which will have the effect of cutting the wattage requirement of your dropping resistor.
Correct, you do not need any caps, and you do not need a transformer unless isolation is desired, as in outdoor use of an unenclosed string. You did not give much info about your application, could you please elaborate?
You will use a resistor and diode only. Assuming that LED having 0.3 V drop and the diode having 0.7 V drop. So the resistor will able to drop the rest of the supply voltage.
finding the resistance value,
Vpeak = sqrt(2) x Vrms
Vpeak = sqrt(2) x 220
Vpeak = 311 V
This is the maximum value of the rectified supply voltage.
The total load drop is only 1 V. So the reistor must drop the remaining 310 V. Assuming that you will use 1/4 W resistor so,
R = V^2/P
R = 310^2 / 0.25
R = 384 K ohm
To avoid the heating of the resistor quickly so used around 400 K ohm.
try their suggestions, but as for me, i have a circuit diagram for a 110volts ac powering one led, i used a 300ohm, 1/2 w resistor in series with a 0.47 microfarad capacitor in series with a 1N4004, 1 amp diode, anode is in the end, i paralleled the led at 1n4004 diode, cathode connected to diode's anode and the anode of LED is connected to cathode of diode, i measured the current at LED to be 12 milliamps.
my second circuit is a little bit complicated, but it detects from 120 volt ac to 240 volts ac, and two LED's were used, i'd like to draw diagram but there are lots of parts, this one is useful for power lines and a transformerless circuitry, see this circuit at www.redcircuits.com/page110.ht...
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