I have designed a dead simple LED circuit I calculated the required resistance to be (220-2)/0.02 = ~11k so I used 10k+1k resistors in series but for some reason, the 10K resistor started smoking in like 10 seconds although I estimated the power usage to be 2 V*0.02 A = 0.04 W which is way below the rated power of my resistor.
On the second circuit, I used 4 47k resistors in parallel so I get a resistance around 12k this time the resistors did not burn but still got very hot.
I can't understand what I'm missing or I don't know exactly.
AC LED Circuit – Resistor in AC LED Circuit Gets Fried
acheatledresistors
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Best Answer
First: Your power calculation isn't correct. 2V*0.02a = 0.04w is the calculation if you had 2 volts across the resistor. You don't. You have approximately1 220 - 2 = 218 V. The power dissipation is hence roughly 4.4 W if conducting throughout the AC cycle.
Additionally: LEDs aren't intended to withstand a large reverse voltage. On the negative cycle of the waveform, the LED initially has no current through it, so there is no voltage drop across the resistors. This places up to 220 V * sqrt(2) = 311 V of reverse voltage across the LED, destroying it almost instantly (they are designed to handle around 5 V or so of reverse voltage).
You'll can add a shunt diode which will also protect the LED. It can also be a second LED, in which case you twice the light - one lights on one half-cycle, one lights on the other half-cycle. Of course, twice the resistor current means twice the resistor power dissipation.
Additionally, be very careful with mains circuits in general as they can be extremely dangerous to you and your home (as they can electrocute, start fires, etc). Based on the mistake you're currently making here, I strongly suggest that you practice with battery powered circuits instead - while you may still destroy components, you do not risk deadly electric shocks or house fires. Thankfully, this mistake did not lead to a fire, although it had a risk of doing so (unless you use special flame-proof resistors which are guaranteed not to burn down your home if overloaded).
A much deeper and more accurate understanding is necessary to safely work with 220 VAC (or 110 VAC).
1 Approximate, because in reality 220 is Vrms and we're subtracting an offset of 2 V.