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running LED strip on ac

So i started working on a new project with RGB LED strips and I needed a power source, I dug around my box full of wall warts and saw one that said 12v 3000mA and thought ''perfect!''.
I never hit the wires with my multimeter, just used a spare chunk of led strip to figure out polarity.. Anyway, plugged a line into my strip and into my LM7805 and blew my voltage regulator, only then did I look at the power supply and notice it was 12v AC!
So to my question (sorry for the long winded intro) How are my LEDs working with AC power?? I switched the leads and they turn on with either line attached, so its obviously alternating, but how could these turn on at all?

They can work with AC. They are just turning on and off rapidly. If your in the US then they are turning on and off 60 times a second. This is plenty to make them look like they are constantly on. LEDs are diodes. So when the current is flowing in one direction they light up, when it goes back the other direction then they turn off.

The best way to fade LEDs is using a PWM signal. Basically turning the power on and off rapidly. The slower the frequency the dimmer they get. The higher the frequency the brighter they get. Once they are turning on and off about 30 times a second you've surpassed what the human eye can distinguish so they look like they are at full brightness.