STUPID basic LED questions. XD

[Electric Rain]

Alright, this seems obvious to me, but I wanted to make sure I wasn't missing something...

LEDs must be supplied a relatively specific voltage, otherwise they need a current-limiting resistor so they don't burn up... So, say I had 60 LEDs with a forward-bias voltage of 2, and I wanted to power them from 120VAC... I think you already see where I'm going... Could I not simply use a rectifier for AC/DC conversion and just wire all of my LEDs in series? I don't understand why people always use a 12V PSU with transformers and a bunch of current-limiting resistors in massive LED-count systems. Wire a bunch of them up in series and plug them right into the wall! (Not literally... probably be best to put a rectifier in between, but you get my point.)

Also, say these LEDs draw 100mA each... and say I want 28 panels of 60, each panel being powered off of 120V. That means each panel only draws 100mA @ 120V, which means that the entire system only draws... 2.8 amps, right? That seems a little low for what I'm doing... I'm totally confused as to how any of that makes sense to me...

Edit: Actually, I guess that seems about right... but still, I didn't miss anything, right?

[nitro2k01]

You can rectify 120 V AC and wire up the LEDs in series. The reason why you shouldn't is because 120 V is risky. It could easily zap you to death, either if you touch the two conductors or if you touch one of the conductors and something that is grounded, like a heat radiator. And if you wire something wrong, the risk of starting a fire is bigger.

[timmeh87]

You will still need (pretty large) resistors in series with the LEDs, the mains supply is by no means constant, it can change voltage daily depending on demand and such and such.

120v is the RMS value of AC, if you rectify and filter it, you will be seeing closer to 160, 170v DC peak, with some massive ripple depending on filtering and your load.

And people DO do this. You can buy screw-in LED light bulbs these days.. Hm come to think of it, they might have a controller in there anyways. Apparently its way more efficient to pulse LEDS.

Hm maybe you shouldn't rectify it...

Have you seen LED Christmas lights? unrectified mains right into the LEDs

[Skyone]

Have you seen LED Christmas lights? unrectified mains right into the LEDs

Just about to mention those; bought a 60-strand the other day actually. (120/60 = 2v forward as you mentioned). Even if it were to be a bit off (100/60 = 1.66v), it wouldn't make to much of a difference in brightness.

[Mario]

Check out this Instructable on powering LEDs off 120v with only a .47uf capacitor, a 1k 1/4w resistor, and 2 LEDs. http://www.instructables.com/id/LED_Lightbulb/ It needs two, but you could wire up 30 in parallel on one half and 30 on the other.
Don't need no transformer with this design!

How and why it works:
"First to address an oversight, you need a non polarized capacitor for this project, muy importante.

Now, how does this work? We all know that to run an LED off a higher than rated voltage source we muct limit the current with a resistor. Indeed in this case we could limit the current with a resistor of value approx. 6.8K ohms, however that resistor would need to dissipate several watts!!! Not a good thing.

Since we are using an AC source we can take advantage of a property of a capacitor subjected to AC called Reactance. We can equate reactance to resistance. Calculating the reactance is a simple formula

R=1/(2*Pi*Freq*C) Solving this for C will give us the size capacitor we need to limit the current to the LED.

So why do we have a resistor at all? When the power is switched on there is an in rush of current and the 1K ohm resistor is there to limit that in rush current.

Finally, Why two LED's? Well an LED is a diode and since we are dealing with AC here we need to wire two led's in opposition so that the waveform can complete it's cycle. Essentially each LED is flickering at 60HZ but in opposite phase."