connecting LEDS to 12vac

[aznknightzz]

i want to connect 3 white LEDS into a 12v ac power supply, how would i do it? Would it be the same as connecting them to a 12v dc power supply?

 

[John Sorensen]

Series would be the most efficient way (I'm assuming you want all three to come on together with the 12VAC, and have equal brightness), since there would be less losses in the resistance. Measure the Vf of the LEDs; it is probably around 3V each. Then, using RMS voltages, R=(12-3*Vf)/I, so if the current desired is 10mA (for example), R=300 ohms.

Here's the trick: you need to put a diode in anti-parallel with the LEDs to keep their reverse voltage from being exceeded. This is expecially important for white and blue LEDs, which are more sensitive than green, red or amber. The power dissipated in the resistor on the half cycle the reverse diode will be conducting will be much greater than when the LEDs are on, since less voltage will be dropped across the single diode, the Vf of which will be only 0.7V, so you'll need to size the resistor wattage appropriately.

The alternative would be to use FOUR white LEDs, two each in anti-parallel. Now THAT would be sweeeeeet.

j.

 

[aznknightzz]

Thanks for answering my questions so quickly. If possible, could you draw a simple schematic? I still don't know where the diode(s) go.

 

[niq_ro]

schematic :wink:

 

[John Sorensen]

You know, I think I might do it like this (attached). It limits the reverse voltage across the reverse biased LEDs. The three LEDs in series won't work as well, I don't think, because the Vf of a white LED (the one I looked up) is like 3.6V, so 3*Vf=10.8. The peak currents through the LEDs will have to be higher since three LEDs will conduct for less of the half cycle than two LEDs.

j.

 

[aznknightzz]

Thanks Nig-ro and John, that was very helpful, but isn't there suppose to be a diode somewhere in the circuit?

 

[kitedude]

theres 4 diodes in the picture.

LED= light emitting diode

 

[John Sorensen]

If you do the four led design, then the LEDs that are in antiparallel are performing the function of the diode I was talking about before: limiting the reverse voltage across the reverse biased LEDs, with the added benefit of giving off light :? .

j.

 

[niq_ro]

3 LEDs.. a diode and a resistor :wink: