AC powered LEDs?

[qtommer]

i've noticed that some power strips have LEDs in them directly powered from the LIVE. I see resistors in the power strips as well so I guess their purpose is to limit the AC current into the LED? But how does AC drive the LED? Is it left due fact of the 60Hz too fast for the eye that it is on all the time concept or is there some sort of rectification? Thanks=)

 

[audioguru]

My Christmas tree lights are LEDs and are powered from the mains. They are in series and either use a rectifier or have two strings in parallel with the polarity of one string reversed. I don't see anything that limits the current. Since my mains is 60Hz then I see them flicker when I move my eyes.

 

[qtommer]

hi thanks for the reply.

If nothing limits the current in your lights, I guess the LEDs are specially rated LEDs ? The LED I saw in the power strip is similar to the conventional 20mA/1.8V kind.

 

[DerStrom8]

hi thanks for the reply.

If nothing limits the current in your lights, I guess the LEDs are specially rated LEDs ?

I would assume that segments of the LED Christmas lights are wired in series, so the voltage and current is divided up among the LEDs. Is this right?

 

[qtommer]

if the lights are in series, the current would be the same but the voltage divided...si?

 

[crutschow]

To reduce flicker you could use a bridge rectifier to power the LEDs. That increases the flicker frequency to 120Hz and also gives more output light per bulb for a given peak current. You would, of course, need an appropriate series resistor to limit the current in the LEDs.

 

[audioguru]

My electricity is 117V to 123V so with many LEDs in series their forward voltages will average to be about the peak voltage (170VDC) of 120VAC. Then a current-limiting resistor is not needed.

The LED in a power strip or in a night light has its current limited with a capacitor in series with it.

 

[Diver300]

To reduce flicker you could use a bridge rectifier to power the LEDs. That increases the flicker frequency to 120Hz and also gives more output light per bulb for a given peak current. You would, of course, need an appropriate series resistor to limit the current in the LEDs.

A small capacitor with the right voltage rating will get rid of the flicker if you have a bridge rectifier. You don't need to keep the LEDs anywhere near full brightness each half cycle to make the flicker impossible to see. You can certainly use a capacitor 1/10th the size of one that you would need if you were designing a power supply with that voltage and current rating.

On one set of lights, they contained a bridge rectifier but I could still see the flicker. I couldn't open up the lights, but I could open up the power supply. I added a rectifier inside the power supply, along with a small capacitor. The rectifier in the lights was no longer needed but I can't get it out and it doesn't do any harm.

 

[qtommer]

awesome! good to have experts around=)
thank you all for the replies. I have my bearings now on the topic. Thanks!=)