How do you wire up a led from AC?

kyrill · « **on:** 6 Oct 2004, 07:41 pm »

If you do not have DC but only AC how can you wire a led for ON/OFF?
Or is this not possible?

ginger · « **Reply #1 on:** 6 Oct 2004, 11:30 pm »

G'day Kyrill,

The LED is forward biased and conducts and gives out light on one half cycle and is reverse biased on the other half cycle.

The problem with this is that the reverse breakdown voltage of the LED is very low. You therefore need a power diode (a 1N4007 or similar) in series with the LED to handle the reverse voltage and protect the LED on the non-conducting half cycle.

So from the Active side:
1) Current Limiting Resistor - for 230V AC a 47K will give a bright LED, 100K for less brightness
2) Other end of resistor to anode of 1N4007 power diode
3) cathode of 1N4007 to anode of the LED
4) Cathode of the LED to Neutral

Cheers,
Ginger

kyrill · « **Reply #2 on:** 7 Oct 2004, 10:03 am »

thanks Ginger

zachslc · « **Reply #3 on:** 19 Oct 2004, 07:31 pm »

There are also small neon indicators that look like LEDs. They can be wired directly to the AC. Parts Express has them as I recall.

kyrill · « **Reply #4 on:** 19 Oct 2004, 09:19 pm »

Ah zachsic

thanks for yr advice
Exactly what the man in the shop advised me to do after he inquired further why I need the leds for. I bought now two 5 mm such neon's you describe. The color is red as he didn't have any other.

Greg Erskine · « **Reply #5 on:** 19 Oct 2004, 09:49 pm »

hi kyrill,

Another solution I've seen but not tested is to add a few winds onto one of the toriods for an extra low voltage supply. This seems simple, as you would probably only need a extra resistor in series and everything is electrically isolated.

Greg

kyrill · « **Reply #6 on:** 19 Oct 2004, 09:57 pm »

hi Greg

But it would still be AC, wouldn't it?

Greg Erskine · « **Reply #7 on:** 20 Oct 2004, 03:32 am »

hi kyrill,

I thought a LED was a diode, so on low voltage it would be ok. In any case another diode would fix it. I haven't tried this myself.

Greg