Tuesday, February 27, 2018

AC Line Powered LED

From what I've seen of AC line powered LED circuits, they tend to be pretty elaborate, and they always include a rectifier diode in reverse parallel with the LED, to limit the reverse voltage that the LED will be subjected to. Then I came across this receptacle tripler in my home with an AC line powered LED in it, and thought I'd look into how the LED was incorporated. What I found was the simplest circuit imaginable.

There's no manufacturer's name on it, but embossed on the back of it is "MODEL: CT3-1V". Also embossed is the CSA (Canadian Standards Association) logo. Two M3 threading screws fasten it together, so it was easy to open for an examination of its innards. Here's a view of the interior.

And here's the schematic that I worked out for it.

Ignore the thermal fuse and the MOV for now, and note that the LED with a current limiting resistor is directly across the AC line. There's no reverse diode across the LED. Everything I've read about powering LEDs from the AC line says that you're not supposed to do it that way, but there it is and it works and it has the CSA's blessing.

So, the LED's reverse breakdown voltage must be up to the task; that's the only explanation. And looking into the matter a bit, that's what I found. If you google "led reverse voltage", you'll find some threads that  reveal that led reverse breakdown voltage is commonly well in excess of the 5V figure that most LED data sheets give.

So there you have it -- powering an LED directly from the AC line is actually a cinch. All that's needed is an appropriate value of current limiting resistor.

Notes On The Schematic
  • The MOV (Metal Oxide Varistor) provides transient ('surge') suppression. Power bars that claim 'surge suppression' all have an MOV in them. That's what their surge suppression consists of -- a single MOV.
  • The thermal fuse is there to protect against catastrophic MOV failure. A huge overvoltage surge could conceivably cause an MOV to flame out, hence the nearby thermal fuse.
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