This LED works with up to 80 V AC from mains line 0

Posted In Do it yourself electronic projects,Experiments,In the Lab

Here are a few photos so folks can help me understand how this LED works perfectly with more than 80 V AC, in series with a 10k OHM resistor, directly from the mains line.

In this case I’ve tested with 2 10k resistors, dropping the voltage a further ~5 volts.

The resistor gets pretty hot. It stays only warm with 2 of these 1/4 watt 10k resistors in series. The LED stays cold…left it on for over 1 minute, nothing unusual except for the fact the voltage drop on the LED is over 80 V AC RMS.

Solution
What was I doing wrong? Well, it’s a pretty trivial mistake really but I hadn’t seen it. Every half cycle there was little or no voltage drop across the LED, so the multimeter sampled 223 V AC. Then on the conducting half cycle the resistors produced a big voltage drop, and the multimeter sampled that low voltage as well. The result was some sort of average computed by the multimeter. Unable to figure out how that LED withstood 75 V AC I went to Electro Tech Online and asked. Members of the forum were immediately on the case, offering ideas. It didn’t take long before 2 or 3 folks had it solved: place another LED in opposite polarity and the multimeter will display the correct reading. Result? 2 V – which seems like a correct reading. There I go, one more lesson learned.

A 75 V AC voltage drop across the LED