20th May 2009 11:27 PM
A few questions about LEDs
Hi, my first post on the forum will be to ask about LEDs. I thought I knew all there was to know about LEDs until today I was fooling around in after hours with a LED circuit fed directly from my 220 VAC mains line and observed a few things I'd never known.
1) I fed a LED from the 223 VAC RMS mains line in series with 2 10k resistors. I plugged it in and checked the voltage across the LED terminals and it was 76 VAC, the LED was cool, light was moderate, resistors were warm. Left on for over a minute. How can this voltage be possible and not blow the LED?
2) I fed the same circuit on 122 VAC RMS and voltage drop across the LED was now 60V. If the proportion 122/223 was maintained, the voltage should be 41.5 V across the LED. How is a nonlinear circuit possible with a LED? Again, 60V and the LED was cool, resistor warm to the touch.
3) So I exaggerated he resistor adding a 2M2 resistor, connected to a 223 VAC line. The LED didn't light up, but surprisingly the voltage drop across the LED was still 60VAC
Other resistor values tested:
110K VLED 66V
10K VLED 84V
Any ideas why the LED stays intact? Why is the LED voltage drop nonlinear?
Thanks in advance for any pointers.
21st May 2009 12:49 AM
Are you sure those were LEDs?
Looks to me as if they were neon lamps.
Last edited by Boncuk; 21st May 2009 at 12:50 AM.
21st May 2009 01:59 AM
21st May 2009 02:52 AM
Have you tried another meter?
21st May 2009 03:04 AM
Forgive my ignorance here but the LED is in essence a diode in this circuit. You have your meter set to measure AC. The LED should result in half wave rectified DC if I'm not mistaken. Therefore the your DMM reading is twice what it should be, I think. I'm only a hobbyist so someone with more experience will need to chime in.
21st May 2009 03:15 AM
Hi Jim, thanks for the idea. If we had some other load after the LED, we'd have half rectified wave, yes. In this case the LED is the load and is consuming 1/2 wave. Tried it though, it reads 64.5 V on the DC scale.
Originally Posted by jimlovell777
Hayato: did not try another meter, I only have this one, but other measurements seem right...
21st May 2009 03:23 AM
(230V - 3V) / 20kOhm = 10mA This is why the led is only dim.
My theory for the Voltage across the LED is that it is in fact the multimeter that is taking an average of the DC/AC components, because as previously stated your circuit is a half bridge. The led is only on half the time which makes the 10mA seem like 5mA. When a diode is reverse biased it sees the whole 223 volts or 122V. Perhaps the multimeter is showing some weird average. The 50-60Hz of half the wave must be messing with the calculations.
Try this, another led in parallel with that led but hooked up in reverse with respect to this one, they should both look dim, and if you put the multimeter across the terminal now in DC mode it will maybe show 3V....just a guess from an amateur
21st May 2009 03:40 AM
Mike, you were right(as well as Jim above). At DC now I get 0.001 volts and on the AC scale I get 2.01 volts. What I was seeing was some weird average computed by the multimeter.
Originally Posted by birdman0_o
Actually, both lit up pretty brightly, the reason it seems dim on the photo was the camera flash which overwhelmed the LED brightness.
Edit: as we're now consuming the full wave, the resistors are burning hot, as expected.
Many thanks to everyone who replied.
Last edited by jmaf; 21st May 2009 at 03:42 AM.
21st May 2009 03:45 AM
The resistors are dropping about 227V at 0.01A / 2 thats a heck load of heat!!!! ~ 1.15W per resistor!!!!! The ones you are using are rated at 1/4 of a watt, be carefull.
If you want the led's at theyr maximum brightness perhaps try 4 2k2 resistors in series
Glad I could be of aid
21st May 2009 03:52 AM
Yes, will be careful if this ever becomes part of a real circuit. Thanks again.
Originally Posted by birdman0_o
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