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I have a home-made thingy that I would like to use as a lightning in my living room. It contains 7*4=28 LEDs (red, 1.86V, 20mA), originally in 7 groups by 4, each switched on seperately, powered by 12VDC.

Now, I would like to power the whole thing by 220VAC. I thought of using a simple stabilizer as on the following schematic:

schematic

simulate this circuit – Schematic created using CircuitLab

If the stabilizer were optimal, I would get (220-28*1.86) / 8.2k = 20.4mA, which is fine for 20mA diodes I think. However, I have some issues:

  1. Is my idea of ACDC converter correct? I have a bad feeling that the capacitor is somehow the 1st thing in the circuit and that it's not correct.

  2. How well the signal is stabilized by such layout? Especially at the beginning, there might be high currents through the bridge, can't there?

  3. Can I rely on the LEDs having moreorless correct voltage drop? It seems that 0.1V per LED would make a little difference in the current so I think it should be safe, but I'm not sure.

yo'
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  • Why not just get one of those very cheap Christmas LED light strings and use it? – jonk Mar 26 '14 at 00:39
  • @jonk Because the LEDs are soldered in a complicated way in the thingy (it is actually a home-made star gate `:D` but I thought that's irrelevant, so I didn't include that bit of information `;)` ). – yo' Mar 26 '14 at 00:41
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    It's just that what you propose presents a safety hazard unless you can completely seal up all of the contacts. Even then, there are other considerations that may present still more hazards. If there is any way you can fashion your "star gate" using a tested safe string of LEDs, that would be preferable. If you must do this and you aren't thoroughly versed in safety issues with residential 220VAC then you probably should consider a transformer for galvanic isolation and voltage reduction. Or just get a wall wart with 9VDC@250mA or so and use 7 parallel strings of 4 series-LEDs and resistors. – jonk Mar 26 '14 at 01:04
  • Most LED packages are not rated for 220VAC isolation and are therefore unsafe to use. Even when the voltage across the LED is only a couple volts, the voltage with respect to ground (or your body for that matter) may be significantly larger. I guess what I am saying is, don't be stubborn, use a transformer. – jippie Mar 26 '14 at 06:48
  • @jonk Got your advice. I'll stick with a 12V solution since that's what was used originally, or maybe 24V and powering 8 LEDs at once. Thanks a lot! (Care to make it an answer so that I can accept it?) – yo' Mar 26 '14 at 10:49
  • When one LED dies you have 220v across the dead device, a situation worth thinking about as some LED's can fail open-circuit and some can fail short-circuit. Will the string survive this? How many LED's can fail before it explodes? – John U Mar 26 '14 at 17:50
  • @jonk Thanks. I think we might purge the comments. – yo' Mar 27 '14 at 10:26

3 Answers3

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Safety is a big issue and testing this directly from 220V AC is a hazard you should avoid. That's the warning done.

Instead of wasting over 3W of heat in a 8.2kohm resistor, use a dropper capacitor before the bridge on the AC side. At 20mA LED current and with 220V ac, 50 Hz I'd estimate a capacitive reactance of about 11k8 ohms would be about right.

\$X_C = \dfrac{1}{2\pi f C}\$

Capacitance will be about 270nF. This should produce a peak current from 220V AC into a load that looks like 60V of 20mA. Here's a simulation: -

enter image description here

Input AC voltage of 50Hz and peak of 311 volts (220VRMS) drives via 270nF capacitor a bridge rectifier. A capacitor is on the output but this can be virtually any value and in fact I've just rechecked and it is not needed. A zener diode of 62 volts rating is simulating the 28 LEDs in series. An insignificant 1 ohm resistor tells me what the current is. The response shows a peak current of about 26mA and an average current of about 13mA.

The circuit linked to in the comments is unsuitable for driving strings of LEDs because of reverse bias diode breakdown problems.

MAKE SURE YOUR 270nF CAPACITOR IS X OR Y RATED AT THE APPROPRIATE SUPPLY VOLTAGE.

The diodes in the bridge I've shown as 1N4007 because I know they are rated easily in excess of the potential voltages that might be present. 500V minimum rating for these diodes I'd say.

Andy aka
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  • Just to be clear, do you suggest a 270nF cap in series on the mains? – Dzarda Mar 26 '14 at 10:07
  • @Dzarda: http://electronics.stackexchange.com/a/78130/2191 – RedGrittyBrick Mar 26 '14 at 11:12
  • @RedGrittyBrick the circuit won't work for strings of leds dude because of reverse bias problems. – Andy aka Mar 26 '14 at 11:47
  • @RedGrittyBrick: Also, the individual LEDs in that circuit flicker at 50Hz, which will probably be rather objectionable. Andy's circuit flickers at 100Hz, which is better, but the OP's circuit doesn't flicker at all. Andy, changing your C2 to something like 22uF would fix the flicker, and also bring up the average LED current. – Dave Tweed Mar 26 '14 at 12:37
  • @dave it might but an extra resistor might help., maybe 1kohm. I'll have a look in a short while. – Andy aka Mar 26 '14 at 12:49
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Wiring up a string of LEDs directly to 220VAC presents a possible shock hazard if it isn't completely sealed away. You should probably consider, for safety's sake, a transformer for galvanic isolation and voltage reduction. Or else select a suitable wall wart with 9VDC@250mA, perhaps, and use 7 parallel strings of 4 series-LEDs and resistors. DC voltages higher than about 50V are also considered to be a safety issue. So try and choose a DC voltage below that figure.

jonk
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This doesn’t look good practice to me. You simply do not connect bridge rectifiers to the mains. You need to use a transformer for isolation so you do not have your led’s directly connected to the mains. Rather than connect the led’s in series which requires quite a high voltage with that number usually 3.3V each LED I would redesign your matrix to wire banks of smaller numbers in parallel. You don’t necessarily need full bridge ratification or smoothing, if you use half wave rectification that might be sufficient depending on your transformer selection.

Brian Moreau
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