How to power LED with 120 volt A/C supply
How to power LED with 120 volt A/C supply
(OP)
I am looking for a way to power a LED from a 120 volt A/C source. I have seen some suggestions that it could be done with a resistor only and some saying a diode is needed as well. I am using a 2.2v max V-F, and 35mA max I-F LED and want a simple "power on" indicator light from the 120 volt A/C line. Do I need a diode/resistor on both the anode and cathode of the LED? I tried a suggestion I read on this site that said a 27-47k resistor would work by it self, but it pops the resistor and LED instantly. Any suggestions would be greatly appreciated
Thanks, ShadyRaider
Thanks, ShadyRaider





RE: How to power LED with 120 volt A/C supply
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Sometimes I only open my mouth to swap feet...
RE: How to power LED with 120 volt A/C supply
RE: How to power LED with 120 volt A/C supply
a diode bridge. The LED is in the diagonal.
Alternative : .47 uF 600V is series with the antiparallel
combination of the LED and one diode.
Since the cap costs much more and is much larger,
I suggest the first.
Don't forget to add a fuse !!! It should be a
50 to 100 mA fast acting.
For diode 1N4001 would be a good choice. I would add a R=100 Ohm, 1/4W to protect the diodes in the case of short.
Here is the wire list( a=anode, c= cathode, 1,2=one or
other end)
Parts: D1 to D4, Fuse,Cap,Led,Resistor, ACline.
AC.1---R.1
R.2---F.1
F.2---C.1
C.2---D1.a---D2.c
D1.c---L.a ---D3.c
D2.a---L.c---D4.a
AC.2---D3.a---D4.c
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Please read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: How to power LED with 120 volt A/C supply
RE: How to power LED with 120 volt A/C supply
My power strip neons all flicker if there is ambient light on them and are OFF in the dark.
Oh oh! And the one I plugged into an outlet, without a dropping resistor, with my fingers, during an high school electronics class lecture on, "Do nothing without checking with me!", by the teacher, exploded in my fingers on the word "me". Which leads me to disagree with the indestructibility comment, though properly applied, is probably true,(if you mean the glass envelope).
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to power LED with 120 volt A/C supply
Just what was the value of that resistor? This is a good reminder that a neophyte can take an engineering tip and consider it complete engineering advice. We all tend to assume that the readers knowledge level is sufficient to know appropriate values and wattages. 27-47K can be interpreted as 27 ohms to 47K ohms, 1/8W resistors are as good as anything, and just any old capacitor can be placed across line voltage. I'm fond of capacitors, but it should be UL X2 line rated with a 220 ohm resistor to absorb some spikes. Provide adequate spacing for any component failure.
RE: How to power LED with 120 volt A/C supply
RE: How to power LED with 120 volt A/C supply
Dan - Owner

http://www.Hi-TecDesigns.com
RE: How to power LED with 120 volt A/C supply
I was called in to determine why a dozen control boards and then the replacement boards went up in smoke at a water treatment plant. These used a dropping resistor to power a couple LEDs. A 2W power resistor was burning a hole in the board and cracking. From the pieces I was able to determine the installed resistor was between 10 and 50 ohms. The correct resistance value was supposed to be 1,000 times higher. Those color codes are getting real tough to read! Factory rep swore no one else was having a problem. The correct resistor got too hot for the board anyway, I replaced it with a cap. Interesting enough, none of the LEDs ever burnt out although it did take out a couple of control transformers. There was another little 1/4W (maybe 220 ohm)that protected them and also burnt up.
RE: How to power LED with 120 volt A/C supply
The cap is in series, not parallel. In the circuit I suggested both the cap and the resistor are in series with
the diode bridge.
You can replace the diode bridge with an antiparallel
diode but this circuit uses only half of the current,
the other half is shorted, co you need twice larger C
for the same current.
Yes, antiparallel is oarallel but backwards ( Anode to
cathode so the arrows of the diode and LED point in the opposite direction.
The R in series with the C limits the current if the
C is shorted or when the circuit is initially connected
to the AC, to protect the diodes and give time to the fuse to responde.
----------------------------
Please read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: How to power LED with 120 volt A/C supply
If you want to do it 'by the book', then use an approved, frame-mounted low voltage power supply and a series resistor.
RE: How to power LED with 120 volt A/C supply
RE: How to power LED with 120 volt A/C supply
I hope you used a big enough resister - probably a 1 watt minimum. I calc your circuit is about 5 mA, so the resister is dropping 120v * .005 = .6 watt. If you were using a smaller resister and running the lead at the usual brightness and current (20ma) then you'd drop 120v * .02 = 2.4 watts. That's probably a good way to fry your circuit board. Is that why you are keeping the led so dim?
Because this circuit is so wildly inefficient (the power used by the led is less than 1% of the total) I'm very interested in nbucska's post. I understand the way the cap is limiting the current, but how do I figure it for different leds and different currents. I have a Luxeon 350mA and another Luxeon K2 that's 750mA. I'm really ignorant about A/C. What rules and formulae apply? If I knew their names, I could look them up.
I also love using three more $0.03 diodes to cut the power used in half via the bridge and lower the capacitor value and cost at the same time, probably enough to save money overall. I question the fuse, though. Doesn't the resister in series with the fuse perform the same function? If you were designing it that way, would you use a normal resister or the kind that don't burn up? Could you depend upon it failing gracefully and thereby save a component or would you be risking a fire?
As my handle shows, I'm a computer guy out of his sweet spot when things are not digital.
RE: How to power LED with 120 volt A/C supply
The suggestion of the fuse is obvious after reading these posts. A fuse is a good idea for someone that does not have even a basic understanding of electronics. A resistor of several hundred ohms should always be used in series with a capacitor to limit inrush current. Consider LED driver ICs, the HV9923(50mA)series is one . The new crop of LED regulators are great for powering wide voltage range electronics. I have sen these used for AC-DC operation from 24V to 250V by replacing the LED with a zener.
RE: How to power LED with 120 volt A/C supply
The impedance of a capacitor C is 1/j(2*pi*C*f) where the frequency "F" is in hertz. The "j" is electrical engineering for the square root of negative 1. We don't use "i" becuse we'd confuse it for current.
Plug in the numbers for an impedance of 12000 (it will be reactive, but for this type of thing the effect is the same) and you get nbucska's value 0.22 uF.
So for your 750 mA Luxeon you'll need about 15 to 20 uF. This will have to be non-polarized (no electrolytic caps) of an appropriate voltage rating, X2 rated for safety, fused, series resistor to limit the effect of line transients...The bottom line is that the cap trick works well at the night light level, but you may have difficulty scaling it up.
RE: How to power LED with 120 volt A/C supply
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Please read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: How to power LED with 120 volt A/C supply
Thank you all for helping me understand this, and pointing out why this doesn't scale as far as I was thinking of taking it. I WAS thinking of the Luxeon LEDs as night lights equivalent to the $4 mini fluorescent ones but using much less than their 3 watts and lasting effectively forever without mercury problems at disposal. However, a 20uF 400V polyester cap is over $10 even in some quantity so it isn't the right solution. Smaller caps are cheaper, so perhaps 300mA seems to be the edge right now for this technique depending upon the balance between price, power requirements, heat, size, and quantity. I mean, one $15 cap to use with my one $10 LED isn't so outrageous but I wouldn't want to build and sell night lights engineered like this. I have more enjoyable ways to lose my money.
Now that I know to look for "impedance", I found a great tutorial at http://www.kpsec.freeuk.com/imped.htm that clears things up for me. JimKirk's "square root of negative 1" threw me a bit until I looked at the web site and it triggered enough memories of the plotting of Capacitive reactance and Inductive reactance.
Thanks again, everyone.
RE: How to power LED with 120 volt A/C supply
The dione can't be in series with the cap. The current
in the two halfperiods MUST be the same, otherwise
the DC -- resulting from the unballance -- charges up
the cap and the current stops.
----------------------------
Please read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: How to power LED with 120 volt A/C supply
I also took VE1BLL's suggestion and bought a cheap night light. It turns itself off when there's light, so the circuit isn't too simple, but it is obvious to see it is powered by a 474K 250T cap (.47 uF?) in parallel with a 1M 1/8 Watt resistor and in series with a 200 ohm 1/4 watt resistor, just as some on this thread suggested.
The rest of the circuit has 5 1N4007's, one small signal or zener or something, the photocell, three white LEDs in series, an SCR, and three more resistors including a 1 watt that's high temperature and mounted away from the board. It LOOKS like it turns "off" by shunting the power to the resistor so that any power savings are imaginary. I don't understand the circuit yet, so I just might be paranoid.
RE: How to power LED with 120 volt A/C supply
For some reason I find that discovery to be hilarious. Thanks for sharing.