noob question: 24 5v led's at 120 vac?

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noob question: 24 5v led\'s at 120 vac?

I guess the title is pretty self explanatory...

Is it possible to to run 24 5v led's in series at 120 vac? Can you run them on ac at all? Do I need a capacitor in line?

What happens if the voltage to the series burps +/- 5v?

Any one done anything like this before? pics? plans? linkage?

thanks

 

[Streak]

Re: noob question: 24 5v led\'s at 120 vac?

Hey Stinky.
Yes this can be done using a bridge rectifier and then connecting the LED's in series.
Take a look at http://hometown.aol.com/xaxo/index.html
for a possible circuit diagram.

Be careful working with mains voltage.!!

My Land Rover Horror Story

 

[**DONOTDELETE**]

Re: noob question: 24 5v led\'s at 120 vac?

streak

thanks your schematic should help me flesh out my idea.

mabey some questions?

in your diagram of the line driven led, (is this a dumb question?)
where is the rectifier? ever build this?

thanks

 

[Streak]

Re: noob question: 24 5v led\'s at 120 vac?

The four diodes make up the bridge rectifier. This is not my circuit but one that I plan on using.
I have built one using only one diode but then it was for only one LED.

 

[**DONOTDELETE**]

Re: noob question: 24 5v led\'s at 120 vac?

Hey Streak, gotta question on this for you.

I'd like to power 4 LEDs (something small and bright, of course) and use them as a sort of "cool computer room" lighting fixture, similar to those halogen light bulb contraptions you see in lighting and hardware stores.

Ideally, something like 4 individual LEDs, each hanging from a ceiling connection by just a thin pair of wires.

Any idea how I would wire such a beast, and now to power this from 120V AC?

Just curious, and wondered if you could help. Thanks, either way.

kirkhere

 

[Streak]

Re: noob question: 24 5v led\'s at 120 vac?

Hey I am no expert but I would do it this way.
Buy one of the variable voltage AC/DC adapters, 110volt in variable DC out 500mA should do. Run your two wires across the room from this adapter.

Solder a resistor to each LED, the value will depend on what voltage you choose to run the system at. At 3.5 volts you would not really need any resistor but better to use one to balance the brightness of the different LED's.
You could rate the system at say 12 volts with the appropriate resistors and then switch the adapter to lower voltages to dim the system. Probably safer that than rating it at say 6 volts and then turning the voltage up to 12 and popping the LEDs!!

Hang the leds from your wires.

Have fun.

 

[**DONOTDELETE**]

Re: noob question: 24 5v led\'s at 120 vac?

Coolio, I can handle this... thanks for giving me the starting point.

Dumb guy question... "Solder a resistor to each LED, the value will depend on what voltage you choose to run the system at." How do I determine resistor values for the LEDs I choose? Point me there and I'll go do the legwork.

Thank you, Streak!

kirkhere

 

[Streak]

Re: noob question: 24 5v led\'s at 120 vac?

Kirk,
Here is the formula

R = (Vs-Vl)/Il

Where: R = resistance
Vs = supply voltage
Vl = LED forward voltage
Il = LED forward current

Typically for a white LED,
V1=approx. 3.4 check specs of led.
I1=20mA for normal or more (say 30mA) if you want to overdrive a little.

So, for a supply of 12v you would need a series resistor of 430 ohm per LED.

Good luck.

 

[MrAl]

Re: noob question: 24 5v led\'s at 120 vac?

Hello there!


Using 24 white LED's run from 120vac source


That's a very interesting idea. If you use a bridge
rectifier like Steak was saying, you get a dc output
that will power the LED's quite well i would think.

The characteristic voltage is more like 3.6 volts though,
so i would think you would use maybe 33 LED's unless you
want to over drive them. You also have to look
at the peak current, which would be quite a bit higher then
the average current.

You also have to use a reverse connected diode to protect
the LED's from being reversed biased at a high voltage.

For lead inductance reasons, you should also connect
a capacitor with a value of about 0.01uf to 0.1uf
rated at 400v across the output of the bridge rectifer.

Connect a small value resistor (maybe 100 ohms) from the output
(+) to a diode (1N4006) cathode. Connect the anode to
the (-) rectifier output. Connect the string of LED's across
the diode with the top LED's anode connected to the cathode,
and the bottom LED's cathode connected to the anode.

One practical method might be to connect the above circuit
with a string of 33 LED's in series with a 1k ohm resistor, and
measure the current though the string using a meter. You can
measure the voltage across the 1k resistor and calculate the
current if you want to do that instead.
If you find you get much more then 20ma you can then increase the
value of the 1k resistor and try again. If you find you get much less
then 20ma you can decrease the value of the 1k resistor and try again.
Of course you should unplug the circuit when you do the changes.
You can make changes in 100 ohm increments, ie 1000, 1100, 1200 etc or
1000, 900, 800 etc. untill you get the right current level.


Precautions:

1. Make sure ALL the led's are connected correctly. If one led is
accidentally connected in reverse, this one will blow out when you
plug the circuit in.
2. Made sure the extra diode polarity is correct or it might explode
when you plug in. Use a fuse on the ac side of the rectifier also
with a max rating of 1 amp.
3. When working with 120vac there is always the danger of life threatening
shock. Always unplug when you make changes, dont trust a switch.
Some switches only cut out one line of the ac, leaving the other side
still 'hot'. Touching this and a good ground (like a radiator) could
cause death. Standing in water or on wet ground is even worse.
4. Always wear eye protection. Some electronic parts will explode
violently when connected incorrectly, and sometimes even when connected
correctly. I've seen diodes burst into hot pieces, and high power
transistors explode like fireworks sending hot molten copper and other
metals spewing out in all directions. If any of this stuff hits your
eye, your're blind for the rest of your life. Also seen caps explode
like cannons, sending their guts out into the surrounding area too.
In general, 120vac is not like playing with 9v batteries. The danger
is real and very possible.

Good luck with your LED circuits,
Al

 

[MrAl]

Re: noob question: 24 5v led\'s at 120 vac?

Hello again,

A simulation of the above circuit
was made, using 60v input and
15 led's (that would be 30 led's at 120v).
The 100 ohm resistor was used and the
1k resistor came down to 500 ohms.
The average current was around 20ma.

The led's can take a 2x voltage for a
short time, so that doesnt seem to be a
problem.

You might have to consult your local
electric company to find out the
guaranteed specs on the line voltage.
I wouldnt run the led's without
the series resistance, because a
line surge might blow them out.

Good luck with your LED circuits,
Al