Newbe 8 leds connected to 120 volt ac

Hi, Let me start by saying that I know very little about electronics,but I would like to know how to hook up 8 white leds to 120 volt ac. Is as simple as going to Radio Shack and buying a transformer? If so what size transformer will I need to get? Thank You,Randy

Reply to
Randy2747
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Get a transformer with appropriate DC output voltage (5/9/12 anything). Then calculate resistor you have to hook up in series depending on DC output voltage and forward drop of LED which is around 1.3V. Then you are all set.

snipped-for-privacy@msn.com wrote:

120

transformer?

Reply to
aman

You probably already have an old transformer laying around... look at some of your old wall adapters, maybe an external modem supply, speakers, tear apart an old radio or some kid toy.

High intensity white LEDs are usually around 3.5V, check the package. Most important thing, make sure you limit the current going thru the LED.

Reply to
GotCoffee

120

transformer?

Hi, Randy. Quick answer -- white LEDs usually have a V(f) of 3 to 3.6V across them. Here's the Radio Shack Parts you need:

5mm White LED $5.29 Catalog #: 276-320 (8 req'd) 9V/300mA AC-to-DC Power Adapter $13.99 Catalog #: 273-1767 (1 req'd) 330 ohm 1/4W 5% Carbon Film Resistor pk/5 $0.99 Catalog #: 271-1315 (2 pk req'd)

Connect them up like this (view in fixed font or M$ Notepad):

.-------o---o---o---o---o---o---o---. | | | | | | | | | | | | | | | | | | | D| D| D| D| D| D| D| D| | V~~ V~~ V~~ V~~ V~~ V~~ V~~ V~~ | - - - - - - - - +|9VDC | | | | | | | | --- | | | | | | | | - .-. .-. .-. .-. .-. .-. .-. .-. | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | '-' '-' '-' '-' '-' '-' '-' '-' | R| R| R| R| R| R| R| R| | | | | | | | | | | | | | | | | | | '-------o---o---o---o---o---o---o---'

D = White LED R = 330 ohm, 1/4W created by Andy´s ASCII-Circuit v1.24.140803 Beta

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Have fun, and good luck Chris

Reply to
Chris

Randy,

Do you have the LEDs picked out yet? If so, do you know any of the electrical specs? Specifically you want to know the current thru the LED and the forward voltage drop of the LED.

Typically a data sheet will list a maximum current (say 60mA) and a nominal forward current (say 10-20mA) or one that they used to list other specs like brightness. LEDs are current devices and you control their brightness by how much current you allow thru them. Current control is usually done by simply selecting a proper sized resistor inline with the LED to limit current to whatever you need and below what would damage the LED. If you hook up an LED to a power source without a current limiting resistor, you will likely blow it.

LEDs have a forward voltage drop that tends to be related to color. For example, typical RED LEDs have a forward voltage drop of about 2V or so. I think White LEDs have a much larger forward voltage drop, like

3V-4.5V or there abouts. This is important because your voltage source needs to be a bit higher than this to work with the LED and will help you determine if you can hook up the 8 in series, parallel, or a combination of both. As an example, let's say your White LEDs have a forward voltage drop of 3.3V. If you took 8 and connected them in series, this would be a total forward voltage drop of 8 x 3.3V = 26.4V, so you would need a power supply of at least that, 28V being the typical common size or maybe 32V.

So instead you might hook them up in a combination of 2 or more parallel strings or a couple/few LEDs. As an example, if you used two strings in parallel of 4 LEDs, this would be a forward voltage drop of

13.2V, so a 24V power supply would work. Or 4 strings of 2 LEDs so you need a power supply of over 6.6, so a typical 9V or 12V supply would work or even a nine volt battery could be used.

Anyway, back to your original question, yes, you would be best off buying a small wall-wart type transformer from Radio Shack or somewhere like that. Something about 9Vdc or larger would probably do the trick and have a current rating of maybe 200mA or larger would be plenty. Or you might look in your junk drawer at old chargers and transformers you may already have for cell phones, radio controlled toy chargers, etc. The specs (voltage and current) should be listed on the case.

Then the other thing(s) you will need are current limiting resistors, but you first have to know what current(s) you are going to supply to the LEDs so you can calculate what values and wattages you need.

To give you an idea, again using the 3.3V drop LED as an example and say the LED is best run at 20mA and say you have a 12Vdc source.

Ohm's law is V=IR (voltage = current x resistance). Power is P=VI (power = voltage times current).

For one LED off the 12V source at 20mA you have...

R = V/I = (12-3.3) / 20x10-3 = 435 ohms. Then you just pick a standard resistor value that is in that ballpark (430, etc.).

Of course any errors I might have made above will quickly be corrected by the rest of the group. :)

PT

Reply to
PT

If you don't mind waiting two weeks for the LEDs, check out besthongkong on ebay. I purchased 50 ultra bright white LEDs for $8 + $5 shipping, 11000mcd. They work great and you can order a 1/4w resistor for each one for a penny each. The LEDs are the same specs as the rip off shacks at $5 each.

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Reply to
GotCoffee

LEDs are not the best of diodes...when the AC reverses polarity, it will be bad news. You will need to connect the LEDs in inverse parallel which means + to - and - to + so that at least one LED is lit on each half cycle of the AC line or you need to install a protection diode that essentially lops off one half cycle.

The way you connect the LED is important as well. They can all be in series or in parallel or some combination. The forward voltage drop of the LEDs all add if the LEDs are all in series. Lets say each LED drops an average of

3 volts when lit. 8 lamps in series would drop 24 volts plus the minimal ..6 volt drop of the protection diode plus the drop across your limit resistor. You wouldn't have enough voltage.

The answer is to make several smaller series strings, each with their own limit resistor and connect these in series with the protection diode.

I suggest you move this question to alt.binaries.schematics.electronic because that is where people can post a picture with the wiring.

What tools do you have for electronic work?

Reply to
Lord Garth

You could use a 2.7K, 5 watt resistor to connect the LEDs directly to the 120VAC line. Put 4 LEDs in series, and the other 4 LEDs in series the opposite way in parallel with the first. Like the drawing below if it shows up right.

(+) (-) |----LED---LED----LED---LED----| 2.7K 5 watt 120VAC-| |-----/\/\/\-------120VAC |----LED---LED----LED---LED----| (-) (+)

-Bill

Reply to
Bill Bowden

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Reply to
Lord Garth

Thank You all for writing to help me out,sorry I didnt get to check back and reply but my dad caught me surfing p*rn on his computer and he grounded me :-) LOL

I already have my leds here are the spec that came with them

DATA SPECIFICATION

5mm Reverse Voltage (VR)5V Reverse Current ( at VR=5V)
Reply to
Randy2747

Like I said I know very little about electronics but let me see if this is correct: R=E/I

9 voltage

-6.4 (3.2 volt bulb x 2 since its in series) =2.6 volts

2.6 volts/0.040 amps (.020 amps x 2 in series=.040) =65 ohms

so the colors on my resistor would be Blue,Green,Black,Gold

Is this correct and am I on the right path?

And would the 65 ohm resistor work for the 10mm leds as well because I see that the Max Continuous Forward Current is 30mA. Would this make the leds dimmer? Thanks again for your help

Rheilly Phoull wrote:

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Reply to
Randy2747

On Thu, 28 Apr 2005 10:37:56 -0700, Randy2747 top-posted:

NO. They're in series, so the current through the whole series string is still 20 mA. The current is the same through every point in a series circuit. Remember, volts are pressure, amps are rate of flow.

Just do the math again for 20 mA.

The total current through the 4 series strings (of two LEDS and R) will be the sum of the currents through the legs, or 80 mA.

Other than the resistor value, (which you should calculate as above, using the forward V of the other LEDs), your setup should give equal currents through the LEDs, which is actually what you're looking for: The dimmer LEDs are dimmer LEDs.

My Pleasure! Rich

P.S. Many people prefer bottom-posting, for continuity of the conversation for newcomers to the thread.

Reply to
Rich Grise

Yes.

No. When you wire LEDs in series, the voltage drops add up but the current is the same for all of them. 2.6/0.020 = 130 ohms.

Yes, just dimmer.

Notice that your LEDs actually specify a range of forward voltage drops, from 3.2 to 3.8 V for the 10mm LEDs. That means that the voltage drop might be anywhere in that range. If it is on the low end, 3.2 V, then the current will be perfect, because you used 3.2 V in the calculations. If it is on the high end then the current is (9 - 3.8 - 3.8)/130 = 1.4/130 = 0.011 A or 11 mA, which is maybe a bit low. You probably don't care though.

Jonathan

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Reply to
Jonathan Westhues

One day snipped-for-privacy@msn.com got dressed and committed to text

The power adaptor should be OK if you run the LED's at 20mA. Since the forward voltage of the LED's id 3.2v 3 in series would not be good for the

9v supply, so you will have to have 4 series sets of 2 LED's connected in parallel (if that makes sense ?). Each group would need its own resistor (= 4 resistors). You should work out the values of the resistors and the current dissipation yourself for yourself as a start in your experience. Folk in this NG will help if you show some indication of effort.

-- Regards ..... Rheilly Phoull

Reply to
Rheilly Phoull

In a series circuit, the current is the same through both lamps so do not multiply by 2.

130 ohms is your target value.

The power dissipated by the resistor is P=I*E ..02*2.6=.052 Watts so a 1/4 watt resistor is fine.

Reply to
Lord Garth

Okay now I have a better understanding what I am doing,I will give these two circuits a try,Thanks again for all of your help,Randy

Reply to
Randy2747

okay I have one more question about this led circuit. Can I use this sound activated switch to hook up to my leds?

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Reply to
Randy2747

Yes. The relay output is rated for 12 VDC, 10 A, and the current drawn by your LEDs is much less than that. Just wire the normally-open relay contacts in series with the power supply that you use to run your LEDs; it works just like a switch.

Jonathan

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Reply to
Jonathan Westhues

Okay I figured it would but I just wanted to make sure.Thanks a million to all of you,you all are a great bunch of people here in this group!!!!

Cheers, Randy

Reply to
Randy2747

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