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

 

[aman]

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.


Randy2...@msn.com wrote:

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

 

[GotCoffee]

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.

 

[Chris]

Randy2...@msn.com wrote:

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

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 www.tech-chat.de

Have fun, and good luck
Chris

 

[PT]

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

 

[GotCoffee]

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.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=7966592595&category=33713

 

[Bill Bowden]

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

 

Thank You all for writing to help me out,sorry I didnt get to check
back and reply but my dad caught me surfing porn 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)<=10uA
Operating Temperature-40C ~ 85C
Storage Temperature-40C ~ 100C
Lead Soldering Temperature 260C for 5 seconds (Max)
Power Dissipation 5.5 mW
Emitted Color White
Chromaticity Coordinates (20mA) X : 0.32Y : 0.31
Lens Appearance Water Clear
Absolute max Rating Pd : 120 mW IF : 20 mA
Peak IF : 120 mA
Electro-optical Date (20mA)
VF : 3.2V (Max 3.4V)
IV : 15,000 mcd
(14,000/16,000 mcd Min/Max)
Viewing Angle 15 +- 5
Life (normal usage) 80,000 - 100,000 hours
___________________________
my other leds I ordered are:

20 x 18000mcd SUPER WHITE 10mm LED Lamp
Product Description :
Emitted Colour : WHITE
Size (mm) : 10mm
Lens Colour : Water Clear
Peak Wave Length (nm) : N/A
Forward Voltage (V) : 3.2 ~ 3.8
Reverse Current (uA) : <=30
Luminous Intensity Typ Iv (mcd) : Average in 18000
Life Rating : 100,000 Hours
Viewing Angle : ?10?
Absolute Maximum Ratings (Ta=25?C)
Max Power Dissipation : 80mw
Max Continuous Forward Current : 30mA
Max Peak Forward Current : 75mA
Reverse Voltage : 5~6V
Lead Soldering Temperature : 240?C (<5Sec)
Operating Temperature Range : -25?C ~ +85?C
Preservative Temperature Range : -30?C ~ +100?C
_________________________________________

I would like to make two differant circuits,one using the 5mm leds and
another using the 10mm leds to see which leds are whiter. I prefer to
use a transformer instead of 120 volt.

Now my questions are:

1. Is the 9V/300mA AC-to-DC Power Adapter going to put out enough
voltage for 8 leds in each of the two circuits I want to make?

2. Should I still use the 330 ohm 1/4W 5% Carbon Film Resistor for all
leds in each circuit?

Thank you all again for helping a rookie like myself,Randy

 

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:

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

 

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

 

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

http://www.electronickits.com/kit/complete/elec/k126.htm

 

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