# Newbe 8 leds connected to 120 volt ac

Discussion in 'Electronic Basics' started by [email protected], Apr 12, 2005.

1. ### Guest

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

2. ### amanGuest

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.

3. ### GotCoffeeGuest

You probably already have an old transformer laying around... look at
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.

4. ### Lord GarthGuest

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?

5. ### ChrisGuest

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

6. ### PTGuest

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
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
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
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

8. ### Bill BowdenGuest

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

9. ### Lord GarthGuest

The kids going to hurt himself like this....best to let him use a transformer.

10. ### Guest

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

11. ### Rheilly PhoullGuest

One day 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.

12. ### Guest

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?

13. ### Lord GarthGuest

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.

14. ### Jonathan WesthuesGuest

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
http://cq.cx/

15. ### Rich GriseGuest

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

16. ### Guest

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

18. ### Jonathan WesthuesGuest

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
http://cq.cx/

19. ### Guest

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