Hlep with LED project.

[AAMoy]

Would like some help picking the values for a circuit to drive a bunch of LEDs.

I would like to take 120vac60Hz and rectify it directly to DC. Filter that
with a PI or maybe a C-R-C filter to smooth out and limit current. Then use
that to directly power 32 series 3.4vdc LEDs.

I would also like any help with safety.

Thanks in Advance.

Alan

 

[CFoley1064]

Subject: Hlep with LED project.
From: aamoy@aol.com (AAMoy)
Date: 5/16/2004 7:37 AM Central Standard Time
Message-id: <20040516083754.25458.00001509@mb-m29.aol.com

Would like some help picking the values for a circuit to drive a bunch of
LEDs.

I would like to take 120vac60Hz and rectify it directly to DC. Filter that
with a PI or maybe a C-R-C filter to smooth out and limit current. Then use
that to directly power 32 series 3.4vdc LEDs.

I would also like any help with safety.

Thanks in Advance.

Alan

Good morning, Alan. Here's something that will work for you, and should be
relatively safe (view in fixed font or M$ Notepad):

"Safer" 120VAC Diode Strings
1/4A .------.
____ | |
o-|_--_|---. ,------. ,----o~ +o-------o------o
120VAC )|( )|( | | | |
)|( )|( | | .-. .-.
o----------' '------' '----o~ -o-. R| | R| |
Pri. Sec. Pri. Sec.| | | | | | |
| | | '-' '-'
'------' | | |
| V V
| - -
| | |
| . .
| . .
| 2 Strings.
| 16 In Series
|
| | |
| V V
| - -
| | |
'-------------
Here's the ticket. First, find two identical transformers of more than 10VA --
that would be 12VAC at greater than 800 mA, or 24VAC at greater than 400 mA,
you get the picture. Just multiply secondary volts times rated current to get
the VA rating. Set them up so that the secondary of the first feeds the
secondary of the second. You'll get something approaching 120VAC out of the
"primary" of the second transformer. This type of thing is called
"back-feeding" a transformer. It will provide isolation from the line, which
is always a big plus with safety issues. You should spec your transformer for
greater than required VA rating when doing this kind of thing, because
transformers tend to misbehave a little when they're being backfed and are
loaded down. Make sure you use a 1/4 A fuse in series with the power at the
primary for safety.

Now that you've got your 120VAC, get a bridge rectifier of appropriate voltage
rating (400V is a lot better than 200V) to rectify the AC. Or you can use 4
ea. 1N4004s for this. You will now want to look at powering your LEDs. You
actually don't need a filter if your LEDs will be lit most of the time. You
can do that by having two strings of 16 LEDs, which should give you a series
voltage of about 54V on each string when they're lit. That will mean your
120VAC rectified voltage (170V Peak) will have at least some current going
through the LED more than 3/4 of the time. 120Hz flicker with this duty cycle
is not visible -- persistence of vision will ensure that the LEDs will be
viewed as constantly on. Now just get 2 ea. 3300 ohm, 2 watt or greater power
resistors to limit current for each string, and you're on your way to a
brighter future.

A gentle reminder -- if you're not confident about what you're doing when
playing with higher voltages, don't. Always double-check that your toy is
unpowered when you're fiddling with it, and make sure you havent got any metal
or conductive material near when it's powered. One really good piece of newbie
advice is to make sure your toy is really bolted down before you call it done.
A good safe circuit can become unsafe quickly if it's rolling around in an
enclosure. Don't depend on hot-melt glue to do this for you, either. Be
responsible and use screws or some other permanent means. Remember, even if
it's isolated from the line, high voltage can hurt.

Oh, and by the way -- sci.electronics.basics is a great forum for this type of
question.

Good luck, and be safe
Chris

 

[John Fields]

On 16 May 2004 12:37:54 GMT, aamoy@aol.com (AAMoy) wrote:

Would like some help picking the values for a circuit to drive a bunch of LEDs.

I would like to take 120vac60Hz and rectify it directly to DC. Filter that
with a PI or maybe a C-R-C filter to smooth out and limit current. Then use
that to directly power 32 series 3.4vdc LEDs.

I would also like any help with safety.

---
We make (and sell) isolated and non-isolated LED ballasts for up to 40
white LEDs. For more info and prices, email me. (Our long overdue
web site overhaul is in the works, so the data's not there yet, but it
will be, soon.)


--
John Fields

 

[AAMoy]

Thank for the input, but, what is so wrong with my original ideas?

All I want to do is limit the number of component. Diodes to change AC to DC,
a resistor to limit current and a couple of caps to smooth out what the LED
have to work from.

Is that so wrong?

I know LED love to suck current if you don't put that in you design, and they
work best of DC, that's why I'm looking for JUST the values of the bridge and
CRC to run 32 3.5v LED off of LINE.

Thanks for any help on what I want or what I and about to do wrong.

Moy

 

[Ian Stirling]

AAMoy <aamoy@aol.com> wrote:

Thank for the input, but, what is so wrong with my original ideas?

All I want to do is limit the number of component. Diodes to change AC to DC,
a resistor to limit current and a couple of caps to smooth out what the LED
have to work from.

Is that so wrong?

I know LED love to suck current if you don't put that in you design, and they
work best of DC, that's why I'm looking for JUST the values of the bridge and
CRC to run 32 3.5v LED off of LINE.

Thanks for any help on what I want or what I and about to do wrong.

What is this for, is it one-off, or production?
Can the user touch the LEDs?
Are they inside a plastic enclosiure/..

 

[Stefan Heinzmann]

AAMoy wrote:

Thank for the input, but, what is so wrong with my original ideas?

All I want to do is limit the number of component. Diodes to change AC to DC,
a resistor to limit current and a couple of caps to smooth out what the LED
have to work from.

Is that so wrong?

Not necessarily. It seems to be something like a FAQ, similar
discussions have appeared here in the past. Quite often, the way people
ask leaves little confidence in their ability to make responsible
judgments regarding safety vs. component count/price. So don't be
surprised if you're getting treated like a clueless newbie. Clueless
newbies who want to mess with mains voltage are frowned upon.

I know LED love to suck current if you don't put that in you design, and they
work best of DC, that's why I'm looking for JUST the values of the bridge and
CRC to run 32 3.5v LED off of LINE.

So what is wrong with using a line transformer? It improves safety a lot
and isn't very expensive. If you don't use one you have to provide
safety insulation for the whole circuit, including the LED chain. Why is
that any better?

Thanks for any help on what I want or what I and about to do wrong.

What you want can be done. It is just that you come across as being a
bit too naive for the job. If you can demonstrate that you know what
you're doing you'll get all the details you need. For that, it would
help a lot if you provided more information, such as design constraints
(cost/size/production volume/safety rule compliance etc.). But then, you
would probably be able to work it out by yourself, or at least google
for it.

--
Cheers
Stefan