LED display design

C

captsomer

Guest
I need to build a sign. It will have 7 letters and I want to use LED's
to backlight it. I'm going to need to light about 40 of them. I have
tinkered around with LED's in the past and I have a basic knowledge,
but I thought I'd ask others for thier opinion before I spend all of
this time and money.

This circuit needs to be powered by 110 VAC. The LED's are 3.6v @ 20mA.
Should I use a lower voltage power supply? Are there schematics for
projects like this on the web that might come close to what I'm doing?
Series or paralell?

Help!
Shawn
 
captsomer wrote:
I need to build a sign. It will have 7 letters and I want to use LED's
to backlight it. I'm going to need to light about 40 of them. I have
tinkered around with LED's in the past and I have a basic knowledge,
but I thought I'd ask others for thier opinion before I spend all of
this time and money.

This circuit needs to be powered by 110 VAC. The LED's are 3.6v @ 20mA.
Should I use a lower voltage power supply? Are there schematics for
projects like this on the web that might come close to what I'm doing?
Series or paralell?

Help!
Shawn
Click to expand...
The basic way would be to use say an off the shelf 12V DC 1A supply
driving 3 LEDs and a 68ohm resistor (to give you about 17mA) in series.
Then you parallel up as many of these series strings as you need from
the one 12V supply.

Regards
Dave :)
 
On 27 Jul 2005 22:00:02 -0700, in sci.electronics.design "captsomer"
<destinyyacht@fuse.net> wrote:

I need to build a sign. It will have 7 letters and I want to use LED's
to backlight it. I'm going to need to light about 40 of them. I have
tinkered around with LED's in the past and I have a basic knowledge,
but I thought I'd ask others for thier opinion before I spend all of
this time and money.

This circuit needs to be powered by 110 VAC. The LED's are 3.6v @ 20mA.
Should I use a lower voltage power supply? Are there schematics for
projects like this on the web that might come close to what I'm doing?
Series or paralell?

Help!
Shawn
loads of stuff here
Click to expand...
http://www.epanorama.net/links/opto.html


martin
 
On 27 Jul 2005 22:00:02 -0700, "captsomer" <destinyyacht@fuse.net>
wrote:

I need to build a sign. It will have 7 letters and I want to use LED's
to backlight it. I'm going to need to light about 40 of them. I have
tinkered around with LED's in the past and I have a basic knowledge,
but I thought I'd ask others for thier opinion before I spend all of
this time and money.

This circuit needs to be powered by 110 VAC. The LED's are 3.6v @ 20mA.
Should I use a lower voltage power supply? Are there schematics for
projects like this on the web that might come close to what I'm doing?
Series or paralell?

Help!
Shawn
Click to expand...
Do you need isolation? (If you were using bulbs, you probably would
not have it, either). If you don't need it, there is a cheap
alternative with very few capacitors, resistors and diodes. Let me
know if you want to go this way.
 
On Fri, 29 Jul 2005 00:18:53 +0200, Mochuelo wrote:

On 27 Jul 2005 22:00:02 -0700, "captsomer" <destinyyacht@fuse.net
wrote:

I need to build a sign. It will have 7 letters and I want to use LED's
to backlight it. I'm going to need to light about 40 of them. I have
tinkered around with LED's in the past and I have a basic knowledge,
but I thought I'd ask others for thier opinion before I spend all of
this time and money.

This circuit needs to be powered by 110 VAC. The LED's are 3.6v @ 20mA.
Should I use a lower voltage power supply? Are there schematics for
projects like this on the web that might come close to what I'm doing?
Series or paralell?

Do you need isolation? (If you were using bulbs, you probably would
not have it, either). If you don't need it, there is a cheap
alternative with very few capacitors, resistors and diodes. Let me
know if you want to go this way.
Click to expand...
Easy. Two series strings of 20 LEDs each, with about a 2.4K, 2W resistor
all in series, in antiparallel. (i.e., one string with the cathodes up,
the other with the cathodes down, and the two strings in parallel right
across the 110V line). This is assuming LED current of ~20 mA.


L1 -------------+-------+
| |
[2K4] [2K4]
| |
--- ---
\ / / \
--- ---
| |
. .
. . <- 40 total, 20 per string
. .
| |
--- ---
\ / / \
--- ---
| |
L2 -------------+-------+


Cheers!
Rich
 
Hey everyone,

Thanks for all of the ideas! You guys are a big help. I appreciate it.
Hey Mochuelo, what was your idea?

Thanks all,
Shawn
Mochuelo wrote:
On 27 Jul 2005 22:00:02 -0700, "captsomer" <destinyyacht@fuse.net
wrote:

I need to build a sign. It will have 7 letters and I want to use LED's
to backlight it. I'm going to need to light about 40 of them. I have
tinkered around with LED's in the past and I have a basic knowledge,
but I thought I'd ask others for thier opinion before I spend all of
this time and money.

This circuit needs to be powered by 110 VAC. The LED's are 3.6v @ 20mA.
Should I use a lower voltage power supply? Are there schematics for
projects like this on the web that might come close to what I'm doing?
Series or paralell?

Help!
Shawn

Do you need isolation? (If you were using bulbs, you probably would
not have it, either). If you don't need it, there is a cheap
alternative with very few capacitors, resistors and diodes. Let me
know if you want to go this way.
Click to expand...
 
On 28 Jul 2005 20:28:16 -0700, "captsomer" <destinyyacht@fuse.net>
wrote:

Hey everyone,

Thanks for all of the ideas! You guys are a big help. I appreciate it.
Hey Mochuelo, what was your idea?
Click to expand...
Without isolation, I see two options:
-------------------------------------
1) "Low-efficiency" option.

With only two resistors, arranged as in
http://80.35.195.131/ng/20050729_Solution_1.png
(server not always up)
Each resistor needs to be 3.9 kohm, 0.5 W.

Approximate results:
I_led_max = 21.4 mA
Efficiency = 52 %
P_out_avg = 0.60 W (average total output power, at the LEDs)
Flicker (at each LED) = 60 Hz
-------------------------------------
2) "High-efficiency" option.

With 4 resistors, 3 capacitors (actually, one is to fine tune the led
current), and 4 diodes, arranged as in
http://80.35.195.131/ng/20050729_Solution_2.png

All resistors may be 0.125 W, or even smaller. C1 needs to sustain
around 230 V, and C2, 100 V. For instance, Digi-Key ref numbers:
C1a: EF2185-ND (1.8 uF, 250 V, 0.9600 USD)
C1b: fines tune I_led_max. Around 100 nF, with my model.
C2 : 493-1668-ND (100 uF, 100 V, 0.4500 USD)

Approximate results:
I_led_max = 20.5 mA
Efficiency = 90 %
P_out_avg = 2.2 W (average total output power, at the LEDs)
Flicker (at each LED) = 120 Hz
-------------------------------------

Solution #2 is not only more efficient, but it also delivers much more
average power to the LEDs, without increasing their stress (actually,
with a slightly lower stress), thanks to the averaging effect of C2.
You will have about 4 times more light with sol #2. Thanks to the
bridge, you will also have a much better flicker. Of course, it is a
little bit more expensive, but for me, it is worth. RS is an inrush
current limiter, and R1 "bleeds" C1. The diode bridge can be a cheap
and small one (even smd). It must sustain 200 V. Its current rating is
not a problem. I was thinking of solution #2 for you.



Thanks all,
Shawn
Mochuelo wrote:
On 27 Jul 2005 22:00:02 -0700, "captsomer" <destinyyacht@fuse.net
wrote:

I need to build a sign. It will have 7 letters and I want to use LED's
to backlight it. I'm going to need to light about 40 of them. I have
tinkered around with LED's in the past and I have a basic knowledge,
but I thought I'd ask others for thier opinion before I spend all of
this time and money.

This circuit needs to be powered by 110 VAC. The LED's are 3.6v @ 20mA.
Should I use a lower voltage power supply? Are there schematics for
projects like this on the web that might come close to what I'm doing?
Series or paralell?

Help!
Shawn

Do you need isolation? (If you were using bulbs, you probably would
not have it, either). If you don't need it, there is a cheap
alternative with very few capacitors, resistors and diodes. Let me
know if you want to go this way.
Click to expand...
 
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