Re: LED display design

On 28 Jul 2005 20:28:16 -0700, "captsomer" <destinyyacht@xxxxxxxx>
wrote:

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

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@xxxxxxxx>
>> 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.

.

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