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?
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.
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.
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.
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.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.