Hi all I will be building myself a small lightstrip shortly, perhaps more than just one. At the moment i am having a little trouble designing the power circuitry. the light strip will be composed of the following: 6X - 2 rows by 5 leds for a total of 60 LED's altogether. below is the technical information for the LED's now, the issue is thus I have no clue how to connect these led's up such that they all get a nice even glow to them should i use a seperate supply for each of the arrays, or should i just hook all of them up to a seventh 'power supply' board? Ideally i would prefer it if i could run it off the mains but i do not know how i would go about calculating the correct resistor values and of course the necessary circuitry to convert the AC mains supply to the DC supply the LED's require any help is much appreciated, i am just starting out with my 'electronics' learning now so please be gentle! many thanks
http://ledshoppe.com/Product/led/LE2005.htm these are the led's i will be purchasing (mainly because of the $25 price tag for 100!)
Do you have a power supply picked out? Whether you have 12v, 5v, etc available will determine how the LEDs are wired up. Current will also be an issue: if you're using a lower voltage, there will be more LEDs in parallel and thus the current draw will be higher (for the same overall power).
Depending on how adventurous you are, you can technically run 70 LEDs in series directly of UK power (240vAC) with no other circuitry whatsoever (see half-wave rectifier) . This works because the total voltage drop of components wired in series is a sum of each component's voltage drop (see series), and 70 LEDs wired in series gives a total forward voltage = 245V. Code: Now before someone says "but the 240 volt mains run at an RMS of 240 volts, which peaks at 340 volts, so wouldn't the LEDs burn out!?" I will just answer the question now. Since the AC in mains power is approximates a sine wave, and given the LEDs forward voltage is 245 volts, we know that in each period the LEDs will conduct from pi/2 -> 3pi/2, giving a 25% duty cycle. At 50Hz, the period is 20mS, so the LEDs conduct for a total of 5mS. [IMG]http://theeshadow.com/h/rms.jpg[/IMG] Further, they are only over their maximum voltage (3.8v) for aprox. 3.5 mS, which is well below the maximum time on a 130% pulse over current. The problem is the 50hz flicker due to the alternating current, which can be quite noticeable. One solution would be to add a diode at one end of the string and have a 300 volt filter capacitor run across the ends of the string to achive this: Alternately you could arrange two strings of 70, with reversed polarity, so the flicker frequency is effectively 100hz, which "should" be past human perception. If you are adverse to having all the LEDs in one big string, you could try a circuit like this, which uses a capacitor in series with a shorter string. There is some serious calculus behind how this works, but essentially a small amount of current is stored in the cap during each half cycle, and is discharged through the LED array when the voltage returns to zero. The number of LEDs in the string is irrelevant, the current is governed by the capacitance of C1. I cannot emphasize enough that working with mains voltage SHOULD NOT BE TAKEN LIGHTLY. . Never work on the circuit with it plugged in, insulate all connections well, and fuse it with the smallest fuse you can find. That being said, it's not as bad as some people make it out to be, especially with LEDs. 9/10 times if you've wired something wrong, you'll know it immediately: one or two LEDs will explode well before the circuit breaker trips. Positive reinforcement If working with 240v is not something you'd like to do, the other option is to get a wall-wart or switchmode power supply and wire the LEDs the old fashion way. Hope this helps. JSD
