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Only up to four LEDs light up. Answered

I've got loads of LEDs and resistors from eBay and using the resistors, only up to 4 LEDs in serial will light. I can't just use more resistors, as I broke open a mains lighting bulb that uses 20 LEDs to provide lighting. Looking at the printed circuit, they are all soldered in series. If I put + at one side of an LED and - at the other with these resistors I have, it lights, two three and 4 work, but on the 5th, it drops dim, after that it won't light at all. Do I need a different resistor, of is something else wrong. Could someone kindly tell my what resistor I need for 20 5mm White LEDs running off 18v (two 9V batteries), and 9v (one)? Thanks! Josh.

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guyfrom7up
guyfrom7up

12 years ago

well, the resistors you probably got are 560 ohms and used with a 12 volt power supply to light 1 led, the more in series of LEDs the lesse resistance you need. If you have 4 LED in series you can actually put 4 of your resistors in parallel to get 1/4 the resistance.

well, since white has a voltage drop of 3, 18/3 equals 6, and for simplicity lets just put sets of 5 together.

so 5 LEDs, that'd be 18-15= 3, 3 X50= 150 ohms, so for 5 in series hooked up to an 18 volt power supply you'd need a 150 ohm resistor in series.

So if you have 4 sets of those in parallel (you have to use a seperate resistor for each set of series, do not use 1 resistor to drive all 4 sets of parallel's) then you'd have 20 white LED's lighting up