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LED power source in a combination of series and parallel.?
I'm planning to use a laptop power supply to power(19.5V 4.7amp 91.65W) an array of LED(2.1V 20miliamp) lights. The series LED will be 9 in a row with a 30 ohm 1/4 watt 5% Carbon Film Resistor.
(2.1V)*(9LED)=18.9V .
R=V/I
[(19.5V) -(18.9V)] / (0.02amp) = 30 Ohm
1)The question is, how much of these series can I put into a parallel circuit?
I'm thinking--- (power 4.7amp)/ (LED .02amp)=235circuits
2)Another question, Do I need to consider the amperes from the power supply when choosing the resistor if I use only 3 circuits?
"9 in a row" means 9 in series? That adds up to 18.9 volts, but LED voltages have a tolerance, perhaps ±0.2 volts, so you may have too many in a string. Plus the difference between 18.9 and 19.5 is only 0.6 volts, and you should have 2-3 volts across the resistor to get some protection.
so I'd drop it down to 7 or 8 in series with a larger resistor. At 7, that is 14.7 volts, leaving 4.2 volts for the resistor. At 8, that is 16.8 volts, leaving 2.7 volts for the resistor. for the latter, R = 2.7/30 = 90 ohms.
You can put as many strings in parallel as you want, limited by the amount of current your supply can deliver. Note that each string needs it's own resistor. In this case it is 4.7 amps. 4.7 a / 30ma = 156, you can have 156 strings.
Resistor is calculated as above, supply voltage – sum of LED voltages divided by LED current.
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