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LED light resistance

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New Member
I am building a light using High Intensity LED's (if anyone remembers me, I'm re-doing my emergency flasher lights).
I have 18 LED's and I don't know what size, and how to position the resistors. I will be using 12VDC to power them. I have thought of three ways:

1. Wire all 18 LED's in parallel using one resistor at the end of the line.
2. Wire 6 LED's in parallel with a resistor at the end of those six, and having the 3 sets of six all in parallel with my 12VDC.
3. Wire a single LED, and a resisitor in series with it, and have all 18 LED/resistors in parallel with the 12VDC.

I know that it would make a cleaner circuit if I use option 1 but when I try it, the LED's are not as bright. I am probably not using the correct value resistor though, just digging through my junk. My thought is that all three options would work but I would have to compensate in resistance to keep the same brightness for all three options, if that is correct, then it's an issue of how many components I want on the board right.

If that is true, what values of resistors do I need. The only rating I have for the LED's are 5 mm, 3000 mcd, from All Electronics Corporation.


New Member
well its very simple to calculate. first you need to mesure the current trough the led, but i know that 20-25mA is the best to use.
now the leds you are using are they the same color or different colors. if you use different colours you musnt connect the different colous leds in paralel because they require different voltages
if you connect 6 of them in paraled, i thing the resistance would be around
150-160 ohms, bu you need a high power resisor, because you will disipate 3. something watts, so a 4W would be great. but this is for 6 leds only.
but if you have 12 volts why dont you connect 3 leds in series and a series resistor for the group?


New Member
all LED's will be the same color. So you suggest putting 3 LED's in series with a resistor in series with that huh... so I would have a total of 6 resistors on the board. You say that would be better than the other options huh?


Active Member
The normal red LED about 1.2V, hi-brite red LED about 2V. For best efficiency put the LED-s serial.If You have normal red LED-s, make only two serial groups with 9-9 LED-s (9*1.2V=10.8V need for LED-s, remain only 1.2V for resistor. For 25mA need serial 47ohm small resistor, and the power loss only 30mW.)


New Member
here is a problem, the red leds dont require onle 1.2 volts, in that way you can make it light with a 1.5 volts battery, but if you use only one you wount make the led light.
the voltage is 2.2 to 3 for red leds. so i thikn that if you connect 3 in series and this group in series with a 200ohm resistor and connect it to 12 volts, it will be the best. connect the 6 groups in paralel.
you could use 5 leds in series but you will have 3 groups of five and group of 2 leds, so i think that connectring in grouops of 3 is the best for the nnumber of leds you have(18)


New Member
here is what i mean. of course you might connect the fround to the collector of the transistor, but you must use a transistor that can hold a bigger current, like the bc639(1 amp).



New Member
if you can't figure out where to put the switch then what makes you think you have the necessary education to install the LED's? The switch is the easiest part. I'm not going to tell you where to put it but I will tell you a switch's purchase is to make or Led Light Accessories Connector / Power Wire LED Power Supplies break power. So put it in line with the power source of the LEDs.
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Well-Known Member
Most Helpful Member
Why do people guess wrongly about the forward voltage of an ordinary 5mm red Led?
It is not 1.2V and it is not 3V. Instead it is from 1.7V to 2.2V.

LEDs are NEVER connected directly in parallel unless their forward voltages are matched.
The forward voltage of LEDs are different even if they have the same part number so if they are in parallel the the one with the lowest voltage will hog all the current and burn out soon, followed by the one with the next higher voltage then it will burn out. Then the remaining LEDs will have too much current and they will also burn out.

I agree that with a 12V supply then four red LEDs should be in series and in series with a current-limiting resistor making one string. Lots of these strings can be used.
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