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My plan is to connect 3 series of 3 blue leds in parallel, all connected to a 9 volt battery. Would this be possible to do without a resistor? Because from what i read, blue leds use 3 volts Here's a rough sketch of what i want to do. Any help is appreciated

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  • \$\begingroup\$ Please provide a link to the datasheet for the LEDs. We need to see the specifications for them. \$\endgroup\$ Apr 5, 2020 at 16:26
  • \$\begingroup\$ Nogt a real design question! Of course it is possible. \$\endgroup\$ Apr 5, 2020 at 16:32
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    \$\begingroup\$ Why are 7 of the LED's shorted out? \$\endgroup\$
    – Ron Beyer
    Apr 5, 2020 at 16:34
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    \$\begingroup\$ Well, of course it's possible.. but would it work? Ask yourself what happens if your 9v battery isn't exactly 9v. Or if your LED forward voltage isn't exactly 3v for every single diode.. \$\endgroup\$ Apr 5, 2020 at 16:38
  • \$\begingroup\$ @Elliot, if the OP didn't say anything, I think that is very reasonable to think that he/she is talking about generic LEDs, there's no need to see a datasheet to answer this question thinking about the specs of a generic LED. \$\endgroup\$
    – mguima
    Apr 5, 2020 at 21:29

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Of course it is possible. Whether you want to do that is another thing.

9V batteries can have more than 9.5V when new and are considered empty around 5V.

Blue LEDs have approximately 3V forward voltage, but it won't be exactly 3.0V and it will vary due to manufacturing tolerances etc.

So assuming all the LEDs have exactly 3.0V forward voltage at their rated current, a fresh battery with 9.5V connected to 3 LEDs can give the LEDs quite high current so that their useful life is reduced, or they might burn out immediately.

And assuming the battery voltage has dropped to say 7V, maybe no LED will glow even if there is still energy left in the battery.

Also the LED manufacturing tolerances will cause the LED chain with smallest sum of LED forward voltages to be brighter than the other chains, and maybe the other LED chains won't light up at all.

So, in light of this, maybe you don't want to use chains of 3 LEDs without resistors on a 9V battery, even if it is possible.

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  • \$\begingroup\$ I have red keychain LED lights that use one button cell (no resistor), and white keychain lights that use two button cells (no resistor). They work, but they unnecessarily use too much power when the batteries are new, and the brightness fades more than it should when the batteries are near depletion. \$\endgroup\$
    – Mattman944
    Apr 5, 2020 at 20:59
  • \$\begingroup\$ @Mattman944 Just to point out for the OP that the reason your keychain light works is due to the relatively high internal resistance of the button cell. Try that with a fresh AA alkaline battery with very little internal resistance and that LED is probably toast. \$\endgroup\$
    – DoxyLover
    Apr 6, 2020 at 6:01

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