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How do I wire 4 LEDs (3.5v), a small pc fan (12v), using a switch and 2 9V batteries or 3 if needed? Answered

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What i did was, using 4 wires (since there are two leds on each side of the helmet) i connected the LEDs together by connecting the positive sides to each other and the negative sides together then i connected the positive wires to the positive wire of the fan and those wires to the negative wire of the switch. Then i connected both positive battery wires to the positive wire on the switch, and then connected all the negative wires from the LEDs and the fan and the battery together. I hit the switch and the white lights turned blue and dimed to nothing all almost instantly. the fan worked and still works but the LEDs died... so is the math/voltage wrong or what am i missing. i thought i would need and additional battery to make it 27v vs 26v but if 18 overloaded then IDK wtf to do.

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steveastroukBest Answer (author)2011-09-26

YOu've destroyed them because you have them in parallel, and you have no current limiting resistor.

If you put them in SERIES, as Caerntedd says, you should be OK, because you will have a voltage drop of 3.5+3.5+3.5+3.5 thorugh them. As you HAVE it, you have just 3.5 V.

In fact you have the batteries in PARALLEL too, so they are only giving you 9V out, not 18V, which is too low to get the maximum blow from the fan, but 18 will be too much - so you could really do with a different battery setup for good results.

Exactly which LEDs did you buy ?

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HUNTER II7 (author)steveastrouk2011-09-27

ok thanks! i bought some 10mm white LEDs from raido shack. FW supply 3.5V, 4.0V max, FW current 20mA

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steveastrouk (author)HUNTER II72011-09-27

Either try all 4 in series, and use my 8, AAA batteries, or 2 chains, with 250 Ohms in each chain.

The picture on the LEFT shows the wiring FOR 8, 1.5 V batteries, and 4 LEDS in series, the one on the RIGHT 2 parallel chains - the one on the right will be a lot brighter, and safe for the LEDs.

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HUNTER II7 (author)steveastrouk2011-09-27

so about the battery setup, will the way Caarntedd did it work? i know you said it would, but then you said do with a different battery setup for good results.

If i wire them in a series will the batteries over heat? i had that problem before.
I really dont want to use 3 9V so in your opinion could i run everything with two as shown in Caarntedd's drawing?

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steveastrouk (author)HUNTER II72011-09-27

9V rectangular batteries are very, very poor for big loads like fans and bright leds, that may be why things got hot. How about some AAA ? 8 of them would be just right for the fan, and no heavier or bigger than 2 9V'ers ?

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HUNTER II7 (author)2011-09-27

IT WORKED!!!!!!!! FINALY!!!! :)

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HUNTER II7 (author)HUNTER II72011-09-27

Thank-you everyone! your help went to a good cause :)

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astroboy907 (author)2011-09-26

Try connecting the LEDs in series, or better yet, add a resistor to the LEDs
Right now the LED's are being run about 3v too high (I think)
If you could make a little diagram in MS paint it would help :)

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HUNTER II7 (author)astroboy9072011-09-26

I dont know how to do anything like that idk what i have wired now series or parrellel. I put a picture up but dont laugh lol i did it real quick and its not in the traditional circuit diagram. i have not taken a electronics class in about 7 years. so uh thats why i am here...

so as you can see:
The (+) from the LEDs and Fan are all connected to then (-) on the Switch.
The (-) from the LEDs and Fan are connected to the (-) on the Batteries
The (+) from the Batteries are connected to the (+) on the Switch

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caarntedd (author)HUNTER II72011-09-26

Try connecting the leds : 1st led (+) to fan (+), 1st led (-) to 2nd led(+), 2nd led (-) to fan (-). Do the same for the other two leds and see what happens.

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HUNTER II7 (author)caarntedd2011-09-26

I will have to do this tomorrow as raido shack is closed and I only bought 4 LEDS all wich are now fried.

Can someone just explain to me how they got destoryed???
They use 3.5V there are 4 so 3.5*4= 14 + the 12v fan so 26v. I was using only two 9v batteries= 18v how could they fry when the battery is not even powerfull enough??? i just dont understand. Im guessing becuase of the way i had it wired.

But does that math sound correct? i mean thats how it all works right you just add up all the V needed to use the item then try to match it with a power source?

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caarntedd (author)HUNTER II72011-09-27

Try this. Might work, might not.

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HUNTER II7 (author)caarntedd2011-09-27

ok but the switch has a black and red wire. does it matter which one i connect it to? (left or right) or will it just change the side of the switch thats on and off? (plug in one way down is off plug in the other way up is off)

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caarntedd (author)HUNTER II72011-09-27

Yeah, that's right. I would connect the red to the battery and the black to the fan (+), but it will work either way.

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lemonie (author)2011-09-26


Your math is wrong.
If you wanted to add 3.5*4= 14 + the 12v fan to 26v, you should have strung them in series.
The LEDs are fried, you'll need to replace them.

L

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