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So I'm hoping someone can help me understand my problem here! (Also sorry if this question has been asked before !)

I've done a simple led circuit before , a switch battery pack connected to led with no resistor and it worked fine considering it was powered by two AA batteries.

I'm making a prop weapon (I do costumes and props) and it needs three "glowing gems" so I casted resin and inserted white led lights into them. I need all three lights to be able to turn on and off with a switch while being on different sides of the weapon.

I currently wired the lights (extra ones ) to test and I can't seems to be able to power the second led light when wired this certain way. Positive wire of battery (two AAs) to positive of led. Negative pole of (first)led to positive of next led. negative pole to switch. Other side of switch to negative side of battery. When taking the second led light off everything seems to work fine but when adding the second light it stops working.

Would I be able to power the 3 white led lights with a 3volts battery pack ( 2 AA batteries ) or would I have to connect it to a 9 volt battery ?

Any help would be greatly appreciated!

Thanks!

A Stackhouse
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  • Post the part number of the LED, a hand drawing if you can, and try wiring them in parallel does it work then? You should probably also include a resistor eventually so you don't burn them out but give that a try. Sounds like a voltage drop problem. – Some Hardware Guy Dec 10 '14 at 17:11
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    You've either blown out the LED because you don't have a current limiting resistor or the forward voltage drop of the LEDs are two high for the ~3V you're getting from 2 AA batteries. – Dan Laks Dec 10 '14 at 17:13
  • What do you mean by ..."being on different sides of the weapon."? – EM Fields Dec 10 '14 at 17:33
  • http://www.speed4game.com/upload/image/one-handed%20mace/Light%27s%20Justice2.jpg A picture of the prop weapon i am making. On either side on the hammer heads are "glowing gems" – A Stackhouse Dec 10 '14 at 17:46

1 Answers1

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Firstly, you need to go back to school to learn the basics. What you are trying has no hope of working.

An LED is a current driven device, and it has a fixed forward voltage drop. Quite frankly, you are lucky the LEDs are even working at all any more - you must have LEDs that have a forward voltage at (or around) the 3V the batteries are supplying.

You have to run the LEDs with some form of current limiting device. At the moment that current limiting device is the batteries themselves being in the non-linear portion of the LEDs IV curve, which is very fortunate for you, or they would have died.

So your LEDs have a forward voltage of (around) 3V. Putting two in series makes that forward voltage 6V, so you will need at least 6V to drive them in series. You also need to limit that current to the right value to run the LEDs (no idea what that should be for your specific LEDs, but 20mA is common for normal LEDs).

So running from 9V would be sensible in this situation. Assuming 20mA and 3V forward voltage, you will need a resistor, of:

$$ V_{LED}=9V - 6V = 3V $$ $$ R_{LED}=\frac{V_{LED}}{I_{LED}} = \frac{3}{0.02} = 150\Omega $$

schematic

simulate this circuit – Schematic created using CircuitLab

You could also run from 4.5V (3xAA) and have as many LEDs in parallel as you wish. Each LED must have its own resistor in this case:

schematic

simulate this circuit

The resistor of 75Ω is worked out in the same way:

$$ V_{LED}=4.5V - 3V = 1.5V $$ $$ R_{LED}=\frac{V_{LED}}{I_{LED}} = \frac{1.5}{0.02} = 75\Omega $$

As a side note, it is more common to put the switch in the + side of the circuit, not the - side. It doesn't really make any difference, it's just that's where others would expect it to be who don't know the circuit you have designed.

Majenko
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  • Thanks for your help. I don't normally work with led lights in prop weapons. Would I still need a resistor if using three led lights and the 9 volt battery – A Stackhouse Dec 10 '14 at 17:38
  • @AStackhouse try [some research](https://www.google.com/search?q=how+to+use+an+LED&ie=utf-8&oe=utf-8)? – Phil Frost Dec 10 '14 at 17:47
  • @PhilFrost As stated before I dont normally work with led lights (being a blacksmith by trade) I dont see the shame in asking for help when you dont quite understand something. – A Stackhouse Dec 10 '14 at 18:03
  • As I said in my answer, you have been lucky. You have to have some form of current limiting, and a resistor is the easiest. I am going to post another option in my answer. – Majenko Dec 10 '14 at 18:05