# Resistor Help.



## The Watcher (Sep 13, 2008)

I am trying to get a led to run on 9 volts. The calculator says, 1/4 watt or larger 330 ohms. When it says larger, dose that mean a 1/4 watt 470 ohm will work? Are did it just mean a larger wattage? I don't have a 330 and hate to drive 40 miles in this rain to get one.


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## Otaku (Dec 3, 2004)

The calculator meant that you you could use a 1/2 watt resistor, but an 1/8 watt would likely overheat and fail. If you use a 470 ohm resistor the LED will still work, but will be dimmer. You could go down to a 290 ohm - the LED will be overdriven a bit, but will probably not fail. Kinda depends on the quality of the LED.


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## The Watcher (Sep 13, 2008)

Thanks Otaku, I kind of figured it was talking about the watts. But was hoping I could find a substitute.


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## heresjohnny (Feb 15, 2006)

Watcher, I am curious, are you using the 9 volt battery for something in addition to the LED? If not you might consider using an AA or AAA battery. As a rule of thumb I often will use 2 AA batteries for 2 LEDs (in parallel) without a resistor. Using the handy LED calculator says you should have a 56 ohm resistor, so the result will be a little overdriven as Otaku says but has worked just fine for me.

The reason I bring all of this up is to drive an LED (that needs about 2 volts across it) with a 9 volt battery uses up a lot of battery power as heat from the resistor.


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## corey872 (Jan 10, 2010)

All good info from Otaku as usual. A couple of additional tidbits:

Using the numbers you mentioned, 9V source, 330 ohm resistor - that will give you about 2.4 volts across the LED at 20mA. Based on that assumption, you're burning about .13 watts in the resistor and .05 in the LED. It's not going to black out half the country or anything, but most of your power is going away as heat. 

If you're wanting/needing more light/LED's you could add up to 2 more in series and that heat would become essentially free light. If this is battery power, you could step down to 2 'AA' or 'AAA' cells which would give 3V and be a much closer match for the single LED. Also, don't forget you can parallel and/or series resistors you have to make values you don't have. (been there, done that many times!)

Either way, good luck.


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## Otaku (Dec 3, 2004)

Good advice, corey872. I never run a single LED from a 9 volt - I use those for small 3 LED spots.


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## The Watcher (Sep 13, 2008)

Thanks everyone, I am running other things as well. This is for my Nemesis Gun. This led will be firing down the barrels. But I am also running a fan motor to turn the barrels and a amp to make the machine gun fire louder. But I think I will need the amp on a separate 9 volt battery. For some reason When I hit the momentary switch with the motor hooked up, it cuts the amp off. I can't figure that out, they are both wired correct. So I am going to just turn the amp on by another switch. I got 1 of those littler 10.00 recorders from Elec123. They are very simple to use and would work great for a lot of places in a haunt. But they are somewhere close to the Gemmy Heads.


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## fravak (May 16, 2009)

I think the problem is happening because your motor is pulling all of the available current from your battery and not leaving enough for your amp. A 9 volt battery can't supply much current. I tried to find an example to show you, but everything I found was filled with so many equations that it was hard to see what's happening. (Everyone, feel free to correct me. My high school electronics is pretty rusty...)


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## Doc Doom (Aug 28, 2008)

I've followed this thread to try to learn as much as I can. I typically run LEDs on 12 volts since I use those little air vent motors. For most LEDS I can get at my local Radio Shack (they have a very limited supply), 330 ohm resistors are called for. I use 300 and they work fine. My question is, since I'm out of 300s, can I use two 150s in series?


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## bradbaum (Jul 26, 2008)

of course.

or you could use two 600 ohm resistors in parallel.


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## Doc Doom (Aug 28, 2008)

bradbaum said:


> of course.
> 
> or you could use two 600 ohm resistors in parallel.


Thanks.


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