--Am working on a friend's art car and we're going to need a few LEDs in the mix. I know they don't need a full 12v as we've done experiments and it looks like proper brightness happens with a 12v pack of AAs and a 220 ohm resistor on one leg. The trouble is I'm using 1/4 watt resistors and those suckers get *hot*! I plan to grab some higher wattage resistors tomorrow, but how big is big enough? Most likely we'll be running the LEDs off of a car battery; will this affect resistor size or will the larger battery (with same voltage) only affect run time?
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SF Bay Area
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Re: Resistors?
Sat, August 11, 2007 - 7:07 AMOne reason for this is that the LED is going to use a fixed number of volts, probably something like 3 or 3.5, so the resistor is going to have to dissipate the other 9 V as heat. If you are running more than 1 LED you can improve this situation by putting several of them in a row on each. It will also save on battery life. Incidentally, you probably don't need a car battery - LEDs just don't use much power, you can run them as long as you want off some AAs.
Here is a website that will design the circuit for you: led.linear1.org/led.wiz
You need to know the diode forward voltage, which if you don't know you can just measure with a voltmeter in your existing circuit. With 12V and 220 ohm for one LED your "diode forward current" in your present setup is something like 40 mA. -
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Re: Resistors?
Sat, August 11, 2007 - 6:26 PMWatts = Volts * Amps = Amps * Amps * Ohms = Volts * Volts / Ohms
Assuming 3v forward voltage, I=40mA, P=360mW - a half watt resistor should be fine.
But like the man says, 12v is overkill and you're wasting power.
4.5V battery, 37R resistor gives you 40mA, P=60mW on the resistor.
or put 3 leds in series (9v) and a 75R resistor gives 40mA, 120mW
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Re: Resistors?
Sat, August 11, 2007 - 9:43 PMAfter reading your post a little more carefully, I realized that you're probably using a car battery because you have a car handy, and you say you have a 220-ohm on "one leg" which suggests that you already have several LEDs in series. What is "one leg", exactly? (In my response above, I was assuming you had the LEDs in parallel).
In general if you reduce the wasted voltage, you'll reduce the power dissipated as heat in the resistors. Using a different type of battery with the same voltage won't help. It will affect how long the battery lasts, but it won't change the heat dissipation.
I still recommend that LED design thing. A word of advice: if it recommends a circuit using any 1-ohm resistors, don't use that one, pick one of the other solutions. -
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Re: Resistors?
Thu, August 16, 2007 - 9:57 PM--The project is a flaming hearse art car. LEDs will be on top of little flagpoles on each of the front bumpers. There are others on the tips of the bat wings (long story...). We're running 12v from car battery into a power bus that's distributing the output to various devices which are switched via a (coffin-shaped, natch) control panel that's a ways from where the LEDs will be. I'm thinking I'll use one of those Radio Shack voltage reducing transformers, the ones that're designed to plug into a cigarette lighter socket. I'll pop off the external trappings, then rewire the thing to a switch in the panel, then route the output to the two LEDs. But it turns out the local RS doesn't have a 3V step-down transformer so I had to get one of the 4.5v ones (which is enough to poof an LED) so I'll be able to use a smaller resistor to get it down to 3v at the LEDs. I'm soldering the resistor to one leg of several LEDs so that I'll have spares in case something goes poof. Does that sound like a sensible approach?
Here's a link to the Blood Vessel Tribe for more on the project: tribes.tribe.net/816634d4-...de0ed8f8a4 -
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Re: Resistors?
Thu, August 16, 2007 - 10:06 PMTransformers are for AC, not DC. -
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Re: Resistors?
Thu, August 16, 2007 - 10:13 PM--Well it's a transformer-like dingus then! All I know is it drops the voltage to something an LED can *almost* handle! :-) -
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Re: Resistors?
Thu, August 16, 2007 - 10:27 PMIt might well contain a transformer - many DC-DC converters do.
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Re: Resistors?
Thu, August 16, 2007 - 10:30 PMOh, like I say, if you want 40mA through your LED then 37ohms or so should be sweet and don't worry about power resistors, you're under the 125mW easily!
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Re: Resistors?
Fri, August 17, 2007 - 7:47 AMI assume this is some sort of DC-DC transformer you've got, as LEDs want DC.
It's really not necessary to reduce the voltage like that, if you wire up the LEDs in the right order. If you have 12V and the LEDs want 3.5V, you can just put three LEDs in series. Three-LEDs-in-series will want 3.5*3 = 10.5V.
Also, I think a better way to think about what fries LEDs is that they want a certain amount of current, not they want a certain amount of voltage. If the LED is rated to a current of 40 mA, then if you give it more than 40 mA it will fry. If it wants a voltage of 3.5V, then if you "give it" more than 3.5V there will still be 3.5V across the LED; what will happen is that the rest of your circuit (i.e., your battery and wiring) will adjust itself so that the rest of the voltage drop occurs there. Which is bad, because in order for a battery and some wiring to create this voltage drop, it will have to move a whole lot of current, no doubt more than 40 mA, and THAT is what will fry your LED.
It's not a good idea to run an LED without a resistor, even if your Radio Shack DC transformer has reduced the DC voltage to just a bit more than the LED wants. The reason is: you want the LED to be bright, but not fry. That means you want to put the rated current through it: if it wants 40 mA and instead you give it 10 mA it will be dim, and if you give it 80 mA it will be crispy. But how do you control the current? The LED won't control it - the LED will take 3.5V, and let the current be do as it will. Therefore you have to use the resistor to control the current.
Here is a bunch of math; if you do not love a bunch of math, then I recommend using that website I mentioned earlier, it will do it for you. But anyway, if you want to know what that website is doing under the hood, here it is: If the LED is taking 3.5V, the rest of the voltage will be across the resistor. So say you just hooked it up directly to the car battery (12V); since the LED uses 3.5V, the other 12-3.5=8.5V is across the resistor. You then use "voltage = resistance * current" to figure out the proper resistor. In this case, we know the voltage is 8.5V and the current we want is 40mA which is 0.04A, so resistance = 8.5V / 0.04A = 213 ohms. Now, you can't buy a 213-ohm resistor, they come in standard sizes, so you'd get a 220-ohm resistor (which is the next standard size). (The website does all of this for you, including the "round up to the next standard size" part).
If you want to just post (1) how many LEDs you have, (2) how they are laid out physically, i.e. how many flagpoles, (3) what is the voltage drop across the LED and (4) what is the rated current of the LED, I can just design the circuit and post it for you. -
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Re: Resistors?
Sun, August 19, 2007 - 12:45 AM--Thanks for the math refresher; it's what I needed, as the memories were fading! Yeah, I prefer to do it with pencil and paper, so's I can do it 'in the field', so to speak. Will grab some 220s out of the bin and have a go at it. Will try to use ammeter (hooked in parallel, yes?) to see what the LED is seeing. I like the idea of running several LEDs in series to get the voltage right; this will give an overall brighter 'point' of light (when they're bundled together) too. -
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Re: Resistors?
Mon, August 20, 2007 - 5:04 PMNo Ed, your ammeter needs to be in series to measure current. For most LEDs, 20mA is a safe level. Some can be driven higher but it depends on the device. You really need to look at the data sheet because you might like the brightness you get, but you'll be replacing the lamps in a few weeks instead of a few years. As others have said, wiring them in series is a good way to use up the 12V. If you need a lower voltage for some reason, the simplest thing is to use an IC regulator like the 7805. It costs less than 50 cents. -
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Re: Resistors?
Tue, August 21, 2007 - 9:14 AMWhere can I get said regulator? -
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Re: Resistors?
Tue, August 21, 2007 - 1:41 PMFry's will have something that's analogous. Unfortunately Fry's (at least the one near me) carries some weird off-brand chips that have their own numbering scheme, so you have to go to their ICs isle and find the big, well-worn catalog and figure out what they call the 7805s. If you have time to mail order, you can get them from e.g. Jameco online. Radio Shack might carry something, but their IC selection is limited.
Note that if you use a regulator to get the voltage down to 5V and then attach LEDs to it, you still need to use a resistor to limit the current. It will just be a smaller resistor, and more of the voltage-lost-to-heat will be in the regulator instead of in the resistor.
There are lots of other applications (for example, some other fancier ICs) that need a steady 5V power source, so that kind of regulator chip is great for those.
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Re: Resistors?
Thu, August 16, 2007 - 9:58 PM"Resistance is futile", eh? Good one, hyuck hyuck.. :-)
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