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Old May 23, 2009, 11:58 AM
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Default HELP! Led circuit.

Long story short, made a circuit with 12V+ going to a 1/4W resistor goes to + side of LED, the - side goes to ground. Tested the outlet from PSU for the 12V, got 11,7. Seems fine.

Plug everything up, test it. LED goes kablooey! Way too much power went to it and it lit up for half a second before producing magic smoke.

What did I do wrong? Thx!

For the long story, check the last post : http://www.hardwarecanucks.com/forum...-v2-0-a-2.html
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Old May 23, 2009, 12:12 PM
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How many ohm is the resistor, and what color is the LED?

Oh, and if you're running a single LED, why not go off of the 5v instead of the 12v?
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Old May 23, 2009, 12:19 PM
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*facepalm*

forgot about th ohms ... im trying on the 12V because...i dunno, the wire was already plugged like that and my real circuit will have more then one LED, just want to test it all see if it works b4 i blow many LEDs to smitherizes...

bought a 150 resistor value pack ... gonna dig through it to find the one with the right ohm .....
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Old May 23, 2009, 12:24 PM
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I guess I could offer a bit more advice without the answer anyway. What you're essentially trying to do is balance Ohm's Law. It states:

Voltage=amps * resistance

In this case, you're trying to find your correct resistance so let's make that R=V/A

Now, most LEDs are 20 milliamp (.020amps), and the voltage depends on the color. Reds are 2.1v, IIRC, and blues are 3.3v.

So, if you're coming from 12v power and a blue LED, you would have R=(12v-3.3)/0.02 R=435 So you need a 435ohm resistor.

I suggest, however, that you take it off of 5v, so R=(5v-3.3v)/0.02 R=85ohm


Now, I'm not a specialist with these things, so take it with a grain of salt, but the logic is sound, I think.
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Old May 23, 2009, 12:28 PM
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Oh, OK. Keep in mind that if you're running the LEDs in series (neg of LED #1 to pos of LED #2, etc) then the current is divided, which may work out for you.

For two blues:
R=(12v-3.3-3.3)/0.02 R=270ohm (put the resistor between power and first LED pos)

You can also run them parallel (all the pos of the LEDs to a pos power, all negs to a neg power), in which case, you're still better running them off of the 5v line, you will need a resistor for EACH LED, as the voltage will not be divided between the LEDs.

As a sidenote, where did you get the resistor pack?
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Old May 23, 2009, 12:57 PM
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Thx for the heads up! I'll make it run most probably on the 5V line when the final project is done, but for the sake of testing since its all hooked up to the 12V line already, why not.

Bought the resistor pack at Radio Poop, 150 of them in a pack...2 of each ... from 1 to 3.3m ohm ... so I went fishing on the living room floor finaly found the color coded one!

It was a real pita ... but 150 resistor for 5$ hey wth!

So now I'll re-try with the correct ohm resistor. And weirdly enough, everyone says that LEDs are normal 20mA but my white ones are 10mA for some reason...but my reds are 20mA ... hope it works !
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Old May 23, 2009, 01:43 PM
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Sweet it works! ... so wadayakno ... ohms do matter thx for the help Vulcan!
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Old May 23, 2009, 02:40 PM
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Good to hear; glad to help!
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Old May 23, 2009, 03:06 PM
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If I take some LEDs from a little broken toy I have, safe to bet they're 20mA ?
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Old May 23, 2009, 04:03 PM
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Hard to tell really, only way to find that out is to test them and see if they blow up, most of the LEDs ive worked with have ranged from 10mA to 20mA, but ive also blown up a few that ive pulled out of random toys and what not as well. Chances are if its roughly the same size as the ones your working with already then it will be the same mA... Colors also make a difference.
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