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OK. I'm confused about resistors and LEDs.

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Bach On

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I bought a bunch (50) of these:

https://www.electro-tech-online.com/custompdfs/2013/04/2096851.pdf

They are warm-white LEDs on a small brass circuit of some sort. Each consumes roughly 1 watt (there is also a 3 watt version.) Mine are the warm-white version - it isn't the bluish thing that passes for white. I won't say it is warm - but it isn't bluish-purple like you're in a welding shop.

For 12 volt use (that's automotive, for example), I've connected 4 of them in a series (+ to -) with four in the series. I connect the Positive wire from my 12 volt power source to one and the Negative wire to the last in the series. They give off a very bright light for all kinds of situations. They are rated for up to 3.6 volts each with 10 mAs listed in the datasheet as their current rating.

This circuit was without a resistor. Yeah. I know - VERY BAD. But the truth is that I'm not certain how to treat current limiting resistors in a circuit like this. I have searched around on this board and I've read the various threads on the subject. But I'm still confused about using LEDs in a series like this.

So, here are MY questions:

1. I'm not sure if these should be treated like regular LEDs. Each has a + and a minus marking.
2. I'm not certain what to do with the resistor in this series circuit. Does each of these little guys need a separate resistor? Or is one resistor in the circuit the best thing? (I'm thinking not.)
3. I'm not real sure how to use the many varieties of the laws of Mr. Ohm to figure out what value of resistor is needed and just how to add it (them) to this circuit..
4. And finally, 1 watt is roughly 10mA at 12 volts (Isn't it?). Will simple 1/4 watt resistors work, or do I need to go for parallel resistors to handle the 1 watt needed for each of these units.

That's right. I'm not an engineer. I've messed around with electronics for 50 years. And I clearly know just enough to be dangerous. (But I'll bet I'm not the only one who frequents this board who fits into that category.) I get that I need to limit the current. It's the details and theory that confuse me. Any help is always appreciated.

Can somebody address this issue? I hate to burn these little guys up.

Bach On
 
1) Yes. + =anode, - = cathode.
2) If LEDs are in series only one resistor per series is needed.
3) Try this site tool:
https://www.electro-tech-online.com/tools/led-series-resistance-calculator.php
4) No. 10mA x 12V = 120mW (for which a 1/4 watt resistor is fine). If the forward voltage is 3.6 (NB this is a typical value and subject to manufacturing tolerance) and the rated power is 1W then the maximum allowed current is 1 / 3.6 = 0.277A = 277mA. But it is never a good idea to run things at their maximum.

Edit: A current of >200mA will almost certainly require a series resistor of > 1/4W rating. Watts = current squared x resistor value.
 
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Thanks. That calculator is a good tool. Does the placement of the resistor in the circuit matter? I mean does it matter if it goes on the anode or cathode side of one of the LEDs? I'd think not.

BO
 
Thanks. That calculator is a good tool. Does the placement of the resistor in the circuit matter? I mean does it matter if it goes on the anode or cathode side of one of the LEDs? I'd think not.

BO

Nope, it doesn't matter in a simple LED circuit like this.
 
Thanks, guys. I redid two of the lights for my outdoor landscaping lights. They aren't as bright, but they do look more natural. I used a 1 watt 100 ohm resistor. (Radio Shack only carries a fraction of the available values. These were as close as I could get there.) After running it for an hour, the resistor and the metal plates for the LEDs were not hot. So that is a good sign. Now, only 10 more to do.

BO
 
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