Thread: Electronics (LED) Help

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I could use some help with some electronics I am putting together. I am working on some LED spots. I intend to post them when they are complete. I used an LED calculater referred to elsewhere on HalloweenForum and am satisfied with it.

I am using a 12 volt power source, the LEDs are 1 watt, 3.2-3.6 volt, 300 mA and the LED calculator has me running a 33 ohm 3.0 watt resistor. I hooked it all up and the light is brilliant and wonderful. My problem is the resistor, it is getting hot. I mean, it can burn me if i contact it long enough, and grabbing it would cause a quick scald. (think changing a running light bulb hot).

Can I lose the resistor if I find a transformer that will put out 3.5 volts? The LED won't get hot at 3.5 volts will it? I have done a search for 110 volt AC to 3.5 volt DC transformers/wall warts and I am not having any luck. I would like to run 4 of these spots so I figure that is 1.2 amps, right? Besides losing the resistor by using a 3.5 volt transformer instead of the current 12 volt, where could I find a 3.5 volt DC 1.5 amp wall wart/transformer?

(Is this the type of conundrum Otaku is so good at?)

Hi

The problem is due to the power you need to lose, in this way, you lose it as heat.

One bit of advice would be to get a higher wattage resistor, although doing the same job, it will be physically larger and the heat will dissipate better.
A typical approach is to use aluminimum clad, say 5W.

The other approach is as you say, to drop the power supply, I'd always prefer to have a resistor in circuit with higher wattage leds, the led performance changes with age/use, so better than direct off the supply - also without a resistor, any mains voltage spikes can cause reduced led life.

I'd look for another power supply, preferably regulated (better led life) - you can get loads of phone/mp3 type supplies that give out 5v and 500ma upwards via a usb connector - doing that, you'd need only a 6.8 (or 5.6) ohm resistor, 1W would be fine, it would be losing about 1/2 watt, so much better.

Si

It's really not a good idea to run a LED without a resistor. The resistance of the LED decreases exponentially as the voltage rises, so if you're even a little over voltage the LED will draw way more current than it should. You'll drastically shorten the LEDs lifespan, or worse yet burn it out. Ghoulish Gadgets has the right idea - drop your source voltage down and use the appropriate resistor. A 5 volt power supply with a 5.6 ohm 1 watt resistor would run nice and cool and still give you the full output of the LEDs.

First, I have to say, having some strangers take the time to help you sure can give you the warm fuzzies. Thanks!

Whew, you saved my bacon. I had no inkling that running LEDs without a resistor could lead to problems. I am taking your advice, both of you, and I will look for a 5volt transformer and the 5.6 or 6.8 resistors. It means going back to ordering, disassembling what I have done so far, and starting over but.... hey, at least it will be right and perform for me for a long time. And that will make me happy! Thanks again for helping me out.

I just got done ordering new resistors and a bonafide power supply instead of wall wart. Woopee, Chirstmas is coming in the mail in July! I'll post my results in a how to when I get that far. Thanks again!

Hi,

Happy to help, good luck, looking forward to hearing your results.

Si

Hey, that's how one learns...by burning stuff up!

If you want a nice power supply try an ATX power supply conversion I did one here:
Converting an ATX power supply
I set mine up for all 12volts, but you can set yours up for 5 volts or any combination of the available voltages and it real easy. I posted the link to the websites I used for my conversion give it a try. And don't forget the resistors with your LED's