I have an LED light and it would be perfect for my LNL AP press my problem is that I'm not sure if a wall wart will mess it up or do I need to ger a 12 V LED driver? This is all the info I have on the bulb:
3W 6000K White LED. I am not sure how miliamps it requires.
The wall wart I have puts out 12 V DC at
500 mA. I tried it on one and it burned out the LED after a few minutes so I think it is over amped.
Hope someone can help.

Steve........

You will be drawing .25 Amps. ( 1/4 ) your power supply is 1/2 amp.
Make sure your LED is rated for 12 VDC and not 3 or 5 VDC.
If it is a lower voltage let me know and I will tell you how to drop the voltage. Many ways to do it.

I did find some more info on the bulb, it is:
12 Volt
3 Watt
0.8A
Does it look like the wall wart I have will be ok to run this LED?


Steve.........

quote:

Originally posted by Steve E.:
I did find some more info on the bulb, it is:
12 Volt
3 Watt
0.8A
Does it look like the wall wart I have will be ok to run this LED?

Steve.........

A white LED drops about 3.8 volts at its rated current. If it's rated for 12 volts, the enclosure might contain three of them in series, or one of them with a current-limiting resistor that drops 8 volts at 0.8A. The latter case is a fair amount of power dissipation, so I'd guess that it's three of them in series.

An unregulated wall wart's output will be 12 volts at its rated current, and something more like 16-18 volts with a lighter load. A regulated wall wart will be 12 volts for any load up to its rated output, and will cost more.

About the math. Power is voltage times current. Something about the numbers isn't right. The current through a 12-volt device dissipating 3 watts should be 0.25 amp. If it really draws 0.8 amp, it is dissipating close to 10 watts.

This is the case for some Chinese LED lamps that I've bought through ebay. I've also bought some that draw 0.08 amp, about 1 watt, and they are used with a standard 12 volt 8 amp-hour sealed=lead-acid battery. A friend of mine is using 3-watt 12-volt LEDs for a night-light, they draw 0.25 amp without a current-limiting resistor.

This is an LED from E-Bay. How would I tell if the wall wart is regulated? I believe it is one that came with a spotlight I had at one time. The typical little black square device that plugs into the wall.
The information for it is:

AC/DC Adapter
Class 2 Power Supply
Model: LF12500D-41
Input: 120V AC 60 Hz
Output: 12V DC 500mA


Steve.....

quote:

Originally posted by Steve E.:
This is an LED from E-Bay. How would I tell if the wall wart is regulated? I believe it is one that came with a spotlight I had at one time. The typical little black square device that plugs into the wall.
The information for it is:

AC/DC Adapter
Class 2 Power Supply
Model: LF12500D-41
Input: 120V AC 60 Hz
Output: 12V DC 500mA


Steve.....

There is a little symbol that identifies a regulated wall wart, not sure how to describe it. If it doesn't say anything about being regulated on the back, it probably isn't.

You can find out quick with a voltmeter. With no load, it will either be 12 volts (regulated) or something like 16-19 volts (unregulated). Most likely it is unregulated, and it will take a 500 mA load to bring the output voltage down to 12 volts.

Thanks for the info, it doesn't even hint at being regulated so I won't use it. I hooked it to a 12V LED a while back and it worked for about 10 minutes and then the LED heated up and quit working. In your opinion would it be better to use one with a lower mA value such as a 200mA?

Steve......

quote:

Originally posted by Steve E.:
Thanks for the info, it doesn't even hint at being regulated so I won't use it. I hooked it to a 12V LED a while back and it worked for about 10 minutes and then the LED heated up and quit working. In your opinion would it be better to use one with a lower mA value such as a 200mA?

Steve......

If it's really rated for 12 volts/3 watts, it's rated for 250 mA, and a 12 volt, 200 mA wall wart should be about right.
Alternatively, you could solder a 100-ohm 2 watt resistor between the wall wart and the LED and just limit the current from the 12 volt, 500 mA wall transformer.

I checked the unloaded voltage of the wall wart I have and it was putting out 19.58 volts, no wonder it fried out the other 12V LED.

Steve...........