Want to power LEDs with outlet. Help understanding current.

[guy121]

So here's the thing. I want to power some Infrared LEDs with a power adapter for my Wii.

I understand that a 12VDC power adapter divided by 6 LED's will have 2V going to each LED. Might add two more to make it 1.5V.

Lets say the IR LEDs are rated for 100mA. My question becomes, do I need a 12VDC 100mA adapter, or can I use a 12VDC 600mA adapter? I know that the current use does not change with more LEDs attached, but does that mean that the LEDs will only draw 100mA, or is it the adapter that is giving 100mA.

I'm thinking is it similar to watts, where you can put a 65 watt lightbulb in a light socket and it will only draw 65 watts. Or is it that if the adapter is rated for 600mA then the current through all the LEDs will be 600mA?

 

[TigerhawkT3]

Yep. The current rating on a wall wart is its max, and the device will pull whatever it needs - just like a light bulb socket, as you guessed.

 

[guy121]

Awesome! Thanks! I've been spending hours trying to find that out.

I was also wondering how does it work when the LED states that it can run at 100mA free-air, 133mA mounted?

How is the current regulated then?

 

[KrisP]

It's still a good idea to put a resistor in line with the LED string to limit the current. As LEDs warm up, they draw more current, and the more current they draw, the warmer they get... In the end you can get thermal runaway which will cook your LEDs, so it's just safer to use a current limiting resistor.

Here are a few calculators to determine what value resistor to use:
http://led.linear1.org/led.wiz
http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator
http://ledcalc.com/

 

[guy121]

:twothumbsThanks for the help!