Powering a single LED

Discussion in 'The Projects Forum' started by ptaylorc, Mar 25, 2010.

1. ptaylorc Thread Starter New Member

Mar 25, 2010
2
0
I'm working on a project where I need a pinpoint source of bright white light that will be continuously on, so it needs to be plugged into a wall socket. I don't have any electrical knowledge, hopefully someone here can give me advice on the easiest/cheapest way to do this.

I see there are LED lights on ebay (such as this one: http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=180485485402#ht_2853wt_941)) and I have a few extra power adapters lying around. Can I just connect the power adapter to such an LED and plug it into the wall?

The LED lights I'm seeing have specs like this, but probably vary a lot:
3.5V~4.0V or 3.8~4.5V, etc
700mAh
3Watts

One adapter I have from an old cellphone has this output:
5.0V, 0.7A, no word on watts.

What would I need to do in order to power the LED light? Can I just attach the wires from the adapter to the contacts on the LED? If I'm thinking about this the wrong way, any advice is greatly appreciated.

Thank you!
Paul

2. R!f@@ AAC Fanatic!

Apr 2, 2009
8,792
771
and it is perfect for that led you posted, but you will need a current limiting resistor to protect the led

Mar 24, 2008
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4. ptaylorc Thread Starter New Member

Mar 25, 2010
2
0
I had looked at that posting, but it's a bit above my level. So that's perfect that the adapter I already have will work! Can I ask for one last hint: what strength of resister do I need?

I see Ohm's Law is R = V/I, so... I guess I = 0.7 (amps), and is V = 4 (the adapter) or 3.5 (the LED) or some combination?

Thanks, and sorry for the lack of knowledge on my part.

5. Audioguru New Member

Dec 20, 2007
9,411
896
700mA is the absolute max allowed current for the LED when it is cooled properly.
It will need to be bolted to a pretty big heatsink that is in free circulating air.

6. R!f@@ AAC Fanatic!

Apr 2, 2009
8,792
771
The resistor is used to limit the current which the led can handle and by that drops the excessive voltage and keeps the voltage at the led at it's rated value.
So a resistor value is calculated after the led and the supply is chosen