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I'm trying to build a small table lamp with a 1 watt led inside. I've got this working with a matching LED driver, however, this driver is not nicely packaged and would require me to create some kind of additional housing: led driver

I would like to replace this driver with a more typical power adapter. The one you plug into a wall to charge your cellphone.

The problem is, I don't really know how to make that work. I know enough to be dangerous, which is why I'm asking this here.

From what I read on the package, the LED driver is supplying a constant current (300mA). So you can connect up to 3 1 Watt LEDs in series.

A basic USB power adapter is, I think, supplying a constant voltage of 5V. So, using a LED calculator and assuming that the LED is 3.3V (see below). I would need to add a 6.8 Ohm resistor in series.

So my question:

  • Is it as easy as adding a 6.8 Ohm resistor or is there something more I need to worry about?

This is the only info I currently have on the LED (I'm using the top one): enter image description here

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You're right in thinking you can just use a resistor, but I'm not sure how you get 6.8 Ohms. 5V supply with 3.3V across the LED means 1.7V across the resistor. Divide 1.7V by 0.3A and you get 5.6 Ohms.

The power dissipated by the resistor will be 1.7 x 0.3 = 0.51 Watts. It will get slightly warm so I'd observe it under load for a while to make sure this wasn't going to cause a problem.

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  • \$\begingroup\$ Instead of actually thinking about it, I mindlessly put the values in some random online LED resistor calculator. You're absolutely correct about the resistor value (1.7/0.3=5.6) \$\endgroup\$ – Peter Mar 20 '17 at 10:49
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Like everyone else, I'm pretty sure you have, somewhere, a box full of old wall warts which have long been separated from the stuff they once powered.

Grab one of suitable size, open it, throw away the contents, and replace them with your current LED driver. You can get rid of its plastic enclosure and only use its PCB.

Add a wire and connector to your lamp.

You can also purchase such enclosures directly.

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    \$\begingroup\$ That's actually not a bad idea. Sorry I can only accept one answer. Both are very useful \$\endgroup\$ – Peter Mar 20 '17 at 10:48
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Just reducing the AC voltage supply using a resistor may not be enough to light the led lights. You may have to bridge the resistor with a suitable capacitor, a make and correct value to make the lights to work. I am not an expert but had seen circuits the way I described. May someone give the correct type of capacitor and its value for you to try. Aru

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  • \$\begingroup\$ Could this be used to clean voltage peaks? I've done a bit more Google-ing on the matter and the main reason you'd use a constant current transformer is because small changes in forward voltage over the led have big impact on the current going through it: amperor.com/products/led/… \$\endgroup\$ – Peter Mar 28 '17 at 7:22

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