Something simple for you, but not me. Please help.

[billbyrd1945]

I decided to take BrokenYugo's advice and order one of these: https://www.ebay.com/itm/122604065816
Obviously, I don't know what the hell I'm doing and didn't know which watt to order and didn't know which 'k' value to order. I finally decided to order the 3w with 'cold white' which had the highest number 'k' value.
Would one of you be so kind as to tell me how to power it? I'd like to use USB if possible, but a battery pack would be just fine if that's what you think I need. And I'd also need some advice on which resistor if I need one.
Thank you again for all the explanations.

[TimFox]

In color measurement, "K" (majuscule) means the color temperature in Kelvins, which is the temperature of a black-body radiator that (at least roughly) corresponds to the spectrum of the light source.
Daylight is normally considered 5500 K.  Old-fashioned incandescent lights are roughly 2400 to 3200 K.  A common misnomer is to call that "warm" because it is reddish or yellowish, although the color temperature is lower.
When you get above 5500 K, the color appears blueish, misnamed "cool".
see:  https://en.wikipedia.org/wiki/Color_temperature  for an elementary discussion.
The best choice depends on your application.  This is rarely critical, except in lighting for color photography (film or digital).

[tunk]

A resistor in series with the LED.
You need to drop 1.8V (5-3.2) over the resistor and with 0.7A
that's a 2.6 ohm resistor (1.8/0.7), and it has to be at least
a 1.26W resistor (1.8*0.7). Or use a step-down converter.
And the LED needs to be mounted on a heatsink.

Edit: Was it with or without a PCB?
It should be easier to mount it on a heatsink with a PCB.
You could also run it at a lower wattage to increase the
life time (by using a higher value resistor).

[billbyrd1945]

Thank you. Precisely the kind of info I was looking for. I got it WITH the PCB. I think you're saying that the PCB alone will not provide the heat sink, that I need to do that myself. If so-- you got any suggestions for where I can get one and a link if I'm not asking too much? Also-- I wonder (I'll soon know) if the PCB option includes the needed resistor.

[tooki]

No, since the resistor value depends on what current you want to run it at and what supply voltage you have — and that’s assuming you’re not using a constant-current driver, which is common with power LEDs.

[billbyrd1945]

Thanks for all the info. Regrettably, I think I'm just going to have to scrap the whole project. The bulk of a 2w resistor and a heat sink simply won't allow me to maintain the compactness of the project. And it was looking so good.

[IanB]

You don't necessarily need to run the LED at 700 mA. That would be pushing it to the limits. If you run it at a lower current like 100 mA or 200 mA it will still be plenty bright, and it will need a smaller resistor and less heat sinking. You can tell what kind of heat sink you need by feeling how hot the PCB gets that the LED is mounted on. If it doesn't get too hot to touch it is probably OK without extra cooling.

[billbyrd1945]

Outstanding! That's great news! And I'm just now getting my electronics workstation all set up so that I can do testing for brightness and heat. Thank you!