I'm a total NOOB, but I am teaching myself electronics while in retirement. I want to change the color of the "Power ON" LED in my mod of an ATX-PSU, but I don't know the characteristics of the LED. Current to the LED is controlled by a Fairchild C1815 NPN transistor. Can I use the C1815 data sheet to ferret out the LED's numbers?
That said: there is usually significant latitude when swapping one LED for another that has vaguely similar characteristics. Most LEDs are supplied with a voltage source that is significantly higher than the LED forward voltage. Changing the LED to a different color or package will most likely work just fine because a small change in the forward voltage won't affect the LED current noticeably.
Usually, that is.
The easiest way to tackle this project is to just do it and observe the results.
The transistor will usually be used as a switch. There should also be a resistor in series swith the LED which will limit the current.
You can determine the current used by the LED by measuring the voltage across that resistor, and its value - then calculate the current using Ohm's Law.
For common small LEDs, you can look at any distributor's website to find the "usual" maximum current and forward voltage rating for each colour of LED - but LEDs used as indicator lights are usually operated a much less than the maximum current - perhaps 5 - 10 mA where the Absolute Maximum current in a datasheet will be 25 - 30 mA.
Can I use the C1815 data sheet to ferret out the LED's numbers?
No you can't and you also need to (more than likely) know the value of the current limiting resistor used so the steps are: -
Measure the open circuit voltage (when supposedly on) and this means disconnecting the LED. Then measure the current flowing into the LED. And to be absolutely sure, you need to establish if the LED is being driven from a full-wave rectified supply that is current limited by the resistor because this makes a difference to how you choose the LED.
Of course you can just swap it out and hope for the best but where's the learning in that?