I have been working on a small linear power supply, capable of regulated output between 2V to 30V, using the LM338 IC. For the most part it is a simple implementation taken straight from the datasheet.
I want to include a "power" LED on the board indicating when the board is live. The LM338 requires the input voltage to be ~5V above the output voltage. This means that the input power may vary depending on usage scenario from approx. 5V to 35V. I'm not sure what the best way to drive a power LED on the board given this wide ranging input voltage.
Apparently the 7805 5V regulator can tolerate up to 35V input, which would provide a 'known' voltage to drive an LED from. However this seems a little overkill/expensive and would probably have heat issues. I don't think I can use a large resistor either as at lower voltages the LED would not illuminate or be very dim? I did wonder about a constant current LED driver but could not find a suitable part.
I would appreciate some design suggestions on how to include a 'power on' LED in my design bearing in mind the wide input voltage range. Thanks
Clarification Edit: My power supply board has a three way header and 2-pin jumper to change the operating mode from either (A) fixed voltage or (B) adjustable voltage. For the fixed voltages, such as 3.33V, 5V, 12V, a piano DIP switch is used in conjunction with various 0.1% or 1% resistors. For the adjustable voltage operation, the jumper connects the LM338 ADJ pin through a 5K linear potentiometer.