Power LED Implementation?

[Helsing]

I want to add a power indicator LED to a mic pre design using a +/18v supply.

I have considered the following:

1. taking the feed off of the preregulated +18v line directly after the large filter cap and running the led with an appropriately large resistor.

2. Running the LED from the +/-18v post regulator

3. Adding an LM7805. Can I take the feed to the 7808 right after the large filter caps, in parallel with the 7818, or does it have to be in series "after" the 7818?

Helsing

 

[tmbg]

seems to me you'd be safe enough pulling it off the +18V regulated line, assuming your LED drops 2V, you need to kill 16V with roundabout 20mA of current, so that'd be 16v/0.020A = 800ohms, right?

 

[tmbg]

forgot to mention that you can adjust that for whatever brightness you want... I wouldnt go much above 20mA, but you could probably get away with 5mA, which will be plenty bright enough. 2.2K gets you 7.2mA, which would probably be Just Fine.

 

[Flatpicker]

Depends on the LED and it's application, too. I've got over 100K in series with a bright blue LED on 18V. Any more current and it's too bright. You don't want a power indicating LED too bright or it becomes annoying.

 

[tmbg]

yeah of course... I hope you will sit down and play with the LED in question and find out just how much current it needs to be the intensity you want, then do the math around that.

 

[CJ]

or if you really want to get slick, put in an LDR circuit to control the brightness.