I've been reading about resistors vs. voltage regulators, but can't settle on which would be better in my case. I have a power supply for a mic preamp that has 15V and 48V. I want to add an LED to the power switch. Would it be best/most efficient to use an appropriate 1/2 Watt resistor, or a voltage regulator? I have both so I'm not worried about cost, but which is the "right" way to do something simple like this? The regulator I have on hand is the LM317T. I also have a 7805CV, and I think any of the 3 options will work fine with the 15V, but if someone could briefly explain which works best here or point me to some literature on the topic, I would appreciate it!
I want to add an LED to the power switch.
Any linear regulator or any resistor will "lose" the same amount of power when dropping a higher voltage to a lower voltage. If the input voltage is 15V and the output voltage is 2V then the power dissipated as heat is 13V x current. If the current is 20mA (standard red LED) then the power given off as heat is 260 mW. If you are happy to dissipate that power to activate a LED then a resistor is by far the superior choice because it current limits the LED naturally.
What are the characteristics of the 15V supply? is it a nice stable 15V or does it vary? If the supply voltage is unstable is corresponding instability in the brightness of the LED a good thing (because it indicates a problem to the operator)? a bad thing (because it's a distraction to the operator)? or something you don't care about?
The resistor solution is simpler. The linear regulator solution isolates the LED from variations in the supply voltage which may or may not be a good thing.
The linear regulator solution and the resistor solution are both very inefficient (approximately equally so). The efficient solution would be a switched mode LED driver but that comes at a significant cost in terms of extra complexity and is probablly not worth it for a low current indicator LED.
Resistors Vs Regulators ...They waste the same amount of power as does an analog constant current source .The expected efficiency is 3/15 or 20% of the options so far.In fact this will make your LED less efficient than the incandescant lamp of last century.If efficiency doesnt bother you then stick to the resistor .If you think a SMPS is an overkill you are probably right.Well you have volts to burn in your circuit so place the LED in series with something that doesnt mind losing 3V and already has a sensible LED current already flowing .Double check that there wont be big current surges and now you have actually powered the LED for free .I have used the input of a lightly loaded 7805 so the idle current + the load current runs the LED .Remember that the human eye responds in a logarithmic way to brightness just like the ear registers sound .This means that the current doesnt have to be precise.