Originally Posted by keiths
Is this OK? When I used the LED resistor calculator at LED center
to calculate the current-limiting resistor it warned me that a supply voltage >24v would dissipate excessive heat through the resistor (LED forward Voltage = 2.5v, Forward Current = 20mA, Supply Voltage = 48v. Resistor value = 2k7 2W)
It's OK as far as it goes, but you don't need 20ma - especially just for a power indicator. The chances are much higher that the LED will burn out over a short time. Plus, as you note - the power rating of the resistor becomes an issue.
You could pick a common-size resistor such as a 5K, then plug it into the Ohm's Law equations:
I = 48V/5000ohms = 9.6ma. P= (0.0096^2) * 5000 = 0.46W,
You'd want to have a 2X safety factor for heat, so a 1W resistor would do fine.
Or, you could go all the way to 10K:
I = 48V/10000ohms = 4.8ma. P= (0.0048^2) * 10000 = 0.23W,
a fairly typical 1/2W resistor would do fine in that case.
4.8ma for a panel-mounted power indicator LED is not at all unusual. For instance, amps such as the PIMETA or Mini3 use power indicator LED's that are down in the ~1-3ma range.