Detecting a load on a power source without modifying voltage

[izua]

I'm trying to build a simple ±12 (and switchable to 24)V power source for use in audio apps. Just as an experiment, I'm trying to make a led light up when I power something from it. I'm not sure how to do this. I was thinking of connecting over one power line a resistor, and on the resistor more positive supply, a base of a NPN transistor, on the other end, the transistor's emitter.

This way, when current will be applied, it will go through the resistor and there will also be a potential between Base and Emitter, allowing current to pass from the collector to the emitter, and with the led connected in the emitter, it would theorethically light up.

Not sure it will work, though. Any ideas?
thanks

 

[VVV]

Re: Detecting a load on a power source without modifying vol

You got the right idea, but a simple transistor needs too much voltage to work (≈0.6V). That means a large voltage loss.
Use a low-resistance shunt and monitor the voltage across it with an opamp or comparator. That will work with just a few mV across the shunt.

 

[melc]

Re: Detecting a load on a power source without modifying vol

Put the LED in series with R3