Using an LED's power supply as a seperate power source-no?

[ExpAnonColin]

True or false:

It is not possible to use the voltage after an LEDs current limiting resistor as a "starved" part of the circuit because of the LED's connection to ground.

-Colin

[niftydog]

sure... but depending on the LED you use it'll be REALLY starved! The voltage drop accross a LED is in the order of 1 to 4 VDC.

So depending on exactly where the LED is versus the resistor, you'll either get;

just the LED forward voltage drop of between 1 & 4VDC

or

Just what's left over accross the resistor (subracted from your supply volts) say; 9V - [1 to 4] = 5 to 8VDC.

The current loading of that point in the circuit will also effect the voltage across the resistor and hence the brightness of the LED.

[Brian Marshall]

the resistance of the LED will change as the load of what ever you are doing with the other device changes.... which yes, will equal chaning in brightness, but there is also one other problems.  depending on how you set it up... what ever you are loading could back feed voltage to the led, and the device may not like being turned on suddenly when you turn the stomp box on and off.  turning an opamp on and off could lead to some rather loud pops.

[R.G.]

True or false:

It is not possible to use the voltage after an LEDs current limiting resistor as a "starved" part of the circuit because of the LED's connection to ground.

??
No, it is not false that it is not impossible to do that...

Any component that has a fairly constant voltage over a range of currents may be used as a shunt regulator. This includes
- the forward voltage of all semiconductor junctions: germanium at 0.2 to 0.3V, silicon at 0.5 to 0.7, LEDs of various colors at their characteristic forward voltages, silicon carbide (!) at its 3.5-5V...
- the reverse voltage of an device that has a constant breakover voltage, including but not limited to: germanium diodes, silicon diodes, transistor junctions of all stripes, neon and other gas tubes, etc.

Note that a connection to ground is not necessary. You can use the voltage across the diode(s) as a floating power supply even if one side is not at ground, although that imposes the ground voltage change on any signal that you get out if it.

Stacked diodes and zeners are commonly used as DC level shifters in the internals of ICs.

[ExpAnonColin]

Thanks guys.

-Colin