LED as reference voltage

[kingswayguitar]

I recently saw a thread regarding a 1.5V fuzz.  If you scroll a little bit down to The Fuzz E-One Small Bear Clone - Schematic at

htt ps smallbearelec dot com/Projects/FuzzE-One/FuzzE-One dot htm

you see a 9V supply that feeds a node where one branch has a resistor and 3 diodes (D2,D3,D4) hung in parallel with a 1.7V output.  Coincidentally, I do this on my test bench with small transformers to get a fairly quick and steady"ish" 9V from a 10-18V output.  I use a few LEDs that drop the required voltage but they suck up 30-50mA.  Am I correct in assuming this is still more efficient than using a 78L09 that only needs to drop 1-3V on average after a 1N4001 bridge rectifier.  Aren't they less efficient at that dropout?

thanks

[R.G.]

you see a 9V supply that feeds a node where one branch has a resistor and 3 diodes (D2,D3,D4) hung in parallel with a 1.7V output.  Coincidentally, I do this on my test bench with small transformers to get a fairly quick and steady"ish" 9V from a 10-18V output.  I use a few LEDs that drop the required voltage but they suck up 30-50mA.  Am I correct in assuming this is still more efficient than using a 78L09 that only needs to drop 1-3V on average after a 1N4001 bridge rectifier.  Aren't they less efficient at that dropout?

Regulating devices which are in parallel with the load are shunt regulators; they "eat" a variable amount of current to keep the voltage across them constant(ish). They require dropping resistors in series with them to limit the maximum current they get, and burn up if they don't have some kind of series dropping resistor or other current limiter.

This is in contrast to series regulators like the 7800 series, which regulate by only letting through enough current to keep the voltage on their output at the right level.

Shunt regulators are most efficient when nearly all the current is going into the load. Their regulation is worst there, especially for simple shunt devices like diodes. Shunt regulators are least efficient at light loads where the shunt element is forces to conduct the full available current and the load isn't taking any. At zero load, they still produce the right voltage, but get hottest.

Series regulators are most efficient when they have the least voltage across the regulating element, and/or have very light loads. Series regulators have a minimum voltage that they need to be able to regulate; this "dropout voltage" is where there's not enough voltage for them to continue to make the output constant any more.

So with that as background, no, the LEDs are not more efficient than a series regulator in this instance. Series regulators are most efficient very near dropout and at light loads. Shunt regulators are least efficient at light loads, and as you noted, the LEDs are pulling 30-50ma all the time, load or not.

[Earthscum]

So circuits like the Tytewad and my Lowaht circuit, that are meant to use very little power, if regulated would continue to be most efficient with series regulation over shunt regulation, correct... Would limiting the current to the shunt to say, twice what the circuit consumes at max, make the shunt regulator any more efficient than series (phone won't do question marks, you'll have to imagine them).

[kingswayguitar]

Good question Earthscum.  My 30-50ma was only limited by a 39ohm resistor parallel to a 220uF capacitor for ripple.  And thanks for the insight R.G.  I guess I had it backwards...good regulation (which I can attest to) but inefficient use of current.