Would a LED turn on only at voltages above Vz + Vf + Vdrop if hooked in series ? (Vdrop for the series current-limiting resistor)
Have not seen that in circuits...
I used it in a circuit I designed for an audio amp. I kinda wanted all of the rails (4 of them) to be within 2/3 of 50V, but the 2/3 didn't have to be anywhere near exact,
The design did exactly what it was supposed to do, but the design could have been better. If any rail fails to be >2/3 Vcc my line current limiting resistor will fry. During start-up audio is removed by an optoisolator and the speakers are disconnected. That resistor and 40,000 uF of capacitance determines the turn-on delay.
If a rail fuse pops, that fusible resistor pops too.
No method was employed to remove power if (so many seconds passed by on start-up) AND (immediately after a Power Good signal) AND any supply was <2/3 of the final value was implemented along with "some light".
The home made AMP didn't actually have a power switch either.
I accidentally reversed the PNP and NPN output transistors. The only damage was that resistor that's supposed to pop.
Now, there was ONE ISSUE: The opto's would break until a ZNR was placed across each capacitor. There was a 12 V logic supply that came up immediately. I had a relay that shorted the RESISTOR, but no means to disconnect the AC line power totally. The amps "switch" was external and was a triac built into a "fancy timer".