5 volt to 3.3 volt

[indulis]

Audioguru

While you didn't exactly answer the question directly, your reply kinda has the answer if you "read between the lines".  OK, now let gets back to why you think  "dynamic impedance " comes into play in this particular circuit, as in your reply, you basically stated that the circuit DOESN'T have "any" (virtually non-existent) dynamic properties? Now if the regulated 5V feeding the circuit wasn't so regulated... that would be a different story as there would now be some "dynamic" circuit behavior.

I misspoke earlier in regards to not using the series diode with zeners below 5.1V. The diode also does a "good" thing!! The diode in series with the zener diode has the effect of cancelling changes due to Vbe changes with  temperature. 

 

[audioguru2]

I misspoke earlier in regards to not using the series diode with zeners below 5.1V. The diode also does a "good" thing!! The diode in series with the zener diode has the effect of cancelling changes due to Vbe changes with

 

[indulis]

Audioguru

First you said

A 3.3V zener diode doesn't regulate very well. A 6.8V zener diode regulates 11.5 times better...

Then you brought "dynamic impedance " into the picture for some reason (NEVER applied to this circuit) and now your saying...

The zener diode isn't regulating anything because the input voltage is regulated...

If the zener isn't regulating anything, then what is controlling the output voltage??? Whether the load is changing or not in this circuit is irrelevant. The point was that because the input voltage was regulated and the typical Hfe of a small signal transitor is high enough to not affect the the current in the zener current limit resistor, ergo no dynamic behavior!!!!

 

[audioguru2]

The 2nd reply was from Gazza who recommended a simple voltage divider.
The next reply was from me adding an emitter-follower transistor to the voltage divider.
Another reply from somebody recommended a lousy zener diode that is not needed and it was tested to perform wrong.

 

[mvs sarma]

The 2nd reply was from Gazza who recommended a simple voltage divider.
The next reply was from me adding an emitter-follower transistor to the voltage divider.
Another reply from somebody recommended a lousy zener diode that is not needed and it was tested to perform wrong.

Hi Audioguru,

We regard you very high for the strog fundas you have and demonstrate. while so we feel some times let down by expressions

 

[indulis]

John, John, John

 

[audioguru2]

Indulis,
I showed and explained why a low voltage zener diode makes a lousy regulator.
IF the input voltage changes (but it doesn't here), and IF the load current changes (we didn't know that it doesn't here) then it is a lousy regulator when the temperature changes because its voltage changes.
Also, IF the load current drops then a zener diode wastes current.