Electronics > Projects, Designs, and Technical Stuff
How slow are zener diodes?
StillTrying:
If I go to 3A for 10ns I can get close, after the clamping the zenner supplies 0.8A back into the source. :o
duak:
Some info on Zener diodes: https://www.onsemi.cn/PowerSolutions/document/HBD854-D.PDF
SiliconWizard:
--- Quote from: StillTrying on April 30, 2020, 04:12:09 pm ---If I go to 3A for 10ns I can get close, after the clamping the zenner supplies 0.8A back into the source. :o
--- End quote ---
Well yeah - isn't that just because of the diode's capacitance? When Vin gets back to 0V, you're actually discharging this capacitance.
graybeard:
The spike at the beginning is the charge emptying out of the transistor base. It is probably a lower impedance path than the diodes. Once the charge has emptied out of the BJT the diodes turn fully on.
free_electron:
You are reverse biassing the b-e junction. This will "poison" the transistor over time and trap electrons in the junction. The gain will collapse and the transistor will die. this process is irreversible ( unless you cook the transistor at high temperatures ( like 200 degrees + ) to knock the trapped electrons loose.
NEVER-ever let a b-e junction see a reverse voltage larger than about 6 volts.
Your zener construction will not work. zeners are slow. They can have a capacitance of 1 to 10 nanofarads. ( as reverse voltage increases , the 'plates' get pushed closer , so capacitance increases )
use a stack of silicon diodes. Two or three fast silicon diodes will work wonders and the reverse b-e will not go above 2 volts.
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