Is my zener diode labeled wrong or am i seeing a strange failure mode


Posted on Feb 5, 2014

A power supply circuit to bring 24VAC down to 5VDC. I have an L7805CV, but rectified (a bridge) and smoothed (a 33 F electrolytic), the input is still $40V_{peak}$, which is more than the max input voltage the regulator can take. Looking at the regulator`s datasheet, it suggests the following `High Input Voltage Circuit`, which notes `$V_{in} = V_i - (V_Z + V_{BE})$`, but doesn`t offer any guidance on Q1 or R1: because


Is my zener diode labeled wrong or am i seeing a strange failure mode
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when i orient it as indicated, with the striped cathode end toward $V_i$, the voltage at $V_{in}$ is the full 40V minus approx. 2 diode-drops and the voltage across the zener is only about 0. 7V. (Note that at this point, i have a 33k resistor to ground in place of the 7805 so i don`t damage it until i get the limiter right. ) It is possible that it`s really just marked wrong I made some mistakes wiring it up the first time and something let out a wisp of smoke before i cut the power, but i wasn`t able to figure out which component, and everything i`ve been able to test measures okay. Could overcurrent have somehow magically reversed this diode




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