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Reverse engineering what appears to be a zener diode network, unusual design?

David0101

May 10, 2020
4
Joined
May 10, 2020
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I've been playing around with this digital clock, which uses this network to do an interrupt on the MCU to turn on the LEDs when the wall wart is plugged in. It goes dark, and uses a cr2032 to keep the time when unplugged. Is this an unusual design for the zeners? I'm assuming those are zeners...

The clock is from probably 2011, and uses a full bridge to take in 6v to 9v ac or dc. So this network could see from about 5.5 to 18v. I've modded input voltage to allow 5v from a usb, and piggy backed a 1k resistor on r38 so it gets the 3.3v out still and works fine.

Attached are the pcb and the schematic from my tracing of the circuit.
 

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  • 2022-09-06_14-24-34 (1).png
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  • zenernetwork.png
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