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I know this subject has been covered but I have a specific question regarding zener characteristics for microcontroller GPIO protection in vehicles where nominal voltage could be 12 or 24VDC. CircuitLab asks for Rs and Iz. I wanted to use as small a package as possible for space consideration but 100mW packages advertise a dynamic impedance of 100ohms or more. Running the DC sweep simulator made the voltage at Vout easily rise well above 5V. With the 220mW package, the impedance is advertised at 19ohms. Running the simulator shows me good up to 60+ volts which is my goal. I feel like the simulator can't really function properly without a more detailed model and that the smaller 100mW package would probably be just fine? Would appreciate any thoughts on this. This circuit is more or less based on this Digikey white paper. I didn't feel the input clipping diode to be worth the space it took so I omitted it.

GPIO Protection Circuit

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  • \$\begingroup\$ Is a sot-23-3 too big for the clipping? These are in the BAT54 family of parts and also the BAV99. (I'm sure if you look someone has them in the size of a tiny salt crystal somewhere.) I'm sure there are shottky pairs in the same form factor. \$\endgroup\$ – jonk Mar 3 '18 at 4:25
  • \$\begingroup\$ "...as small a package as possible for space consideration" - so are you making all the other components as small as possible too? Good protection is important, and deserves some space. The Zener in your schematic looks like a good choice. \$\endgroup\$ – Bruce Abbott Mar 3 '18 at 5:42
  • \$\begingroup\$ @Bruce I agree good protection deserves space. My go-to device for multiple inputs is the MC33972. This is a wonderful device. In this case though, it is an OEM application where i need to add to extra inputs to an existing form factor. \$\endgroup\$ – john8791 Mar 3 '18 at 15:42
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Running the DC sweep simulator made the voltage at Vout easily rise well above 5V.

The MMBZ zener diode is rated at 4.7 volts at 20 mA. At this current there is 19 ohms of resistance hence you can model the zener as a 4.32 volt device in series with 19 ohms and get a reasonable result. The 4.32 volts is as a result of reducing 4.7 volts by the volt drop across the 19 ohm resistor at 20 mA.

However, given that you cannot drive more than about 5 mA into the zener (24 volts/4.7 kohm) you will never exceed 4.7 volts.

I feel like the simulator can't really function properly without a more detailed model and that the smaller 100mW package would probably be just fine? Would appreciate any thoughts on this.

Without a link to this device, darkness reigns over this question.

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  • \$\begingroup\$ I chose 4.7K to stay well under the 1/4W of my resistor rating in a 24V application where charging voltages can reach 32V. \$\endgroup\$ – john8791 Mar 3 '18 at 15:52
  • \$\begingroup\$ @john8791 very sensible. \$\endgroup\$ – Andy aka Mar 3 '18 at 15:57

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