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Hello everybody... I need the electronic explanation of Zener diode.... I researched the Zener Diode I understand how it work .... But still don't understand the relationship with the zener with tip120, what is the function of the zener diode in this circuit with the TIP120? Thanks!

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The Darlington will have two diode drops from the NANO output logic level and thus the Vmax is reduced by 1.2 to 1.4V but it has a very low output impedance.

The Zener normally has no effect except when inductive loads are switched off , then the diode side conducts and clamps the out to 12+0.7V and if the power is turned OFF when the output is high, it will result in a negative kickback clamped by the Zener side conducting at 12-Vz. to prevent negative pulses. It could have been done with 2 diodes instead, one to each rail.

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  • \$\begingroup\$ Oh thank you... So the gauge is considered a inductive load? didnt know that... So the function of zener is protect the negative pulse after the power off, to protect the......... Arduino pin or tip120 or both? Thank you \$\endgroup\$
    – Mangostain
    Commented May 16, 2020 at 23:13
  • \$\begingroup\$ Usually it's just the wire ... which is around 1 nH/mm or 1uH /m forward and return may be much less using a chassis. To protect the TIP and load. Although it violates the Automotive specs if -14V reverse charge test were applied (not sure) \$\endgroup\$
    – D.A.S.
    Commented May 16, 2020 at 23:33
  • \$\begingroup\$ Yes Diode for Low side switch and Zener for power cutout when high, to clamp voltages and protect the "bus wire". \$\endgroup\$
    – D.A.S.
    Commented May 16, 2020 at 23:43

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