I am analysing a pulse transformer which is required to transfer below communication protocol pulse from primary to the secondary side. The pulse duration is 32 μs and off time available to reset the core is just 7 μs. Calculation shows that 22.85 V are required to reset the core within 7 μs.

When -22.85 V is applied across TTL logic pins like microcontroller, they will get damaged. For simulation, I used 33 V Zener and 1N4148 blocking diode.

How to isolate this large kick back voltage (-22.85 V) of magnetizing inductance from microcontroller?

Is there any way to reset a core without having GND terminal pulled towards negative voltage?

Pulse information

Pulse transformer schematic

Primary side Vtg waveform

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    \$\begingroup\$ A diode? Also, it's us, seconds, not uS, Siemens. And you can write the coupling as 1 since LTspice, behind the scene, uses that float value, anyway (it's not fully 1). \$\endgroup\$ Commented Sep 1, 2022 at 13:52
  • \$\begingroup\$ @aconcernedcitizen Technically, it's μs, not us. \$\endgroup\$
    – Hearth
    Commented Sep 1, 2022 at 13:59
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    \$\begingroup\$ It is OK to use "us" but not \$\mu S\$ which are Siemens or uS, , it's a common misteak ;) \$\endgroup\$ Commented Sep 1, 2022 at 14:10
  • \$\begingroup\$ ... when used with a value. But without a number "us" just looks like "some people" or a slang abbreviation of the USA. \$\endgroup\$ Commented Sep 1, 2022 at 14:16
  • \$\begingroup\$ @Hearth Well, yes, but I thought that there's no need to be too pedantic about it in this case, it was the message that was important. \$\endgroup\$ Commented Sep 1, 2022 at 18:29

1 Answer 1


Replace the SPICE switch with a transistor switch. Probably move it to the low (GND) side as well.

Effectively a duplicate of: Zener Diode Selection for Relay but the different applications (relay / solenoid / pulse transformer / gate drive / etc.) need enumerating.

  • \$\begingroup\$ mentioned thread is about the calculation of power rating of zener. My question is not about the rating but totally different. \$\endgroup\$
    – Ronnie
    Commented Sep 1, 2022 at 17:58

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