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Maybe this question is non-sense. If I have a commom 12V relay (12V, 400 ohms coil) WITHOUT having a flyback diode connected to it, and driven by a low-side protected transistor/IC like ZXMS6004FF (the internal over-voltage protection would do the diode job). The datasheet of the transistor shows it withstands to a 90mJ "Unclamped Single Pulse Inductive Energy".

This is for a very specific project question. Would the transistor get damaged?

Datasheet of ZXMS6004FF: https://www.diodes.com/assets/Datasheets/ZXMS6004FF.pdf

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You need to know the inductance of the relay coil. Then you can calculate the total energy of the coil by \$ E= \frac {1}{2}LI^2 \$ and compare that to 90 mJ.

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est.L=4H +/-50%, I=12/400=0.03A \$ E= \frac {1}{2}LI^2 \$ =1.8mJ +/-50% seems well under 90mJ

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  • \$\begingroup\$ only that 1.8mJ? So the transistor will be ok! \$\endgroup\$ – abomin3v3l Feb 2 at 10:38
  • \$\begingroup\$ @abomin3v3l Don't jump to comclusions, he estimated L to be 4 H, even if it would need to be considerably greater to do damage. Maybe the word should be emphasized so that a quick glance cannot be tricked. \$\endgroup\$ – a concerned citizen Feb 2 at 22:24
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    \$\begingroup\$ Yes, it will be OK and will electrically switch at a much faster rate than with a Vdd diode clamp for dV/dt. The 50% was an estimated tolerance on my estimate. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 3 at 1:26

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