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Coilgun back-emf trouble

Discussion in 'General Electronics Discussion' started by paulbohm, May 24, 2021.

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  1. paulbohm


    May 24, 2021
    Hello everyone!

    I'm building a simple multistage coilgun, directly powered by some Li-Po batteries. The coilgun works by detecting a projectile in front of the coil, turning on a MOSFET, which lets current flow from the 48V battery through the coil. When the projectile leaves the photogate, the MOSFET is turned off, and the coil is disconnected from the battery.

    The trouble I'm facing is that the MOSFET blows up the moment the projectile leaves the photogate, due to what I think is back-emf from the coil. Since the MOSFET is rated for 55V and 100A, I suspect that it blows up because of some kind of reverse voltage spike. I have tried countless things to prevent this back-emf, such as varistors or silicone diodes but nothing has worked.

    The latest attempt (seen below) involved a beefy schottkey diode, which I used for its fast switching speed. I reasoned that the previously used silicone diodes weren't fast enough to keep up with the fast switching speed of the MOSFET to prevent a voltage spike, which is why I tried schottkey diodes. However, this didn't change the outcome either. Even after soldering in 6 of these diodes in parallel, the MOSFET still blew up.


    If anyone has any suggestions on how to prevent anymore MOSFET casualties, please let me know. For those of you who are interested, I had previously also made some experiments to visalize this back-emf here. In the video, I also optimize the coils I plan on using, a project I had to film for my bachelor.

    Anyway, here is what one module looks like soldered. The coil didn't fit in the picture, so I drew it in orange.


    Lastly, any suggestions to improve my cicuit to make it more compact or draw less current (one module draws about 7mA in idle!) would be greatly appreciated. I am quite new to electronic cicuits like these, so I'm sure I've made some mistakes and redundancies in the design (especially for the NOT-gate).

    Thanks in advance!
  2. Nanren888


    Nov 8, 2015
    Maybe look for the site guidelines. From memory projectile weapons might not fit with this site.
  3. bertus

    bertus Moderator

    Nov 8, 2019

    From terms and rules:

    Some topics may be considered inappropriate, unsafe, unethical or offensive. We are likely to moderate content which strays into following areas (this list is not exhaustive):

    - Weapons
    - Spy tools
    - Hacking/Piracy
    - Personal or public risk
    - Offensive or threatening behaviour
    - Spam
    - General off-topicness
    - Pseudo-science

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