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I have been working on a project controlling a coil to attract a metal ball at high speed. I measure the speed of the ball in front of the coil and then decide how fast I want the ball to accelerate because the speed it needs to have when it leaves the coil is about 10 m/s. I found out I have about 9 miliseconds to use the coil. So what I was thinking is i'll use a MOSFET and a pwm signal in this way i can change the duty cycle and then control the coil. But I read on a site if I want this to work I need to switch high side and low side because else the coil still will be magnetic. now is my question: does somebody know what they mean or what I need and how does is work since I haven't got much experience with coils.

Specs: PWM signal : 3.3v 10Khz Coil: 51v 16A

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  • \$\begingroup\$ Edited for spelling, grammar, and clarity. \$\endgroup\$ – R Drast Oct 23 '15 at 10:34
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What they meant is an inductor has the nasty habit that it wants to let the current continue its path in the same direction even when you turned it off. (storing of energy). You need to provide a feedback path for the current to flow. If you don't the current might reverse (only direction it can go) and will eventually destroy your mosfet.

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Every time your coil is switched off you cause the magnetic field to collapse, inducing current and a voltage spike in the process. To eliminate that, you need a way to safely divert this voltage spike so it doesn't pop your MOSFET by applying an damaging reverse voltage. Use of a flyback or snubber diode, which is placed in parallel to your inductive load will fix this. In your case, parallel to the coil. If you want to know more, here is a good tutorial on flyback diodes and how to place them: http://www.douglaskrantz.com/Flyback_Diode.html

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