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I'm trying to control a (cd rom - head) coil for experimenting with power MOSFETs and transistors. I control it with a potentiometer connected as a voltage divider. It works right I suppose but I get a tiny tremble. Is it normal? Can it be eliminated? (currently I don't use any heat-sink for the MOSFET). The power supply is a wall wart with 1 Amp rating.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Try connecting a 0.1 uF connector between gate and +12. \$\endgroup\$ – WhatRoughBeast Jul 31 '17 at 20:49
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    \$\begingroup\$ The way to dampen an RLC circuit is changing R and C (and potentially L, but you don't want to change that I assume). Adding a C in there on the gate slows the gate turn-on time which causes it to slow down the inductor current which is damping it in a way. \$\endgroup\$ – horta Jul 31 '17 at 21:18
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    \$\begingroup\$ There's inherent capacitance all over the system. Without using discrete components for the R and the C, you're at the mercy of the resistance of the wires and the capacitance of your wires and mosfet. \$\endgroup\$ – horta Jul 31 '17 at 21:19
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    \$\begingroup\$ @JohnAm: Don't delete the question. If nobody posts a decent answer then you can when you've figured it out what was wrong and how to fix it. You can even accept your answer as being correct. Your question and answer may help someone else in future. \$\endgroup\$ – Transistor Jul 31 '17 at 21:32
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    \$\begingroup\$ Are you sure you connected it correct? It's voltage rating should be 25V or higher for a circuit like this (with an inductor). Make sure you didn't connect it backwards. Also, what's the resistance of the inductor (and the inductance)? \$\endgroup\$ – horta Jul 31 '17 at 21:37
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Summarizing the comments: It would appear you're getting ringing in your circuit because the mosfet turning on is allowing your inductor to sink so much current that your supply rail is seeing a voltage droop. This should be solvable using whatRoughBeast's method of slowing down the inrush current of the mosfet by putting a cap on the gate of the mosfet to ground or Vdd. Either way will cause the mosfet to turn on slower preventing the supply rail from seeing voltage droop. Choose a capacitor big enough to slow down the rise time but small enough so that you can turn the inductor on and off quickly. It should be in the 10nF range probably.

Alternately, you could stiffen the power supply rail by adding a decoupling/bypass cap between the power rail and ground. Add this capacitor as close to the resistor going to the mosfet gate as possible. This capacitor should be much bigger than the one used for the previous solution since it's trying to stiffen the power rail. It should maybe be around 10uF.

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