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How to limit inrush current into DC motor simple way?
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Chriss:
I played with this kind of circuit formy soft-start stuff.
The C2 should be replaced with the mentioned dc motor.
I put it just for inrush current testing to this circuit.

How good would be this circuit with this parts parameters to soft-start a 12v car viper dc motor?

Thanks for any advice and ideas.
My best regards.
TerminalJack505:
The motor would be better modeled with a coil.  This way the simulation will show what happens when the motor (an inductive load) is suddenly shut off: the voltage will suddenly rise across the motor--be sure to protect against this.

In the case of your schematic, the motor would need to go on the high-side, between the positive voltage and the N-channel MOSFET.  You will also want to put a resistor between the MOSFET's source and gate to 1) keep stray charge from building up on the gate, and, 2) to discharge C1 when the motor is turned off.

Your circuit (with appropriate changes) might work but watch out for charge injection.  When a voltage suddenly appears at the MOSFET's drain it can cause the voltage at the gate to shoot up which will allow more current to flow then you intend to.  So it can basically foil your attempt to control the surge of current.

The suggestion someone made earlier about using an NTC thermistor is probably the simplest solution.  Although, if you can make it work with a MOSFET then you can solve the problem that the NTC will have--which is that you will need to wait for the NTC to cool down before you can start the motor again.
Chriss:
Thanks for the suggestion. I can manage the stuff with the fet, no problem.
Where I stuck a bit is :
How to set up a testing circuit so I can check how good is my circuit?
I mean, hiw can I monitor the inrush current without a coresponding amp meter ?
I have a scope and could do the math.
I thinking to put a shunt in series with themotor and measure the voltage drop through the shunt and calc the I based of Ohms law.

Would that be a correct procedure?
I can't really manage thesimulation in LTS. maybe somebody will be nice to help me to create a not so complex dc motor part so I can make some simulation in LTS?

I woukd really appreciate that.

I have no imendance meter so I can not check the impedance of my motor but I could set up a circuit with an R in series with my motor and feed some signal into and measure it with scooe and make the calc but all that would be a bit owerkilled now just to make the mechanical test...

Thank you to all of good people who are helping.

My best regards.
Zero999:

--- Quote from: Chriss on February 21, 2020, 08:56:02 pm ---I played with this kind of circuit formy soft-start stuff.
The C2 should be replaced with the mentioned dc motor.
I put it just for inrush current testing to this circuit.

How good would be this circuit with this parts parameters to soft-start a 12v car viper dc motor?

Thanks for any advice and ideas.
My best regards.

--- End quote ---
The only issue with that is the MOSFET is configured as a source follower, which means it will never turn fully on and drop a fairly high voltage, resulting in high power dissipation and the motor never running at full speed. You should put the motor in series with the drain, rather than the source.

One way to reduce the size of the capacitor is to connect it between the gate and drain, making use the Miller effect to multiply the capacitance.
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